TRENDS IN ELECTRON-DEFICIENT BRIDGES

A previously defined procedure, based on orthogonal valence bond decomposition of a correlated wave function, is applied to nine doubly bridged electron-deficient systems isoelectronic with diborane: X2H6 (X = B, Ga), X2H4 (X = C, Si, Ge, Sn, Pb), and X2H62+ (X = C, Si). Basic features are found to be common to all species, such as the high separability of the bridges and the primacy of X-H-X peripheral bonding over X-X central bonding. Other electronic properties exhibit regular and consistent variations. The electron population on the bridging hydrogens H(b) Summarizes these trends and may be used to set up a continuous scale. Simple modeling of a single bridge suggests that its changes can arise from two factors: orbital electronegativity differences, and the architecture of the interactions, ranging between two limiting forms (schematized by the cyclopropenyl cation, with all centers bound, and the allyl cation, with only 1-2 and 2-3 binding). Direct evaluation of the transfer integrals shows that the second factor is not the leading one since the through-space/through-bond ratios are rather constant along the series, except for C2H62+ which undergoes more through-space C-C binding. The distribution of the compounds along the scale is therefore mainly governed by electronegativity factors. The ordering (C2H62+, C2H4, B2H6, Si2H62+, Si2H4, Ga2H6, Ge2H4, Sn2H4, Pb2H4) indicates, among other things, an increase in electron population on H(b) and a decrease in interbridge delocalization (which remains weak in any case). The various components of the energies of fragment dimerization into double bridges are calculated and discussed. Possible views for the diprotonation Of X2H4 are provided. Structures of peculiar singly bridged systems are also examined.