We have studied the [2+2] cycloaddition of ethylene to Mo(E)OCl2 for E = S, Se, O, NH, PH, SiH2, and CH2 based on gradient-corrected density functional theory. The reactant Mo(E)OCl2 has for E = NH, PH, SiH2, and CH2 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)OCl2 perpendicular to the plane bisecting the Cl−Mo−Cl angle. The reaction enthalpy follows the order (in kcal/mol) SiH2 (−24.1) < CH2 (−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) SiH2 (2.3) < CH2 (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 = SiH2 and CH2 but not for E = O.

Density functional theory calculations have been carried out on the trigonal complexes OsO3E and MCl3E (M = V, Ta) and the square pyramidal systems MCl4E (M = Cr, Mo, W, Re) for E = O, S, Se, and Te as well as (C5H5)ReO3. All complexes were fully optimized, and the calculated geometrical parameters are in reasonable agreement with gas-phase electron diffraction data where available. The calculated M−E bond energies decrease from oxygen to tellurium, from bottom to top in a metal triad, and from left to right in a transition series. The trend setting factor is the donation from the dσ metal orbital to the pσ acceptor orbital on the chalcogen atom. The contribution from the chalcogen to metal π back-donation has a maximum for sulfur and selenium. However in relative terms, the contribution from the π back-donation to the total M−E bond energy increases from oxygen to tellurium. Comparisons are made to previous calculations and experimental data on M−E bond strengths.