The synthesis, structure, and chemical reactions of metalloboranes

Abstract

Recent work on the synthesis, structure, and chemical reactions of metalloboranes is reviewed. In the decaborane series, single-crystal X-ray studies indicate that the metal atom can be (a) fully incorporated within the borane cluster, (b) joined to the cluster by an exopolyhedral B-M-B bond in which the metal replaces a bridging proton, or (c) joined by a normal o B-M bond by replacement of a terminal hydrogen atom. Complexes of B 10 H 12 2- with Fe; Zn, Cd, Hg; Mg; Al, Ga, In, Tl; and Sn are specifically discussed. In the pentaborane series numerous complexes have been prepared for a wide variety of transition metals and main group metals (Ti; Fe; Co, Rh, Ir; Ni, Pd, Pt; Cu, Ag, Au; Cd, Hg; and Si) and details of the bonding are elucidated. Kinetic control of products frequently appears to be more important than the achievement of greatest thermodynamic stability. Replacement of bridge protons or terminal hydrogen atoms in the basal borons are the prevalent modes of attachment for B 5 and B 6 systems and no examples of B-H-M bonding have been detected. By contrast, B 3 H 8 is found to bond exclusively via B-H-M bonds in its non-ionic derivatives (examples include complexes of Mn; Fe; Ir; Zn; Al, Ga, In; Si, Ge, Sn, and Pb). NMR spectroscopy proves to be valuable in elucidating the structure and in defining which complexes are fluxional and which have a rigid ‘tetraborane-type’ structure in which a coordinated metal centre formally replaces a BH 2 group in tetraborane (10). The results, taken as whole, establish the general ability of boron hydrides and their anions to act as electron-donating ligands to most, if not all, metals in the Periodic Table.