A(3)Tt(5) phases Sr(3)Sn(5), Ba(3)Pb(5), and La(3)Sn(5). Structure and bonding in a series of isotypic metallic compounds with increased electron count and their comparison with the nominal zintl phase La(3)In(5).
A series of compounds that contain square pyramidal Tt(5) polyanions of tin and lead has been obtained in alkaline-earth or rare-earth metal-tetrel systems by direct fusion of the elements at 570 degrees C (Sr(3)Sn(5)), 1000 degrees C (Ba(3)Pb(5)), or 1300 degrees C (La(3)Sn(5)) followed by slow cooling or annealing. The crystal structures for all three have been refined in the Pu(3)Pd(5) structure type (orthorhombic, Cmcm, Z = 4) with cell dimensions of a = 10.644(2), 11.154(7), and 10.352(5) A, b = 8.588(1), 9.049(7), and 8.290(6) A, and c = 10.895(2), 11.370(5), and 10.652(5) A for Sr(3)Sn(5), Ba(3)Pb(5), and La(3)Sn(5), respectively. Square pyramidal clusters of the tetrel elements are weakly interlinked into chains via two types of longer intercluster interactions that are mediated by bridging cations and substantially influenced by cation size and the free electron count. The new compounds are all metallic (rho(295) approximately 10 (Sr(3)Sn(5)) to approximately 25 (La(3)Sn(5)) muOmega.cm), in agreement with simple valence considerations that predict two and five extra electrons per formula unit, respectively, beyond that necessary for closed-shell nido-Tt(5)(4)(-) anions. Extended Hückel tight-binding calculations on the new compounds as well as on La(3)In(5) reveal that bonding in the regions below and around the Fermi energies are dominated by general cation-anion interactions, that is, lattice covalency. Closed-shell bonding features for the classical Sn(5)(4)(-), In(5)(9)(-), etc. ions are also obvious but subsidiary to the heteroatomic interactions with the cations. The intercluster contacts are relatively unimportant in bonding.