Transition metal complexes with more than one dihydrogen ligand:: Structure and bonding of M(CO)6-x(H2)x (M=Cr, Mo, W, x=1, 2, 3)

Quantum mechanical ab initio calculations at the MP2 and CCSD(T) level of theory have been used to investigate the geometries and bond energies of the complexes M(CO)(6-x)(H-2)(x) (M = Cr, Mo, W; x = 1, 2, 3). The theoretically predicted M(CO)(5)-(H-2) bond dissociation energies are in excellent agreement with experimental values. The M-(H-2) dissociation energies of the bis- and tris-dihydrogen complexes are very similar to the values for the mono-dihydrogen complexes. In M(CO)(5)(H-2) the dihydrogen ligand prefers an eclipsed conformation relative to the equatorial carbonyl groups. For M(CO)(4)(H-2)(2) the cis and trans isomers are nearly equal in energy for M = W, while a cis configuration is favoured for M = Cr. For M(CO)(3)(H-2)(3) the facial configurations are more stable than the meridial structures for all three metals M. The charge decomposition analysis (CDA) classifies dihydrogen as a donor ligand with moderate acceptor properties. In trans-M(CO)(4)(H-2)(2) back donation is increased and the M-(H-2) bonds are stronger than in M(CO)(5)-(H-2). Back donation in M(CO)(3)(H-2)(3) is slightly weaker than in the mono-dihydrogen complexes M(CO)(5)(H-2).