Miriam Colling

Transition metal complexes of boron — synthesis, structure and reactivity

Abstract Over the past decade, transition metal complexes of boron have become established as the fourth class of compounds involving direct metal–boron interactions. In contrast to the other three major groups in this field, i.e. borides, metallaboranes, and π-complexes with boron-containing ligands, transition metal complexes of boron are characterised by electron-precise two-centre two-electron bonds between boron and the metal centre. According to this pattern of metal–boron interaction, a variety of different co-ordination modes for boron-containing ligands have been realised, allowing for a systematic classification of those compounds into borane, boryl, and borylene complexes. Research in this area is stimulated by the interesting applications boryl complexes have, for the functionalisation of hydrocarbons by metal-catalysed hydroboration and CH activation. By now, a variety of synthetic methods are at hand for the preparation of those compounds and the spectroscopic and structural data available provide some insight into the nature of the metal–boron linkage of those compounds. More recently, theoretical studies have become another valuable source of information in this respect. The present review introduces the three compound types (borane, boryl, and borylene complexes), which at the moment constitute the class of transition metal complexes of boron, addressing their synthesis, structure and reactivity.

The chemistry of bridged borylene complexes

The stabilisation of borylenes (BX, X=various ligands) in the sphere of a transition metal was achieved for the first time in the case of bridged borylene complexes of the type [(LxM)2BX]. These compounds are characterised by 2c–2e bonds between metal and boron, thus belonging to the class of transition metal complexes of boron. Over the past 5 years different synthetic approaches were made, yielding the first structurally authentic metal complexes with a bridging BX ligand. Their synthesis, structure and reactivity is the subject of this paper.