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) [1]

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(H₂)_x (M = Cr, Mo, W; x = 1, 2, 3). The theoretically predicted M(CO)₅–(H₂) bond dissociation energies are in excellent agreement with experimental values. The M–(H₂) dissociation energies of the bis- and tris-dihydrogen complexes are very similar to the values for the mono-dihydrogen complexes. In M(CO)₅(H₂) the dihydrogen ligand prefers an eclipsed conformation relative to the equatorial carbonyl groups. For M(CO)₄(H₂)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(H₂)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(H₂)₂ back donation is increased and the M–(H₂) bonds are stronger than in M(CO)₅–(H₂). Back donation in M(CO)₃(H₂)₃ is slightly weaker than in the mono-dihydrogen complexes M(CO)₅(H₂).