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 DEp values follows closely the change of the total interaction energy DEint. The changes in DEint 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.