Electronic Structure and Bonding in Neutral and Dianionic Boradiphospholes: R′BC 2 P 2 R 2 (R=H, t Bu, R′=H, Ph)

Classical and non‐classical isomers of both neutral and dianionic BC 2 P 2 H 3 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 BC 2 P 2 H 3 is based on a vinylcyclopropenyl‐type structure, whereas BC 2 P 2 H 3 2− has a planar aromatic cyclopentadienyl‐ion‐like structure. However, at the G3B3 level, there are three low‐energy isomers for BC 2 P 2 H 3 : 1) tricyclopentane, 2)  nido and 3) vinylcyclopropenyl‐type structures, all within 1.7 kcal mol −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 n C n P 5− n + isomers, BC 2 P 2 H 3 shows a competition between π‐delocalised vinylcyclopropenyl‐ and cluster‐type structures ( nido and tricyclopentane). Substitution of H on C by t Bu, and H on B by Ph, in BC 2 P 2 H 3 increases the energy difference between the low‐lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC 2 P 2 H 3 2− merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes E t Bu 2 C 2 P 2 (E=S, Se, Te), 2) monoanionic heterophospholyl rings E t Bu 2 C 2 P 2 (E=P − , As − , Sb − ) and 3) polyphospholyl rings anions t Bu 5− n C n P 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 BC 2 P 2 H 3 isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the t Bu groups in the HBC 2 P 2 t Bu 2 systems.