On the reactivity of acetylenes coordinated to cobalt. 9. Effects of substitution and coordination on the ¹³C-NMR chemical shifts of the sp Carbons of (μ₂-R¹C₂R²)Co₂(CO)₆ complexes. Molecular structure of (μ₂-PhC₂SiPh₃)Co₂(CO)₆

Thirty-four (μ₂-R¹C₂R²)Co₂(CO)₆ complexes (14 new) were prepared and characterized by their IRv(CO) and ¹H- and ¹³C-NMR spectra. The ¹³C-NMR chemical shifts of the coordinated sp carbon atoms were correlated with those of the corresponding free acetylenes. This indicated that the interaction between the R¹ and R² groups and the C₂Co₂(CO)₆ moiety is very sensitive not only to the donor/acceptor character of R¹ and R² but also to the orbital symmetry of the atoms attached directly to the C(sp) atoms. The changes of the δ(¹³C) values of the C(sp) atoms upon coordination were also analyzed in these terms. It is concluded that although the dinuclear μ₂-coordination of the acetylene results in a high degree of excitation the stability of the C₂Co₂(CO)₆ moiety diminished the reactivity of the organic ligand. Differences in the reactivity of the (μ₂-R¹C₂R²)Co₂(CO)₆ complexes in carbonylation are also discussed. The crystal and molecular structure of (PhC₂SiPh₃)Co₂(CO)₆ was determined by single crystal X-ray diffraction. This compound crystallizes in the orthorhombic Pbca space group with a = 28.790(2) Å, b = 11.577(1) Å, c = 17.833(2) Å, Z = 8. The structure was determined with R = 0.039, R_w = 0.036. © 1995 American Chemical Society.