Nature of the transition-metal-η²-H₂ bond in TM(CO)₃X₂(η²-H₂) (TM = Cr, Mo, W; X = CO, PH₃, PCl₃, PMe₃) complexes

Quantum-chemical calculations at the gradient-corrected DFT level using BP86 of the structures and bond dissociation energies of the title compounds are reported. The nature of the metal-H₂ bonding is quantitatively analyzed with an energy decomposition method. It is found that the metal-H₂ bonds in TM(CO)₅H₂ are ∼47% covalent and ∼53% electrostatic. Two-thirds of the covalent interactions come from TM←σ(H₂) σ donation, while one-third is due to TM→σ*(H₂) π back-donation. Substitution of two CO ligands of the tungsten complex cis to H₂ by PR₃ (R = H, Cl, Me) leads to stronger W-H₂ interactions. The electrostatic and covalent bonding both increase in the PR₃-substituted species. Inspection of the covalent term shows that the W→σ*(H₂) π back-donation becomes stronger when CO is substituted by PR₃. The change of the ΔE_π values follows closely the change of the total interaction energy ΔE_int. The changes in ΔE_int upon rotating the H₂ ligand in W(CO)₃(PR₃)₂H₂ also correlate quantitatively with the strength of the W→σ*(H₂) π back-donation. Thus, the changes in the metal-H₂ bonding situation which are invoked by different substituents and by rotation of the H₂ ligand can be explained with the π contribution of the covalent bonding.