18-electron Os(X)(CHR)(Cl)(CO)L-2 (X = H, Cl): not octahedral and metastable?

The differences in the geometries of Os(X)(CHR)(Cl)(CO)L-2 (X = H, Cl; L = PPr3i; R = SiMe3, Ph), where X and CHR are mutually trans, have been studie d through ab initio MP2 calculations (L = PH3, R = H). The optimized struct ures are in good agreement with the experimental data (P-Os-P = 141.4 degre es for X = H; 167.5 degrees for X = Cl) and show the electronic origin of t he distortion. A molecular orbital analysis explains why the P-Os-P angle i s small for X = H and larger, but not 180 degrees, for X = Cl. For X = H, t he isomer in which H and CH, are mutually cis is shown to be more stable th an the corresponding experimental isomer in which they are mutually trans. However, the 16-electron product resulting from insertion of the carbene in to the Os-H bond is shown to be considerably more stable than the 18-electr on hydrido-carbene parent compound. Thus, the isolated metastable isomer is kinetically protected by a high barrier for site exchange in d(6) hexacoor dinated complexes. Recent experimental observations by Caulton et al. have shown that Os(H)(CH2)(Cl)(CO)L-2 transforms into Os(CH3)(Cl)(CO)L-2 when th e kinetic barrier is removed by loss of phosphine.