Efficient synthesis of the Os-Os dimers [Cp(CO) ₂Os] ₂, [Cp*(CO) ₂Os] ₂, and [ ⁱPr ₄C ₅H(CO) ₂Os] ₂and computational studies on the relative stabilities of their geometrical isomers

The Os-Os dimer [Cp(CO) ₂Os] ₂ was synthesized in good yield by reaction of Cp(CO) ₂OsI with NaK. The related Cp* dimer [Cp*(CO) ₂Os] ₂ (Cp* = η ⁵-C ₅Me ₅) was prepared by reaction of Cp*(CO) ₂OsH with 2,2′-azobis(isobutyrylnitrile) (AIBN). In contrast to [Cp(CO) ₂Os] ₂, which has only terminal CO ligands, spectroscopic and crystallographic data indicate that [Cp*(CO) ₂Os] ₂ has two bridging CO ligands. Reaction of tetraisopropylcyclopentadiene with [(CO) ₃OsI] ₂ provides ( ⁱPr ₄C ₅H)(CO) ₂OsH, which can be converted to ( ⁱPr ₄C ₅H)(CO) ₂OsH through reduction with NaK followed by treatment with EtOH. Reaction of ( ⁱPr ₄C ₅H)(CO) ₂OsH with AIBN gives [( ⁱPr ₄C ₅H)(CO) ₂Os] ₂, which is much more soluble than [Cp*(CO) ₂Os] ₂. DFT (B3LYP) calculations on these Os-Os dimers reveal the energetics of different geometrical isomers. The anti and gauche isomers of [Cp(CO) ₂Os] ₂, with terminal CO ligands, are significantly more stable than the trans and cis isomers with bridging CO ligands, while the stability of the anti isomer of [Cp*(CO) ₂Os] ₂ is similar to that of its trans isomer. Computations on the energetics of the isomers with a [C ₅(CF ₃) ₅] ligand suggest that the difference observed for Cp and Cp* complexes is mainly due to electronic rather than steric effects of the Cp groups.