A Theoretical Insight into the Ability of Group 6 ML5 Metal Fragments to Break the H−H Bond

The interaction between H 2 and M(CO) n (PH 3 ) 5 - n (M = Cr, Mo, W; n = 0, 3, 5) metal fragments has been studied by means of CCSD(T)//B3LYP calculations. Three steps in the dihydrogen addition path that starting from the ML 5 and H 2 separated fragments leads to a stable dihydride have been considered: (i) dihydrogen coordination; (ii) cleavage of the H−H bond in a dihydrogen-like structure, leading to a PB1 cis -dihydride; (iii) reorganization of the pentagonal bipyramidal cis -dihydride formed to a more stable PB2 dihydride structure. From the thermodynamic results and the energy profiles for the oxidative addition the nine complexes under study can be classified in three groups: (i) only dihydrogen observable; M(CO) 5 H 2 (M = Cr, Mo, W); and M(CO) 3 (PH 3 ) 2 H 2 (M = Cr, Mo); (ii) equilibrium between dihydrogen and PB2 dihydride: W(CO) 3 (PH 3 ) 2 H 2 and M(PH 3 ) 5 H 2 (M = Cr, Mo); (iii) only dihydride observable; W(PH 3 ) 5 H 2 . The different behavior for dihydrogen addition is related to the energetics of the M−H 2 and M−H bonds and to the singlet−triplet separation in the ML 5 fragment.