between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal-alkyl bond. The M-CO and MC-O distance are sensitive to other ligands on the metal. Illustrative of these effects are the following data for Mo-C and C-O distances in Mo(CO)₆ and Mo(CO)₃(4-methylpyridine)₃: 2.06 vs 1.90 and 1.11 vs 1.18 Å.

Infrared spectroscopy is a sensitive probe for the presence