Bonding in DiboraneMetal Complexes: A Quantum-Chemical and Experimental Study of Complexes Featuring Early and Late Transition Metals

The coordination chemistry of the doubly base-stabilised diborane(4), [HB(hpp)]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidinate), was extended by the synthesis of new late transition-metal complexes containing CuI and RhI fragments. A detailed experimental study was conducted and quantum-chemical calculations on the metalligand bonding interactions for [HB(hpp)]2 complexes of Group 6, 9, 11 and 12 metals revealed the dominant BHM interactions in the case of early transition-metal fragments, whereas the BBM bonding prevails in the case of the late d-block compounds. These findings support the experimental results as reflected by the IR and NMR spectroscopic parameters of the investigated compounds. DFT calculations on [MeB(hpp)]2 and model reactions between [B2H42NMe3] and [Rh(-Cl)(C2H4)2] showed that the bicyclic guanidinate allows in principle for an oxidative addition of the BB bond. However, the formation of sigma-complexes is thermodynamically favoured. The results point to the selective BH or BB bond-activation of diborane compounds by complexation, depending on the chosen transition-metal fragment.