18-Electron Os(X)(CHR)(Cl)(CO)L2 (X=H, Cl): not octahedral and metastable?

Abstract

The differences in the geometries of Os(X)(CHR)(Cl)(CO)L₂ (X=H, Cl; L=PPr₃ⁱ; R=SiMe₃, Ph), where X and CHR are mutually trans, have been studied through abinitio MP2 calculations (L=PH₃, R=H). The optimized structures are in good agreement with the experimental data (P–Os–P=141.4° for X=H; 167.5° for X=Cl) and show the electronic origin of the distortion. A molecular orbital analysis explains why the P–Os–P angle is small for X=H and larger, but not 180°, for X=Cl. For X=H, the isomer in which H and CH₂ are mutually cis is shown to be more stable than the corresponding experimental isomer in which they are mutually trans. However, the 16-electron product resulting from insertion of the carbene into the Os–H bond is shown to be considerably more stable than the 18-electron hydrido-carbene parent compound. Thus, the isolated metastable isomer is kinetically protected by a high barrier for site exchange in hexacoordinated complexes. Recent experimental observations by Caulton etal. have shown that Os(H)(CH₂)(Cl)(CO)L₂ transforms into Os(CH₃)(Cl)(CO)L₂ when the kinetic barrier is removed by loss of phosphine.