P–Ru-Complexes with a Chelate-Bridge-Switch: A Comparison of 2-Picolyl and 2-Pyridyloxy Moieties as Bridging Ligands

Starting from [Ru(pyO)2(nbd)] 1 and a N,P,N-tridentate ligand (2a: PhP(pic)2, 2b: PhP(pyO)2) (nbd = 2,5-norbornadiene, pic = 2-picolyl = 2-pyridylmethyl, pyO = 2-pyridyloxy = pyridine-2-olate), the compounds [PhP(μ-pic)2(μ-pyO)Ru(κ2-pyO)] (3a) and [PhP(μ-pyO)3Ru(κ2-pyO)] (3b), respectively, were prepared. Reaction of compounds 3 with CO and CNtBu afforded the opening of the Ru(κ2-pyO) chelate motif with the formation of compounds [PhP(μ-pic)2(μ-pyO)Ru(κ-O-pyO)(CO)] (4a), [PhP(μ-pic)2(μ-pyO)2Ru(CNtBu)] (5a), [PhP(μ-pyO)4Ru(CO)] (4b) and [PhP(μ-pyO)4Ru(CNtBu)] (5b). In dichloromethane solution, 4a underwent a reaction with the solvent, i.e., substitution of the dangling pyO ligand by chloride with the formation of [PhP(μ-pic)2(μ-pyO)Ru(Cl)(CO)] (6a). The new complexes 3a, 4a, 5a, 5b and 6a were characterized by single-crystal X-ray diffraction analyses and multi-nuclear (1H, 13C, 31P) NMR spectroscopy. The different coordination behaviors of related pairs of molecules (i.e., pairs of 3, 4 and 5), which depend on the nature of the P–Ru-bridging ligand moieties (μ-pic vs. μ-pyO), were also studied via computational analyses using QTAIM (quantum theory of atoms in molecules) and NBO (natural bond orbital) approaches, as well as the NCI (non-covalent interactions descriptor) for weak intramolecular interactions.


Introduction
Hemilabile ligands are crucial in various kinds of homogeneous catalysis [1,2]. As an example of a relevant Ruthenium-based system, the introduction of a hemilabile site allowed for the development of the well-known Grubbs catalysts (I, Scheme 1), leading to the Grubbs-Hoveyda catalysts (II) [3]. The labile site of the hemilabile ligand stabilizes the metal complex in the absence of alternative electron pair donors, but it may give rise to a vacant coordination site for substrate binding "on demand". The labile ligator function remains a dangling group in close proximity. Recently, we reported on Ru complexes with 2-pyridyloxy (pyO) ligands, in which a chelating pyO group may exert hemilability by means of a coordinative switch between two centers. Rather than simply opening the chelate (with the formation of a monodentate pyO ligand with a dangling second donor site), the κ 2 -pyO motif is converted into a bridging µ-pyO motif within a paddle-wheel-like complex, then buttressing the connection to another ligand site in the Ru coordination sphere (Scheme 1) [4,5]. Starting from the initial [PhP(µ-pyO) 3 Ru(κ 2 -pyO)] system, we have shown that this switch also works for the related As-Ru system, and CO (for P-Ru and As-Ru) and NCMe (for As-Ru) were able to trigger this switch. In the related complex [PhSb(µ-pyO) 4 Ru(NCMe)], the rather strong binding of pyO to the Sb atom did not allow for the release of NCMe with the formation of a corresponding [PhSb(µ-pyO) 3 Ru(κ 2 -pyO)] chelate complex.
As only one pyO ligand is involved in switching, the aim of the current study was the exploration of the role of the remaining buttresses across the P-Ru core. In previous studies, we had explored systems with other (O,N)-bidentate ligands as well (anions of N-methylbenzamide and phthalimidine), which preferred the bridging position in Pn-Ru-complexes (Pn = pnictogen) and showed no tendency to form chelates at Ru [6][7][8].

Scheme 2.
Syntheses of the compounds under investigation.

Single-Crystal X-ray Diffraction
Compound 3a crystallized in the orthorhombic space group Pna21 with two independent (but conformationally very similar) molecules in the asymmetric unit ( Figure  1). The molecular configuration of 3a is the same as in the known compound 3b [4], i.e., the Ru atom is located in a distorted octahedral coordination sphere with a RuN4 equatorial plane and trans-situation of P-and O-donor sites. In both compounds, one pyO ligand chelates at Ru, and the Ru-P bond is bridged by three buttresses ((pic)2(pyO) in 3a, (pyO)3 in 3b).

Single-Crystal X-ray Diffraction
Compound 3a crystallized in the orthorhombic space group Pna2 1 with two independent (but conformationally very similar) molecules in the asymmetric unit ( Figure 1). The molecular configuration of 3a is the same as in the known compound 3b [4], i.e., the Ru atom is located in a distorted octahedral coordination sphere with a RuN 4 equatorial plane and trans-situation of P-and O-donor sites. In both compounds, one pyO ligand chelates at Ru, and the Ru-P bond is bridged by three buttresses ((pic) 2 (pyO) in 3a, (pyO) 3

Scheme 2.
Syntheses of the compounds under investigation.
Compound 4a crystallized from the mother solution as a toluene solvate in the monoclinic space group P21/c (Figure 3a). The molecular configuration of 4a exhibits two striking differences to the presumed analog 4b [4]. Whereas the latter resembles a paddlewheel-shaped molecule with trans situation of P-and CO-ligands at Ru, with all pyO moieties bound to Ru through Ru-N bonds, compound 4a exhibits cis arrangement of Pand CO-ligands at Ru with an only three-fold buttressed Ru-P bond. Furthermore, the monodentate pyO in 4a is Ru-O-bound with a dangling N donor site. This configuration about the Ru-P core is essentially retained in compound 6a, which is crystallized from dichloromethane solution as a DCM solvate in the monoclinic space group P21/n ( Figure  3b). Even though the molecular configuration of 4a was unexpected at first sight (with respect to the known paddle-wheel shape of 4b), [Ru(dppe)(CO)(NCMe)3][OTf]2 [18] is an example of another octahedral Ru(II) complex in which the CO ligand occupies a cis-P,trans-N position, even though sterics would allow for the alternative cis-N,trans-P arrangement. As for the Ru-O-bound monodentate pyO moiety, a paddle-wheel complex with a Ru≡Ru axis, which has been reported by Powers et al., also bears a Ru-O-bound pyO derivative [19]. Because of the configurational analogies of 4a and 6a, Figure 4 provides a direct comparison of these two molecular structures. Their Ru-P-C(phenyl) angles (132.0(2)° in 4a and 132.3(1)° in 6a) are essentially identical, and replacement of pyO by Cl (i.e., replacing of one π-donor ligand by another π-donor) did not alter the Ru-P bond length in a noteworthy manner. Additionally, in both compounds the dangling pyO oxygen atom coordinates the P atom from a remote position with slightly different separations of the P···O contacts (2.701(4) in 4a vs. 2.637(2) Å in 6a), with also only a small  Figure 1) and 3b [4].
Compound 4a crystallized from the mother solution as a toluene solvate in the monoclinic space group P2 1 /c (Figure 3a). The molecular configuration of 4a exhibits two striking differences to the presumed analog 4b [4]. Whereas the latter resembles a paddle-wheelshaped molecule with trans situation of P-and CO-ligands at Ru, with all pyO moieties bound to Ru through Ru-N bonds, compound 4a exhibits cis arrangement of P-and COligands at Ru with an only three-fold buttressed Ru-P bond. Furthermore, the monodentate pyO in 4a is Ru-O-bound with a dangling N donor site. This configuration about the Ru-P core is essentially retained in compound 6a, which is crystallized from dichloromethane solution as a DCM solvate in the monoclinic space group P2 1 /n ( Figure 3b). Even though the molecular configuration of 4a was unexpected at first sight (with respect to the known paddle-wheel shape of 4b), [Ru(dppe)(CO)(NCMe) 3 ][OTf] 2 [18] is an example of another octahedral Ru(II) complex in which the CO ligand occupies a cis-P,trans-N position, even though sterics would allow for the alternative cis-N,trans-P arrangement. As for the Ru-Obound monodentate pyO moiety, a paddle-wheel complex with a Ru≡Ru axis, which has been reported by Powers et al., also bears a Ru-O-bound pyO derivative [19]. Because of the configurational analogies of 4a and 6a, Figure 4 provides a direct comparison of these two molecular structures. Their Ru-P-C(phenyl) angles (132.0(2) • in 4a and 132.3(1) • in 6a) are essentially identical, and replacement of pyO by Cl (i.e., replacing of one π-donor ligand by another π-donor) did not alter the Ru-P bond length in a noteworthy manner. Additionally, in both compounds the dangling pyO oxygen atom coordinates the P atom from a remote position with slightly different separations of the P···O contacts (2.701(4) in 4a vs. 2.637(2) Å in 6a), with also only a small difference between the molecular shapes of 4a and 6a and similar to the corresponding P···O interatomic distance in 3a. Also related to 3a is the slight P-CH 2 bond elongation trans to this P···O contact. In the Ru coordination sphere, the corresponding Ru-N and Ru-C bonds also exhibit similar lengths. In this regard, the Ru-N trans-bond to the CO ligand is significantly longer than the other two Ru-N bonds of the mutually trans-situated pyridine moieties. We attribute this structural feature to weakened Ru→N π-back-bonding trans to the CO ligand, which itself causes strong Ru→C π-back-bonding. The latter is indicated by the C≡O stretching vibration at 1931 cm −1 , which gives rises to a strong band in the IR spectrum of compound 4a. This is just at slightly higher wave numbers than the C≡O stretch found for compound 4b (which is at 1921 cm −1 ) and indicative of strong Ru→C π-back-bonding. Additionally, this structural feature of a long Ru-N(pyridine) bond trans to a CO ligand can be found in other octahedral Ru(II) complexes, e.g., in the cationic complex [Ru(phen)(bpy)(CO)Cl] + with cis-arranged monodentate ligands [20].
difference between the molecular shapes of 4a and 6a and similar to the corresponding P···O interatomic distance in 3a. Also related to 3a is the slight P-CH2 bond elongation trans to this P···O contact. In the Ru coordination sphere, the corresponding Ru-N and Ru-C bonds also exhibit similar lengths. In this regard, the Ru-N trans-bond to the CO ligand is significantly longer than the other two Ru-N bonds of the mutually transsituated pyridine moieties. We attribute this structural feature to weakened Ru→N πback-bonding trans to the CO ligand, which itself causes strong Ru→C π-back-bonding. The latter is indicated by the C≡O stretching vibration at 1931 cm −1 , which gives rises to a strong band in the IR spectrum of compound 4a. This is just at slightly higher wave numbers than the C≡O stretch found for compound 4b (which is at 1921 cm −1 ) and indicative of strong Ru→C π-back-bonding. Additionally, this structural feature of a long Ru-N(pyridine) bond trans to a CO ligand can be found in other octahedral Ru(II) complexes, e.g., in the cationic complex [Ru(phen)(bpy)(CO)Cl] + with cis-arranged monodentate ligands [20].    Figure 3).
Compound 5a crystallized as a DCM solvate in the triclinic space group P1 ̅ ( Figure  5a) and the analogous isonitrile complex 5b crystallized in the orthorhombic space group Pnma (Figure 5b). In this set, the molecules adopt related configurations with transsituated P-and isonitrile-ligands at the RuN4 core. The most striking difference is associated with the different P-O-binding of the pyO bridging ligands. Even though the pyO oxygen atoms approach the P atom in a very direct manner (resulting in O-P-C angles in 5a and O-P-O angles in 5b wider than 170°), in 5a two rather long P···O contacts (2.814(2) and isonitrile-ligands at the RuN 4 core. The most striking difference is associated with the different P-O-binding of the pyO bridging ligands. Even though the pyO oxygen atoms approach the P atom in a very direct manner (resulting in O-P-C angles in 5a and O-P-O angles in 5b wider than 170 • ), in 5a two rather long P···O contacts (2.814(2) and 2.770(2) Å) leave room for a smaller Ru-P-C(phenyl) angle (143.9(1) • ), whereas in 5b the stronger binding of the pyO oxygen atoms (P-O separations of 1.713(3) and 2.309(3) Å) causes noticeable widening of the Ru-P-C(phenyl) angle (158.1(2) • ), thereby approaching a distorted octahedral coordination sphere of the P atom in 5b. In accordance with the pair 3a/3b, compound 5a exhibits a longer Ru-P bond than compound 5b (2.308(1) vs. 2.270(2) Å), which again can be attributed to the weaker π-acceptor phosphane in 5a. The trans-disposed Ru-C bond, however, is slightly shorter in 5a (1.957(2) vs. 1.995(6) Å). As isonitriles are ligands with significant π-acceptor features, the Ru1-C29 bond in 5a is likely to respond to the weaker competing π-acceptor phosphane with enhanced Ru→C π-backbonding. This explanation is supported by the slightly longer Ru-C bond (2.009(6) Å) in complex [Ru 2 (CO) 5 (tBuNC)(bpcd)] (bpcd = 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3dione) [21], which also features a tBuNC-Ru-P trans-arrangement and additional strong π-acceptor ligands. Furthermore, it is backed by IR spectroscopic data (bands of the CN stretch) for complexes 5a and 5b (see Appendix B).  Figure 3).

Solution and Solid State NMR Characterization
In the solid state, compound 3a bears a set of two chemically non-equivalent pyOligands (a chelating and a bridging group), and resulting therefrom two non-equivalent pic-moieties. In dichloromethane solution, the two types of pyO ligands undergo rather rapid exchange (with respect to the NMR time scale), as both the 1 H and 13 C NMR spectra of 3b exhibit one set of signals for pyO-and one for pic-ligands (for 1 H, 13 C and 31 P NMR spectra of 3a and of compounds 4a, 5a, 5b and 6a, see Figures S1-S19 in the Supplementary Materials). Especially in the 1 H spectrum, the former signals are noticeably broadened and do not exhibit coupling patterns (related exchange of pyO-ligands was found in 3b [4]). As to the pic-ligands, only their CH 2 groups suffer severe signal broadening both in 1 H and 13 C spectra. With respect to the phosphorus coordination sphere, the average coordination number in solution is close to the situation in the solid state, the 31 P NMR shift in CD 2 Cl 2 solution (80.8 ppm) is just slightly shifted downfield relative to the values found for the two crystallographically independent P sites in solid 3a (76.3 and 73.8 ppm). With respect to the 31 P NMR shift of the free phosphane PhP(pic) 2 (δ 31 P = −13.7 ppm in CDCl 3 ), the corresponding NMR signal of 3a is shifted downfield by about 90 ppm. The products arising from 3a (i.e., 4a, 5a and 6a) may exhibit different configurations, which will be indexed with superscript-1 and superscript-3 in accord with the isomers under investigation by computational analyses (vide infra). In the crystal structures (Figures 3 and 5), we encountered 4a 1 , 5a 3 and 6a 1 . In solution 1 H NMR spectra, the pic-CH 2 groups are a convenient probe for assigning configurations 1 and 3, as in the former the molecules bear four chemically non-equivalent CH 2 protons, while in the latter the molecules bear two symmetry-related CH 2 groups with diastereotopic protons. Figure 6 shows the section of CH 2 signals of the 1 H NMR spectra of compounds 3a, 4a, 5a and 6a.
ligands (a chelating and a bridging group), and resulting therefrom two non-equivalent pic-moieties. In dichloromethane solution, the two types of pyO ligands undergo rather rapid exchange (with respect to the NMR time scale), as both the 1 H and 13 C NMR spectra of 3b exhibit one set of signals for pyO-and one for pic-ligands (for 1 H, 13 C and 31 P NMR spectra of 3a and of compounds 4a, 5a, 5b and 6a, see Figures S1-S19 in the Supplementary Materials). Especially in the 1 H spectrum, the former signals are noticeably broadened and do not exhibit coupling patterns (related exchange of pyO-ligands was found in 3b [4]). As to the pic-ligands, only their CH2 groups suffer severe signal broadening both in 1 H and 13 C spectra. With respect to the phosphorus coordination sphere, the average coordination number in solution is close to the situation in the solid state, the 31 P NMR shift in CD2Cl2 solution (80.8 ppm) is just slightly shifted downfield relative to the values found for the two crystallographically independent P sites in solid 3a (76.3 and 73.8 ppm). With respect to the 31 P NMR shift of the free phosphane PhP(pic)2 (δ 31 P = −13.7 ppm in CDCl3), the corresponding NMR signal of 3a is shifted downfield by about 90 ppm. The products arising from 3a (i.e., 4a, 5a and 6a) may exhibit different configurations, which will be indexed with superscript-1 and superscript-3 in accord with the isomers under investigation by computational analyses (vide infra). In the crystal structures (Figures 3  and 5), we encountered 4a 1 , 5a 3 and 6a 1 . In solution 1 H NMR spectra, the pic-CH2 groups are a convenient probe for assigning configurations 1 and 3, as in the former the molecules bear four chemically non-equivalent CH2 protons, while in the latter the molecules bear two symmetry-related CH2 groups with diastereotopic protons. Figure 6 shows the section of CH2 signals of the 1 H NMR spectra of compounds 3a, 4a, 5a and 6a. Figure 6. Section of the CH 2 signals of the 1 H NMR spectra of (from bottom to top) 3a, 4a, 6a and 5a in CD 2 Cl 2 (the signal at 5.1 ppm is a 1 J( 13 C 1 H) satellite of CDHCl 2 solvent signal, the asterisks indicate signals of 3a).
Compound 6a (devoid of a second pyO group) must exhibit a configuration with four chemically independent CH 2 protons. Its 1 H spectrum clearly shows the four signals, which are "dd" patterned by 2 J( 31 P 1 H) and 2 J( 1 H 1 H) coupling for each proton (Figure 6, red). Apart from the signals of small amounts of 3a, the spectrum of 4a exhibits essentially the same signal pattern as 6a, showing retention of configuration 4a 1 in the solution (Figure 6, blue). The 31 P NMR signal of 4a in the CD 2 Cl 2 solution (δ 31 P = 68.6 ppm) is slightly shifted upfield with respect to the signal of 3a. In the solid state, 31 P NMR signals of 4a were found at 63.5 and 68.1 ppm (Figure 7). Again, the set of crystallographically independent molecules gives rise to more than just one signal. Furthermore, the 31 P CP/MAS NMR spectrum of 4a indicates the presence of 3a in the solid product. Hence, the signals of 3a encountered in solution spectra of 4a arose from impurities in the solid product rather than from decomposition of 4a with release of CO.
slightly shifted upfield with respect to the signal of 3a. In the solid state, P NMR signals of 4a were found at 63.5 and 68.1 ppm (Figure 7). Again, the set of crystallographically independent molecules gives rise to more than just one signal. Furthermore, the 31 P CP/MAS NMR spectrum of 4a indicates the presence of 3a in the solid product. Hence, the signals of 3a encountered in solution spectra of 4a arose from impurities in the solid product rather than from decomposition of 4a with release of CO. Upon addition of some drops of CNtBu to a CD2Cl2 solution of 3a, which corresponds to large excess of the isonitrile, spectra 5a (initial 1) and 5a (initial 2) were obtained (sample 5a (initial 1) contained somewhat larger excess of CNtBu, while in sample 5a (initial 2) there was only a 15% excess according to integral traces of Ru-bound and free CNtBu), cf. Figure 6 (olive, orange). The appearance of only two "dd"-patterned CH2 proton signals indicates the formation of configurational isomer 5a 3 under these conditions, which is in accordance with the structure of 5a found in the solid state. Within 20 h, the latter sample had undergone some decomposition (pale green spectrum 5a (initial 2′)). Large amounts of 3a formed (thus, 5a 3 must have released CNtBu), and CH2 signals of a new complex appeared. (Unfortunately, we were not able to isolate this new compound by layering the NMR sample solution with n-pentane at this stage.) Upon dissolution of some crystals of the isolated 5a in CD2Cl2, however, a 1 H NMR spectrum was obtained, which indicated the presence of both isomers 5a 3 and presumably 5a 1 (the coupling patterns of the four CH2 signals are very well in accordance with those of 4a and 6a), as well as significant amounts of 3a ( Figure 6, purple). Again, the latter can be explained by dissociation of 5a in solution with formation of 3a and CNtBu in a dynamic equilibrium as the (corresponding to the proton intensities of 3a) relative intensity of 9 H atoms was observed for the signal of free CNtBu in the same spectrum. As compound 5a (isomer 5a 3 ) forms in Upon addition of some drops of CNtBu to a CD 2 Cl 2 solution of 3a, which corresponds to large excess of the isonitrile, spectra 5a (initial 1) and 5a (initial 2) were obtained (sample 5a (initial 1) contained somewhat larger excess of CNtBu, while in sample 5a (initial 2) there was only a 15% excess according to integral traces of Ru-bound and free CNtBu), cf. Figure 6 (olive, orange). The appearance of only two "dd"-patterned CH 2 proton signals indicates the formation of configurational isomer 5a 3 under these conditions, which is in accordance with the structure of 5a found in the solid state. Within 20 h, the latter sample had undergone some decomposition (pale green spectrum 5a (initial 2 )). Large amounts of 3a formed (thus, 5a 3 must have released CNtBu), and CH 2 signals of a new complex appeared. (Unfortunately, we were not able to isolate this new compound by layering the NMR sample solution with n-pentane at this stage.) Upon dissolution of some crystals of the isolated 5a in CD 2 Cl 2 , however, a 1 H NMR spectrum was obtained, which indicated the presence of both isomers 5a 3 and presumably 5a 1 (the coupling patterns of the four CH 2 signals are very well in accordance with those of 4a and 6a), as well as significant amounts of 3a (Figure 6, purple). Again, the latter can be explained by dissociation of 5a in solution with formation of 3a and CNtBu in a dynamic equilibrium as the (corresponding to the proton intensities of 3a) relative intensity of 9 H atoms was observed for the signal of free CNtBu in the same spectrum. As compound 5a (isomer 5a 3 ) forms in the first instance upon adding CNtBu to 3a, but also undergoes dissociation (with formation of 3a and CNtBu) as well as decomposition in solution (most likely driven by free CNtBu), the solution NMR data reported in the experimental section are based on spectra of 5a generated in situ. The identity of the decomposition products (apart from 3a) has not been established yet. This initial formation of 5a 3 and rather quick decomposition observed for this compound in DCM solution (which should include formation of 5a 1 ) serves as an explanation as to why we obtained crystalline 5a (mixture of isomers) in very poor yield, and timely workup of the synthesis mixture was required to isolate 5a at all.
The related compound 5b, in CD 2 Cl 2 solution, produced one set of 1 H and 13 C signals of the pyO moieties, indicating both retention of the paddle-wheel isomer 5b 3 in solution and either the transition toward a more symmetrical arrangement of ligands (with essentially equal P-O bond lengths) or rapid exchange of the pairs of short and long transsituated P-O bonds. Additionally, the 31 P NMR shift of 5b (28.0 ppm), which is noticeably shifted upfield with respect to the starting material 3b (135.0 ppm) [4] and much closer to the shift of 4b (−9.9 ppm) [4], indicates retention of the higher coordination number of the P atom of 5b in solution. Furthermore, the 1 J(P-C(Phenyl)) coupling (225 Hz for 5b in CD 2 Cl 2 solution, 277 Hz for 4b [4]) indicates related coordination spheres about the P atoms of these two compounds. In contrast to 5a, the 1 H NMR spectrum of 5b did not hint at the liberation of CNtBu in solution. (Traces of 3b, the presence of which is apparent in the 31 P NMR spectrum with a signal at 136.1 ppm, must have been contaminations in the solid product).

Relative Stability of Configurational Isomers
The question as to why compound 4a formed the isomer with cis-disposed P-and CO-ligands was addressed with the aid of computational analyses. For that purpose, the molecular structures of seven different isomers of 4a were optimized with consideration of the effects of solvent (COSMO model for toluene), dispersion and relativistic effects. Figure 8 shows the molecular structures of the isomers. (The atomic coordinates and total energies of isomers under investigation in this chapter are listed in the Supplementary Materials, see Figures S20-S32 and Tables S1-S13. Isomer 4a 1 corresponds to the isomer found in the crystal, cf. Figure 3a. Isomers 4a 2 and 4a 3 resemble paddle-wheel-shaped isomers (P-trans-CO arrangement) with mutually transor cis-disposed pic-CH 2 -groups, respectively. Another set of isomers is related to isomer 4a 1 but with Ru-N-bound monodentate pyO ligand (trans to the phosphane moiety.) The hindered rotation about its Ru-N bond gave rise to four different local minima 4a 4 -4a 7 . According to this analysis, the crystallographically encountered isomer 4a 1 represents the favored isomer, whereas the paddle-wheel-shaped isomers 4a 2 and 4a 3 are about 35 and 15 kcal mol −1 less stable, respectively. The trans-arrangement of the CH 2 -P-CH 2 motif (in 4a 2 ) exerts a particularly destabilizing effect. Isomers 4a 4 -4a 7 (with relative energies ranging between 3 and 8 kcal mol −1 ) are noticeably more stable than the paddle-wheel forms. As CO complex 4b and isonitrile complexes 5a and 5b crystallized as isomers with configurations related to paddle-wheels (related to 4a 3 in particular), Gibbs free energy values were analyzed for those isomers (denoted with superscript-3) in comparison with their respective isomer, which corresponds to the cis-P-Ru-CO arrangement and Ru-Obound pyO in 4a 1 (isomers denoted with superscript-1). Surprisingly, 3b 1 , 5a 1 and 5b 1 were found to be the thermodynamically favored isomers. However, their paddle-wheelshaped alternatives were only 4.3 kcal mol −1 (4b 3 ) and 6.2 kcal mol −1 (5b 3 ) less stable for (μ-pyO)4-bridged complexes. The pic-ligands, however, destabilize the paddle-wheel arrangement for isonitrile complex 5a too (5a 3 : +12.9 kcal mol −1 ).
Even though solvent effects, dispersion effects and relativistic effects have been considered in the above analysis, we cannot rule out computational experimental errors which could amount to error bars of some kcal mol −1 for the relative energies and isomer 3 to be slightly more stable than isomer 1 in some cases. However, in the context of the As CO complex 4b and isonitrile complexes 5a and 5b crystallized as isomers with configurations related to paddle-wheels (related to 4a 3 in particular), Gibbs free energy values were analyzed for those isomers (denoted with superscript-3) in comparison with their respective isomer, which corresponds to the cis-P-Ru-CO arrangement and Ru-Obound pyO in 4a 1 (isomers denoted with superscript-1). Surprisingly, 3b 1 , 5a 1 and 5b 1 were found to be the thermodynamically favored isomers. However, their paddle-wheelshaped alternatives were only 4.3 kcal mol −1 (4b 3 ) and 6.2 kcal mol −1 (5b 3 ) less stable for (µ-pyO) 4 -bridged complexes. The pic-ligands, however, destabilize the paddle-wheel arrangement for isonitrile complex 5a too (5a 3 : +12.9 kcal mol −1 ).
Even though solvent effects, dispersion effects and relativistic effects have been considered in the above analysis, we cannot rule out computational experimental errors which could amount to error bars of some kcal mol −1 for the relative energies and isomer 3 to be slightly more stable than isomer 1 in some cases. However, in the context of the observations made in solution (cf. NMR spectroscopy Section 2.3), we attribute the formation of isomers 4b 3 , 5a 3 and 5b 3 to kinetic effects (preferred pyO-chelate opening with dissociation of the Ru-O bond and formation of the complex with trans-P-Ru-C arrangement, and if thermodynamically favorable and kinetically feasible, conversion into a different isomer). Rearrangement into the thermodynamically more stable isomer 1 would involve some steps such as changes of Ru-N-vs. Ru-O-coordination modes of a dangling pyO group and site exchange of the CO or isonitrile ligand. For complexes 4b and 5b, both steps are kinetically hindered to a greater extent. The O atom of the otherwise "dangling" pyO ligand is incorporated in a more or less tight bond with the P atom, and the unavailability of this O donor site appears to hinder Ru-C dissociation as well (no formation of free CNtBu observed in solution of 5b).

P···O and C-H···(O,C) Interactions
Comparison of pairs of related molecular structures (3a/3b and 5a/5b, see Section 2.2) allowed for the conclusion that pic-moieties at the phosphane ligand (in 3a and 5a) destabilize the bridging coordination mode of the pyO moieties in the same molecule. In order to elucidate the effect of pic-vs. pyO substitutions on different facets of bonding, computational analyses in this and the following sections were performed for structurally related molecules, which allowed for comparisons, i.e., the pair of related molecules 3a/3b as well as the more or less paddle-wheel-shaped complexes 4b, 5a and 5b. In the pic-functionalized compounds, bridging pyO groups establish P···O contacts with rather long interatomic separations only, which hint at weak electrostatic attraction. Therefore, we visualized the P-pic-and P-pyO-interplay with the non-covalent interactions descriptor (NCI, Figure 9). In addition to the Ru-N bond, the bridging pyO moieties establish two general types of further attractive interactions, which stabilize them in their bridging position: (A) an attractive P-O-contact and (B) an attractive hydrogen contact between H 6 of the pyO group and the ligand atom trans-disposed to the phosphane. In Figure 9, these interactions are pointed out for 3a with red arrows. Whereas the P-O-contacts in pyO-bridged complexes 3b and 4b are dominated by covalent interactions, the longer P-O contacts in 5b (still exhibiting covalent contributions) are transitioning toward electrostatic interactions. In the pic-bridged complexes 3a and 5a, the P···O-contacts are non-covalent in nature, and according to the color scale are of similar intensity as the C-H···O contacts in 3a. Compounds 4b, 5a and 5b exhibit electrostatic attraction between the pyO-H 6 and the C atom of the monodentate ligand trans to the phosphane (CO or isonitrile, respectively). This attraction, however, is less intense than the corresponding C-H···O interactions in 3a and 3b, which can be attributed to the lower electronegativity of C vs. O. Thus, one contribution to the driving force in the formation of isomer 4a 1 (cf. Figure 8) can be attributed to the retention of the trans-P-Ru-O arrangement, which electrostatically stabilizes three bridging ligands via C-H···O interaction at the cost of an only weakly attractive potential P···O interaction (which could have been established in paddle-wheel isomer 4a 3 ).
isonitrile, respectively). This attraction, however, is less intense than the corresponding C-H···O interactions in 3a and 3b, which can be attributed to the lower electronegativity of C vs. O. Thus, one contribution to the driving force in the formation of isomer 4a 1 (cf. Figure 8) can be attributed to the retention of the trans-P-Ru-O arrangement, which electrostatically stabilizes three bridging ligands via C-H···O interaction at the cost of an only weakly attractive potential P···O interaction (which could have been established in paddle-wheel isomer 4a 3 ). compounds 3a, 3b, 4b, 5a and 5b with color scale (iso-value 0.45; blue zones indicate attractive interaction, red zones indicate repulsive interactions).

Topological Analysis with Quantum Theory of Atoms-In-Molecules
For the analysis of certain characteristic bond features, to supplement the insights from Section 2.4.2, a quantum theory of atoms-in-molecules (QTAIM) analysis was performed for compounds 3a, 3b, 4a, 5a and 5b. Table 1 lists selected characteristic features for selected bonds at their (3,−1) critical points (i.e., bond critical points, BCPs). In all cases, BCPs were detected between the P atoms and the O atoms of the bridging pyO ligands; therefore, the weak P···O interactions are included in this discussion. The Ru-P bonds are similar to one another in terms of electron density ρ at the BCP. A slight decrease in this value is observed from compounds 3 via 4 to 5, which is in accord with the trend of increasing bond length in this order. The ratio of the modulus of the potential energy density per Lagrangian kinetic energy density is in the range of 1 < |V(r b )|/G(r b ) < 2 in all cases and is indicative of an intermediate bond characteristic (i.e., closed-shell covalent bond with additional ionic contribution). The Ru-P bonds do not exhibit any noticeable ellipticity ε of the electron density (≤0.1 in all cases). However, the Wiberg bond index (WBI) indicates certain multiple bond characteristics for compounds 3 (WBI ≈ 1.4-1.5), whereas a WBI close to 1 is shown for Ru-P single bonds in compounds 4 and 5. In compounds 4 and 5, the Ru-C bonds to CO or isonitrile, respectively, exhibit some multiple bond characteristics, with WBI values ranging between 1.4 and 1.7. As for Ru-P, the Ru-C bonds' ellipticity of electron density is also < 0.1, indicating the radial symmetry of the bonding contributions. The π-donor trans to Ru-P in 3a and 3b, as well as the higher multiple bond character of Ru-CO in 4b over Ru-CNtBu in 5a and 5b in combination with the lower WBI of Ru-P in 4b, indicate competing Ru→L π-acceptor ligand contributions as the origin of this variety of WBI values observed for compounds 3, 4 and 5. Table 1. Selected features of (3, −1) critical points in compounds 3a, 3b, 4a, 5a and 5b. Note: electron density ρ(r b ) in au, Laplacian of the electron density ∇ 2 ρ(r b ) in au, Lagrangian kinetic energy density G(r b ) in au, potential energy density V(r b ) in au, ratios |V(r b )|/G(r b ) and G(r b )/ρ(r b ) in au, electron energy density H(r b ) in au, ellipticity of the electron density ε and Wiberg Bond Index (WBI).

Compd
Bond  [4]. 3 This set of P-O bonds in 5b is noticeably shorter than the P-O bonds in compound 4b (1.71 vs. 1.83-1.90 Å) [4]. 4 Because of the symmetry of the molecule (cf. Figure 5b), it features two pairs of chemically equivalent P-O-interactions. Redundant data have been omitted from this table.
The P-O bonds in compounds 3, 4 and 5 can be divided into three groups. The very long P···O contacts (as found in pic-bridged Ru-P-complexes 3a and 5a) exhibit very low electron density values at the BCP (ca. 0.02 au), a ratio |V(r b )|/G(r b ) very close to 1 and a WBI close to 0.1, supporting the interpretation of weak donor-acceptor interactions. Furthermore, the electron energy density H(r b ) is close to zero, also supporting the absence of covalent bonding. In the context of their low total electron density, minor variations in electron density distribution already cause large effects on ε, while the values of ε > 0.3 encountered with these P···O contacts should not be interpreted as the results of multiple bonding. The second group of P-O-interactions corresponds to formally covalent P-O single bonds with a WBI close to 1. The short ones of this group (i.e., the apical P-O bond at the square-pyramidal-coordinated P atom in 3b and the shorter P-O bonds of 5b) exhibit electron densities in the range 0.14-0.16 au, a WBI slightly above 1 and a ratio |V(r b )|/G(r b ) close to 1.5. The latter, as mentioned above, is characteristic of closed-shell covalent bonds with additional ionic contributions. The longer ones of this group, which are part of nearly symmetrical linear O-P-O-arrangements, exhibit WBI values slightly below 1, somewhat lower electron density values (in the range 0.10-0.12 au) and a ratio |V(r b )|/G(r b ) closer to 2. Most of these bonds also exhibit noticeably enhanced ellipticity of their electron density at the BCP. The third "class" of P-O bonds encountered with these compounds is the pair of long P-O bonds in 5b. Their features are intermediate between those of the two former groups: an electron density value of 0.05 au, 1.5 < |V(r b )|/G(r b ) < 1 and WBI of 0.36. This intermediate situation of this set of P-O bonds, as detected by this topological analysis, is in accord with the intermediate situation found for the same bonds in the NCI analysis (cf. Figure 9).

NBO-/NLMO-Analyses
For a closer view of π-back-bonding contributions and weak donor-acceptor-interactions between pyO-ligands and P atoms, as well as for insights into the atomic contributions to the Ru-P σ-bond, we analyzed natural bond orbitals (NBOs) and natural localized molecular orbitals (NLMOs) of compounds 3a, 3b, 4b, 5a and 5b. Table 2 lists the natural charges (NCs) of these compounds' Ru-and P-atoms as well as these atoms' contributions to the Ru-P σ-bond. The NCs of the Ru atoms are only slightly positive, and replacement of the Ru-bound O atom by a monodentate ligand (CO or CNtBu) lowers the Ru atom's NC by ca. 0.15. Even though an anionic π-donor ligand atom is replaced by a charge-neutral π-acceptor ligand, the different electronegativities (O vs. C) appear to dominate the effect on the NC. In accord, replacing two P-bound pyO moieties by pic moieties (i.e., replacement of P-O by P-C bonds) results in lowering of the P atom's NC by ca. 0.4. In spite of the variable number of additional O-donor sites in its proximity, the P-atom's NC is almost identical for the PhP(pyO) n Ru-compounds 3b, 4b and 5b. For those compounds, which exhibit σ-O→P donor-acceptor interactions (3a, 5a, 5b), second-order perturbation theory analysis revealed the relevant orbital interactions ( Figure 10) and the energies E(σ-O→P) listed in Table 2. The increasing intensity of those interaction energies (ca. 4, 6 and 18 kcal mol −1 for 5a, 3a and 5b, respectively) resembles the increasing intensity indicated along this series in the NCI analyses ( Figure 9). Table 2. Natural charges (NCs) of Ru-and P-atoms and contributions to the NLMO of the Ru-P σ- bonds of compounds 3a, 3b, 4b, 5a and 5b, as well as energy levels of selected intramolecular donor-acceptor interactions (obtained from second-order perturbation theory) in kcal mol −1 (∑E(π-Ru→P) = sum of π-back-bonding contributions into relevant σ-antibonding P-O-or P-Cbased orbitals (∑E(π-Ru→C) = sum of π-back-bonding contributions into relevant π-antibonding C-O-or C-N-based orbitals of the CO or CNtBu ligand). 1 Even though NLMO analyses had been performed for 3b and 4b previously [4], we repeated the calculations with the method-basis set combination used in the current paper for the sake of comparability.
The σ-Ru-P bonds, in most cases, can be interpreted as polar covalent bonds with ca. 2/3 phosphorus contributions. Comparison of the σ-Ru-P relevant NLMOs of compounds 3a and 3b shows that the exchange of bridging moieties (pic vs. pyO) has only a marginal influence on the Ru-P σ-bond. For compounds 4b and 5a, NBO/NLMO analyses initially afforded delocalized Ru-P σ-bonds (noticeable delocalization of the σ-Ru-P NLMO across the P-Ru-C axis, involving significant atomic orbital contributions of the trans-disposed ligand, CO or CNtBu). This was not unexpected, as in a previous analysis we had already encountered such a delocalized situation with 4b [4]. For the sake of comparability, in order to obtain corresponding NLMOs with predominant two-atom contributions in compounds 4b and 5a as well, the occupancy threshold of the Lewis structure search was adjusted (from an initial value of 1.65 to 1.57 or 1.51 for 4b or 5a, respectively). The NLMOs thus obtained revealed close similarities between 5a and 5b, underlining that pic-vs. pyO-exchange does not affect the σ-Ru-P bond significantly. (Graphical representations of the σ-Ru-P NLMOs can be found in the Supporting Information, Figure S34.) The composition of the σ-Ru-P NLMO of compound 4b hints at a more covalent situation (equal orbital contributions from both atoms involved).  The σ-Ru-P bonds, in most cases, can be interpreted as polar covalent bonds with ca. 2/3 phosphorus contributions. Comparison of the σ-Ru-P relevant NLMOs of compounds 3a and 3b shows that the exchange of bridging moieties (pic vs. pyO) has only a marginal influence on the Ru-P σ-bond. For compounds 4b and 5a, NBO/NLMO analyses initially afforded delocalized Ru-P σ-bonds (noticeable delocalization of the σ-Ru-P NLMO across the P-Ru-C axis, involving significant atomic orbital contributions of the trans-disposed ligand, CO or CNtBu). This was not unexpected, as in a previous analysis we had already encountered such a delocalized situation with 4b [4]. For the sake of comparability, in order to obtain corresponding NLMOs with predominant two-atom contributions in compounds 4b and 5a as well, the occupancy threshold of the Lewis structure search was adjusted (from an initial value of 1.65 to 1.57 or 1.51 for 4b or 5a, respectively). The NLMOs thus obtained revealed close similarities between 5a and 5b, underlining that pic-vs. pyOexchange does not affect the σ-Ru-P bond significantly. (Graphical representations of the σ-Ru-P NLMOs can be found in the Supporting Information, Figure S34.) The composition of the σ-Ru-P NLMO of compound 4b hints at a more covalent situation (equal orbital contributions from both atoms involved).
Replacing pyO-with pic-bridges, however, has significant influence on the Ru-P bond with respect to Ru→P π-back-donation. For compound 3b, second-order perturbation theory analysis revealed a total of 30.4 kcal mol −1 Ru→P π-back-bonding energy with two major contributions of 20.2 and 7.8 kcal mol −1 into the P-O-based σantibonding orbitals and a minor contribution of 2.3 kcal mol −1 associated with σ*(P-C(Ph)). In compound 3a, the energetically small back-bonding contributions into the σ*(P-C) NBOs result in a total of only 11.5 kcal mol −1 . With respect to 3b, the weaker Ru→P Replacing pyO-with pic-bridges, however, has significant influence on the Ru-P bond with respect to Ru→P π-back-donation. For compound 3b, second-order perturbation theory analysis revealed a total of 30.4 kcal mol −1 Ru→P π-back-bonding energy with two major contributions of 20.2 and 7.8 kcal mol −1 into the P-O-based σ-antibonding orbitals and a minor contribution of 2.3 kcal mol −1 associated with σ*(P-C(Ph)). In compound 3a, the energetically small back-bonding contributions into the σ*(P-C) NBOs result in a total of only 11.5 kcal mol −1 . With respect to 3b, the weaker Ru→P π-back-donation in 3a is in accord with the lower WBI and the longer Ru-P bond found for 3a. Introduction of transdisposed π-acceptor ligands (CO or CNtBu) lowers the Ru-P π-back-donation, as expected. Interestingly, in spite of the stronger π-acceptor CO, π-back-bonding to the phosphorus site is still more efficient in 4b than in 5b. We attribute this to the more symmetrical (nearly square-shaped) PO 4 moiety of 4b, the σ*-O-P-O orbitals of which serve as π-acceptors. This formal flow of electron density in the Ru→P direction in the π-system appears to be compensated for by enhanced σ-Ru←P donation. The contributions of P and Ru to the NLMO, which is representative of the σ-bond, indicate a shift of the electron pair toward Ru. (Graphical representations of the NBOs involved in π-Ru→C, and where applicable π-Ru→P interactions can be found in the Supporting Information, Figures S35-S37).
The geometry optimizations were carried out with ORCA 5.0.2 [30] using the restricted PBE0 functional with a relativistically recontracted Karlsruhe basis sets ZORA-def2-TZVPP [31,32] (for H, C, N, O, P) and SARC-ZORA-TZVPP (for Ru) [33], the scalar relativistic ZORA Hamiltonian [34,35], atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [36,37] and COSMO solvation (toluene, ε = 2.38, r solv = 3.48). Very-TightSCF and slowconv options were applied and the DEFGRID3 was used with a radial integration accuracy of 10 for ruthenium for all calculations. Calculations were started from the molecular structures obtained by single-crystal X-ray diffraction analysis and isomers were created by modifying these structures. Numerical frequency calculations were performed to prove convergence at the local minimum after geometry optimization and to obtain the Gibbs free energy (293.15 K). The calculated C≡N stretching vibrations were taken from the numerical frequency calculations. On the final structures, single-point calculations were performed with a restricted B2T-PLYP functional with relativistically recontracted Karlsruhe basis sets ZORA-def2-TZVPP [31,32] (for H, C, N, O, P) and SARC-ZORA-TZVPP (for Ru) [33] and utilizing the AutoAux generation procedure [38], the scalar relativistic ZORA Hamiltonian [34,35], atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [36,37] and COSMO solvation (toluene).

Syntheses and Characterization
Compound 3a ([PhP(µ-pic) 2 (µ-pyO)Ru(κ 2 -pyO)], C 28 H 25 N 4 O 2 PRu). A Schlenk flask was charged with magnetic stirring bar, [Ru(pyO) 2 (nbd)] (1) (0.525 g, 1.50 mmol) and PhP(pic) 2 (2a) (0.440 g, 1.50 mmol), evacuated and set under Ar atmosphere prior to adding acetonitrile (10 mL). The resultant dispersion was heated and stirred under reflux. Within the first ten minutes of heating, the color of the dispersion changed from yellow to orange. Upon 3 h of heating, the mixture was allowed to attain room temperature. The solid product was filtered off, washed with acetonitrile (2 × 3 mL) and dried in vacuum. (Crystals suitable for X-ray diffraction analysis were grown from a dichloromethane solution of this product upon diffusion of diethyl ether via gas phase in the course of one week.) Yield: 0.589 g (1.01 mmol, 68%). Elemental analysis for C 28   199 mmol) and toluene (2.5 mL) was cooled in liquid N 2 prior to evacuating the initial atmosphere and recharging with CO atmosphere in 3 cycles. (The gas volume in the Schlenk flask (>10 mL) corresponds to excess CO (>0.45 mmol).) Then, the contents were allowed to attain room temperature, and the resultant orange dispersion was stirred at room temperature for two days. On the third day, the contents were stirred at 60 • C for 6 h (the contents remained a dispersion) and then stored at room temperature for 3 days. (Some crystals suitable for X-ray diffraction analysis were taken from the crude product.) Thereafter, the contents were separated from the supernatant by decantation, washed with toluene (1.5 mL) and briefly dried in vacuum. Yield: 0.05 g (0.07 mmol, ca. 35%) of 4a·(toluene). 31 P NMR spectroscopy of both the solid and CD 2 Cl 2 solution of this product indicated the presence of some starting material (contains ca. 15% 3a). Therefore, elemental analysis data are not reported. An attempt at recrystallizing crude 4a·(toluene) from dichloromethane (with gas phase diffusion of n-pentane) afforded some crystals of complex 6a. (The formation of 6a may originate from traces of HCl contained in dichloromethane. Nonetheless, even though pyridine itself is not sufficiently nucleophilic to undergo nucleophilic substitution of chloride from DCM under mild conditions [46], metal-bound pyridyl groups have been shown to undergo nucleophilic attack at DCM [47,48]. Thus, the latter path cannot be ruled out at the current stage. However, side-products of the formation of 6a were not identified.) Analogous synthesis of stirring a dispersion of 3a (0.095 g, 0.16 mmol) in acetonitrile (2 mL) under an atmosphere of CO afforded a clear solution within one day, and the solution remained clear for one week. Gas-phase diffusion of diethyl ether did not result in crystallization of the target product, and a 31 P NMR spectrum recorded from this crude solution (products in MeCN/Et 2 O) indicated the presence of both 4a and starting material 3a with signals at 68.9 and 80.2 ppm, respectively, at an intensity ratio of 2:1. The limited amount of sample available (especially in case of 6a) and decomposition/reaction with solvent (in case of 4a), 1D 13 C{ 1 H} and 2D 13 C NMR spectroscopy did not allow for detecting all 13 C signals or assignment of all signals observed. Thus, the 13 C NMR shifts reported here basically serve as fingerprints of 4a and 6a.
NMR data for 4a (recorded from the crude product of 4a·(toluene)): 1  (in our case a chelating 2-pyridyloxy group, pyO) can undergo chelate opening and take advantage of stabilization of the dangling ligator function by binding to the adjacent P atom. Whereas the pic groups (in the starting material PhP(pic) 2 and in the Ru complexes resulting therefrom) imply the advantage of a more robust building block with respect to lowered hydrolytic sensitivity compared with related pyO-based systems (PhP(pyO) 2 and in the Ru complexes resulting therefrom), they lower the Lewis acidity of the P atom. This affects both the nature of the P-Ru bond (which features significantly lowered Ru→P π-back-bonding contributions) and the tendency for binding of the dangling ligand arm to the P atom. Thus, in PhP(pic) 2 -based systems (3a, 4a, 5a), the latter is noticeably less pronounced than in the related PhP(pyO) 2 -based Ru complexes (3b, 4b, 5b). This fosters reactions back toward formation of the Ru(κ 2 -pyO)-chelate (with release of monodentate ligands in equilibrium, such as Ru-bound isonitrile) or even Ru(κ 2 -pyO)-chelate opening with formation of Ru(κ-O-pyO)-complexes, which feature a dangling pyO nitrogen atom. The dangling N atom of the latter may cause unwanted side reactions (e.g., reaction of 4a and dichloromethane or traces of HCl contained therein with formation of 6a).
In summary, further exploration of related kinds of coordinative switches within Ru-P-systems may benefit from electronegative substituents at the P atom. In general, the subject matter of ligand migration from Ru to an adjacent P atom is worth exploring further. In a recent study, Tanushi and Radosevich showed the migration of an Ru-bound hydride to a special phosphane ligand III (which also bears pyridine anchors as Ru-binding site) with the formation of complex IV (Scheme 3) [49]. This hints at the greater potential of such systems for stabilizing monodentate ligands with an Ru-bound phosphane P atom. hydride to a special phosphane ligand III (which also bears pyridine anchors as Rubinding site) with the formation of complex IV (Scheme 3) [49]. This hints at the greater potential of such systems for stabilizing monodentate ligands with an Ru-bound phosphane P atom.
From an academic point of view, the herein presented compound 5b represents a rare example of a monometallic phosphane complex with hexacoordination of both the transition metal and the phosphorus atom. Scheme 3. Hydride migration to a phosphane ligand [46].

Supplementary Materials:
The following Supporting Information can be downloaded at: www.mdpi.com/xxx/s1. Crystallographic data for the compounds reported in this paper (in CIF format) and a document containing graphics of the 1 H, 13 C and 31 P NMR spectra of compounds 3a, 4a, 5a, 5b and 6a; data sets (consisting of molecular graphic, atomic coordinates and total energies) of the optimized molecular structures of 4a 1 , 4a 2 , 4a 3 , 4a 4 , 4a 5 , 4a 6 , 4a 7 , 5a 1 , 5a 3 , 5b 1 , 5b 3 , 4b 1 and 4b 3 ; graphics of selected NBOs and NLMOs of compounds 3a, 3b, 4b, 5a and 5b.  Sample Availability: The compounds reported in this paper were prepared in small quantities only. Thus, no samples are available from the authors.
From an academic point of view, the herein presented compound 5b represents a rare example of a monometallic phosphane complex with hexacoordination of both the transition metal and the phosphorus atom.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: The compounds reported in this paper were prepared in small quantities only. Thus, no samples are available from the authors.
Appendix A Table A1. Crystallographic data from data collection and refinement processes for 3a, 4a·(toluene), 5a·1.5(CH 2 Cl 2 ) , 5b and 6a·1.5(CH 2 Cl 2 ). The structure of compound 3a was refined as an inversion twin. Without taking the twin into account, the absolute structure parameter χ Flack is 0.30(3). 2 The asymmetric unit comprises four toluene molecules, which suffer heavy disorder. Therefore, the solvent was not refined but treated with SQUEEZE as implemented in PLATON [50][51][52]. This procedure detected, per unit cell, a solvent-accessible volume of 3090 Å 3 and contributions of 840 electrons therein (close to the 800 electrons for the 16 toluene molecules per unit cell, which were omitted from refinement). 3 The asymmetric unit comprises 1.5 CH 2 Cl 2 molecules. One molecule is disordered over three positions and was refined with site occupancies of 0.440(3), 0.234(3) and 0.326 (3). The other solvent site is near a crystallographically imposed center of inversion (0.5 molecules per asymmetric unit). In addition to the symmetry-related disorder in this position, this half molecule was refined in two individual orientations with site occupancy ratio 0.767(5):0.233 (5). 4 The asymmetric unit comprises 1.5 CH 2 Cl 2 molecules. One molecule is well ordered and was refined. The other solvent site is near a crystallographically imposed center of inversion (thus 0.5 molecules per asymmetric unit), and this half molecule suffers heavy disorder. Therefore, this part of the solvent was not refined but treated with SQUEEZE as implemented in PLATON [50][51][52]. This procedure detected, per unit cell, solvent accessible volume of 355 Å 3 and contributions of 83 electrons therein (well in accord with 84 electrons for the two CH 2 Cl 2 molecules per unit cell, which have been omitted from refinement).

Appendix B
In the IR spectra, compound 5a exhibits two strong bands characteristic of C≡N stretching vibrations at 2091 and 2052 cm −1 . In the same region, compound 5b exhibits only one band, at 2087 cm −1 . This hints at the presence of two isomers in this solid product of 5a and one isomer of 5b (which is in accord with 1 H NMR data). Thus, we attribute the bands at 2052 and 2087 cm −1 to the isomers 5a 3 and 5b 3 , respectively, and the band at 2091 cm −1 to isomer 5a 1 . This assignment is based on the C≡N of 5a 1 resonating at somewhat higher wave numbers than 5b 3 (the trend found for the C≡O stretch of complexes 4a 1 and 4b 3 ), while the lower wave number of the C≡N stretch of 5a 3 would be in accord with the stronger π-back-bonding, which is indicated by the shorter Ru-C bond in 5a (vs. 5b). Furthermore, this assignment is supported by computational analyses, which predict a C≡N stretch at enhanced wave numbers (+32 cm −1 ) for 5a 1 with respect to 5a 3 (cf. Section 2.4, optimized molecular structures of selected isomers). In general, the charge-neutral Ru(II) compounds 5 exhibit pronounced π-back-bonding to the CNtBu ligand. For comparison, Ru(II)-compound [Ru(tp)Cl(PPh 3 )(CNtBu)] (tp = tris(pyrazol-1yl)borate), which also bears good donor ligands at Ru(II), still exhibits slightly weaker back-bonding, indicated by a C≡N stretching vibration at 2117 cm −1 [53].