Definition of the Pnictogen Bond: A Perspective

This article proposes a definition for the term pnictogen bond and lists its donors, acceptors, and characteristic features. These may be invoked to identify this specific subset of the inter- and intra-molecular interactions formed by elements of Group 15 which possess an electrophilic site in a molecular entity.


Preface
This paper proposes a definition of the term "pnictogen bond", followed by a list of electron density donors and acceptors of pnictogen bonds and their accompanying experimental and theoretical features. It proposes that the definition be used to designate a subset of the family of inter-and intramolecular interactions formed by the members of the pnictogen family [1], the elements of Group 15 of the periodic table, in molecular entities. This proposal follows from the IUPAC recommendations for hydrogen bonds (HBs) [2], halogen bonds (XBs) [3], and chalcogen bonds (ChBs) [4].
Nitrogen, the lightest member of the pnictogen family, Group 15, has the highest electronegativity and lowest polarizability [5]. It often serves as a nucleophile when present in molecules, such as in N2, NH3, and ammine derivatives, for example [6][7][8][9][10]. The heavier members of the family exhibit similar behavior when they are a constituent of many chemical systems. This presumably applies to Moscovium as well, although little is known about its chemistry. As electron density donors, they are capable of acting as acceptors of, inter alia, hydrogen bonds, halogen bonds, chalcogen bonds, and any other non-covalent interaction. In such cases, the pnictogen atom behaves as a nucleophilic moiety that attractively engages (via coulombic interaction) with its interacting electrophilic partner(s).
The pnictogen atoms in molecular entities also have the ability to act as electron-poor (electrophilic) sites [6,7,[11][12][13][14]. This occurs when they are bonded to electronegative and/or electron-withdrawing groups such as F, CN, NO2, and C6F5. In such instances, they are capable of attracting an electron-rich (nucleophilic) site in the same or in a separate molecular entity when in close proximity.
The difference between the two situations described above depends on the electronic structure profile of the bound pnictogen atom in the molecular entity; it acts as a nucleophile in the first case and as an electrophile in the second. In the latter case, the bound pnictogen atom may be directionally oriented toward the nucleophilic site, resulting in the development of a linear or quasi-linear non-covalent interaction [6,7,[11][12][13][14]. If the entire electrostatic surface of the bound pnictogen atom in a molecular entity is electrophilic, it may lead to the formation of non-linear (or bent) attractive interactions [6,7,[11][12][13][14]. The term "pnictogen bond" is used uniquely to designate the latter set of non-covalent interactions, where the pnictogen atom acts as an electrophile. The presence of an electrophilic 3 of 15 Note 6: Because of its variable electrostatic character, a pnictogen atom in a molecular entity may engage in a number of interactions that lead to the appearance of a variety electronic and geometric features [6,7,[11][12][13][14]19]. The term pnictogen bond should not be used for attractive interactions in which the pnictogen atom (frequently nitrogen and sometimes phosphorous) functions as a nucleophile.
Note 8: Two pnictogen atoms in two different molecular entities may be involved in an attractive engagement to form a pnictogen bond, in which case, one of the pnictogen atoms must act as a pnictogen bond donor, and that in the partner molecular entity must act as a PnB acceptor, such as in NO2HP···NH3 [29].
Note 9: The pnictogen bond should be viewed as an attractive interaction between PnB donor site Pn and PnB acceptor site A of opposite charge polarity (Pn + and A -), resulting in a coulombic interaction between them; the charge polarity  + and  -symbolically refers to the local charge polarity on the interacting regions on Pn and A, respectively. Note 10: The pnictogen bond should follow the Type-II topology of non-covalent bonding interactions; a Type-II interaction, R-Pn···A, is often linear or quasi-linear (but may be non-linear) and satisfies Note 9.

Some Common Pnictogen Bond Donors and Acceptors
Some common PnB donors and acceptors are listed below. We emphasize that the list, which emerged from a search of the Cambridge Structural Database [30] and Inorganic Crystal Structure Database [31,32], is illustrative rather than comprehensive.
The PnB acceptor entity A can be: -A lone pair on an atom in a molecule. There are almost limitless possibilities, for example, the N in pyridines or amines, or even in N2; the O in H2O, CO, CO2, an ether, or a carbonyl group, or a phosphorus oxide; covalently bonded halogens in molecules; As in AsMe3; a chalcogen in a heterocycle such as a thio-, seleno-, and tellurophene derivatives as well as fused polycyclic derivatives thereof; furoxans, 2,5-thiadiazoles N-oxides, sulfoxide, aryl sulfoxides, and tellurazoles N-oxides; derivatives of macrocyclic crown-ethers such as 18-crown-6, 15-crown-5 and 21-crown-7, etc.

A List of Characteristic Features
Evidence of the presence of a pnictogen bond in molecular entities, crystals, and nano-scale materials may emerge from experimental measurements (e.g., X-ray diffraction, infrared, Raman and NMR spectroscopy, etc.), or signatures from ab initio studies, or a combination of both. The evidence could be very similar to that already recommended by the IUPAC for HBs, XBs, and CBs. The following list is not exhaustive but includes some distinguishing features that may be useful as indicators of the occurrence of pnictogen bonding interactions in chemical systems. The more of these features that are met, the more reliable is the identification of the interaction as being a PnB interaction.
On the formation of a typical pnictogen bond R-Pn···A between two interacting entities: a. The separation distance between the PnB donor atom Pn and the nucleophilic site of PnB acceptor A tends to be smaller than the sum of the van der Waals radii of the respective interacting atomic basins [6,7,[11][12][13][14] and larger than the sum of their covalent bond radii [2][3][4]; the deviation of the former is likely since the known van der Waals radii of atoms are only accurate with 0.2 Å [13,113,114]; b. The PnB donor site on Pn tends to approach the PnB acceptor site A along the outer extension of a σ covalent or coordinate bond, and the angular deviation from the 8 of 15 extension is often more pronounced in PnBs [6,7,[11][12][13][14] than in halogen bonds, as in ChBs [4], with the latter possibly being due to the involvement of secondary interactions; c. The angle of interaction, R-Pn···A, tends to be linear or quasi-linear when the approach of the electrophile on Pn is along the σ covalent/coordinate bond extension, but this can be non-linear or have a bent shape when the pnictogen bond occurs between an electron density-deficient (electrophilic) -type orbital of the bonded pnictogen atom and the nucleophilic region on A [6,7,[11][12][13][14] or when secondary interactions are involved; d. When the nucleophilic region on the PnB acceptor site A is a lone pair orbital or an electron density-rich π region, the PnB donor Pn tends to approach A along the axis of the lone pair or orthogonally to the π bond plane [6,7,[11][12][13]115]; e. The distance of the R-Pn covalent bond opposite to the PnB in a molecular adduct is typically longer than that in the isolated (unbound) PnB donor; f.
The infrared absorption and Raman scattering observables of both R-Pn and A are affected by PnB formation; the vibrational frequency of the R-Pn bond may be redshifted or blue-shifted depending on the extent of the interactions involved compared to the frequency of the same bond in the isolated molecular entity; new vibrational modes associated with the formation of the Pn···A intermolecular pnictogen bond should also be characteristically observed [116,117], as observed for ChBs; g. A bond path and a bond-critical point between Pn and A may be found when an electron density topology analysis based on the quantum theory of atoms in molecules (QTAIM) [118] is carried out, together with the emergence of other charge density-based signatures [119][120][121][122][123]; h. Isosurface volumes (colored greenish, blue, or mixed blue-green between Pn and A, representative of attractive interactions [6,7,[11][12][13]124]) may be seen if a non-covalent index analysis based on reduced charge density gradient [125][126][127] is performed; i.
The UV-vis absorption bands of the PnB donor chromophore may experience a shift to longer wavelengths [128]; j.
At least some transfer of charge density from the frontier PnB acceptor orbital to the frontier PnB donor orbital may occur [15,129,130]; when the transfer of electron charge density between them is significant, the formation of a dative coordinate interaction is likely [131]; the occurrence of the IUPAC-recommended phenomena for HBs (see Criteria E1 and Characteristic C5 of Ref. [2]) is also applicable to XBs [132][133][134][135] and ChBs [136][137][138]; k. The NMR chemical shifts of the nuclei in both R-Pn and A [4,128,[139][140][141][142][143] are typically affected, as found for R-X···A XBs and R-Ch···A ChBs [144]; l.
The PnB strength typically decreases with a given acceptor A, as the electronegativity of Pn increases in the order Bi < Sb < As < P < N, and the electron withdrawing ability of R decreases; m. The PnB bond strength increases for a specific PnB acceptor A and the remaining R, as the polarizability of the pnictogen atoms in the molecular entities increases (Bi > Sb > As > P > N) [15]. This is the same as what is observed for the halogen derivative forming XB (I > Br > Cl > F) [145,146] and the chalcogen derivatives forming ChB (Te > Se > S > O) [147]. However, if secondary interactions (e.g., a hydrogen bond, halogen bond, chalcogen bond, tetrel bond, etc.) are simultaneously involved with either the PnB donor or PnB acceptor, the order of interaction strength may also be altered; n. Coulombic interaction occurs between the PnB donor and the PnB acceptor entities at equilibrium, and the energetic contributions to the binding energy arising from electrostatic polarization (and/or induction), exchange repulsion, and long-range dispersion should not be neglected [15,148]. 9 of 15

Concluding Remarks
Pnictogen bonding is a non-covalent interaction with the potential to serve as an electronic glue in the assembly of molecular entities in the process of developing molecular complexes, crystalline solids, supramolecular structures, and functional nanomaterials. Its implications in crystal engineering [6,7,[11][12][13][14]123,[149][150][151], anion transport [152], catalysis [20,[153][154][155], and photovoltaics [7,156,157] are appreciable. A pnictogen bond falls under the umbrella of -and/or -hole-centered non-covalent interactions, provided that it is a result of an attractive engagement between an electrophilic PnB donor moiety Pn containing a -and/or a -hole interacting with a nucleophilic site on A [6,7,[11][12][13][14][158][159][160][161][162]; -and -holes are electron density-deficient electrophilic regions on the PnB donor moiety Pn along the outermost extension of the R-Pn covalent (or coordinate) bond and perpendicular to that bond, respectively. The list of PnB donors and PnB acceptors and the characteristic features of PnB is vast, and only a few are listed in this paper. Several illustrative examples are provided that can assist in recognizing chemical situations where pnictogen bonding in and between molecular entities is likely to occur. The definitions and features proposed in this paper should be useful for researchers and graduate students working in diverse research fields to identify and characterize pnictogen bonding in the novel chemical systems in which they are hosted.  Data Availability Statement: This research did not report any data.