Electronic structure and bonding in neutral and dianionic boradiphospholes: R'BC₂P₂R₂ (R = H, tBu, R' = H, Ph)

Abstract

Classical and non-classical isomers of both neutral and dianionic BC₂P₂H₃ species, which are isolobal to Cp₊ and Cp^-, are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311++G(d,p) for BC₂P₂H₃ is based on a vinylcyclopropenyl-type structure, whereas BC₂P₂H₃^2- has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC₂P₂H₃: 1) tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcalmol^-1 of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp⁺ and H_nC_nP_5-n⁺ isomers, BC₂P₂H₃ shows a competition between π-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC₂P₂H₃ increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC₂P₂H₃^2- merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu₂C₂P₂ (E=S, Se, Te), 2) monoanionic heterophospholyl rings EtBu₂C₂P₂ (E=P^-, As^-, Sb^-) and 3) polyphospholyl rings anions tBu_5-nC_nP_5-n (n=0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC₂P₂H₃ isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC₂P₂tBu₂ systems.