D.S. Nemcsok, A. Kovács, V.M. Rayón, G. Frenking

Nature of the Transition Metal-(h²-H₂) Bond in TM(CO)₃X₂(h₂-H₂) Complexes.

(TM = Cr, Mo, W; X = CO, PH₃, PCl₃, PMe₃)

Organometallics, 21 (2002) 5803-5809

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 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 H₂ to TM  s donation while one third is due to TM to p*(H₂) p backdonation. 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 incease in the PR₃ substituted species. Inspection of the covalent term shows that the W to p*(H₂) p backdonation becomes stronger when CO is substituted by PR₃. The change of the DE_p values follows closely the change of the total interaction energy DE_int. The  changes in DE_int upon rotating the H₂ ligand in W(CO)₃(PR₃)₂H₂ also correlate quantitatively with the strength of the W to p*(H₂) p backdonation. 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 p  contribution of the covalent bonding.