The [2+2] Addition of Ethylene to Metal−Ligand Multiple Bonds:  A Density Functional Study of Mo(E)OCl₂

We have studied the [2+2] cycloaddition of ethylene to Mo(E)OCl 2 for E = S, Se, O, NH, PH, SiH 2, and CH 2 based on gradient-corrected density functional theory. The reactant Mo(E)OCl 2 has for E = NH, PH, SiH 2, and CH 2 a preferred conformation with all atoms of the E ligand in the plane bisecting the Cl−Mo−Cl angle. The [2+2] cycloaddition takes place with ethylene approaching Mo(E)OCl 2 perpendicular to the plane bisecting the Cl−Mo−Cl angle. The reaction enthalpy follows the order (in kcal/mol) SiH 2 (−24.1) < CH 2 (−15.6) < PH (−7.4) < NH (−0.9) < S (5.9) < Se (6.2) < O (12.0) < Cl (13.0). The ligands with the less electronegative heteroatom give rise to the more exothermic reaction, as they have lone pairs of sufficiently high energy to donate charge to π* of the incoming olefin and form a strong C−E bond. The activation energy for the addition follows the order (in kcal/mol) SiH 2 (2.3) < CH 2 (4.7) < PH (5.0) < S(16.6) < Se (16.6) < Cl (19.0) < NH (21.1) < O (25.8). Again, ligands with the less electronegative heteroatoms afford the lowest barrier for the addition since they have lone pairs that at an early stage of the reaction can interact, stabilizing with π* of the incoming ethylene. The investigation affords a rationale for why [2+2] addition of olefin to M E bonds is observed experimentally to be facile for E = SiH 2 and CH 2 but not for E = O.