Coordination modes and hydride exchange dynamics in transition metal tetrahydroborate complexes

This contribution reviews the structural and dynamical properties of mononuclear transition metal complexes with tetrahydroborate ligands. Three different coordination modes involving one (η1), two (η2) or three (η3) bridging hydrogen atoms are possible for the BH4- ligand. A structural classification of the X-ray characterised complexes is presented. The metal-boron distances and the vibrational frequencies of the coordinated borohydrides, which are the key experimental data usually used to determine the hapticity of tetrahydroborate binding, have been surveyed and trends along the Periodic Table established. Electronic factors governing the coordination mode have been rationalized by means of simple orbital arguments supported by quantitative calculations. In solution, most of the transition metal tetrahydroborate complexes show fluxional behaviour, displaying a single resonance for the four B-H hydrogens in the 1H NMR spectrum at ambient temperature. This fast intramolecular exchange between bridging and terminal hydrogens has been analysed. Experimental and computational data for these processes have been collected and the exchange mechanisms are discussed. In summary, several examples illustrating the perspectives on the field are presented. © 2008 Springer-Verlag.