Anisotropic Chemical Shifts in Trigonal Cobalt Carbonyls Containing Metal–Metal Bonds

The ⁵⁹Co NMR was studied in a number of single crystals and solutions of cobalt carbonyl complexes X₃MCo(CO)₄ (M=Si, Ge, Sn, Pb; X=Cl, Br, I, C₆H₅) containing metal–metal bonds. The $z$ component of the chemical shift as determined by single crystal measurements in I₃GeCo(CO)₄, Br₃SnCo(CO)₄, and (C₆H₅)₃SnCo(CO)₄ was found to be constant within the series and showed a paramagnetic contribution of at least 3.6‰. As ligand field theory predicts no paramagnetic shift in this case, the data showed that for low oxidation state complexes the chemical shift can only be explained using MO theory. Good agreement with experiment was obtained using MO coefficients for the isoelectronic Fe(CO)₅. The paramagnetic contributions to the $x$ and $y$ components of the shift vary considerably within the series (3.3‰–5.0‰). These variations were attributed to differences in the metal–metal bonds. It was found that for a given X the shifts do not vary linearly as the atomic weight of M is changed. This was attributed to differences in the importance of π‐bonding effects for the metal–metal bonds. The data suggest that in this series π‐bonding effects are strongest for the Co–Sn bond.