NATURE OF THE HYDROGEN BRIDGE IN TRANSITION-METAL COMPLEXES .6. A COMPARISON OF THE ELECTRONIC-STRUCTURE OF CU-H-B AND CU-H-CU BRIDGES

The electronic structure of a series of homoleptic H-bridged copper-bo ron complexes (Cu-H(n)-BH4-n, n = 1, 2, and 3) and their phosphine sub stituted analogues ((PH3)4-nCu-Hn-BH4-n, n = 1, 2, and 3) has been inv estigated with the Fenske-Hall method. For the sake of comparison para llel calculations have been carried out for H-bridged copper binuclear complexes ([(PH3)4-nCu-Hn-Cu(PH3)4-n]k, n = 1, 2, 3, and k = +1 for n = 1, k = 0 for n = 2, and k = -1 for n = 3). The results of the Fensk e-Hall method are verified by similar calculations with the aid of the non-iterative Extended Huckel theory (EHT). The main conclusions of t he present study follow from the analysis of the electronic charge dis tribution in those molecules. This analysis reveals that the H-bridges in copper-boron compounds differ considerably from those in copper bi nuclear complexes. The former are due to non-covalent electrostatic in teractions between the copper atom (coordinated copper atom) and the n egative-charged BH4 group. The covalency of the Cu-H bond turns out to be negligibly small while in copper binuclear complexes the Cu-H bond resembles the usual, strongly polarized covalent bonds. The new inter pretation of the electronic structure of the copper-boron H-bridge is different from that usually accepted for those systems and facilitates the understanding of the flexibility of the H-bridged copper-boron co mplexes with respect to intramolecular isomerization processes.