R studies of coordinatively unsaturated surface compounds on silica gel. II . CO complexes of nickel(II)

The IR spectra of the title compounds show one absorption band due to the stretching vibration of adsorbed CO (2198.5 cm −1 ) at room temperature and low CO pressure (20 Torr CO) and three bands (2180, 2 195 and 2206 cm −1 ) at low temperatures (−70 to −145°C). The first band is assigned to a trans square‐planar complex with two CO molecules and the latter to an octahedral complex with four CO molecules. Using a different preparation method it was possible to obtain also a cis square‐planar complex with two CO molecules (2 181.5 and 2194 cm −1 ). The reduction of the title compounds to nickel(I) and nickel(0) with CO at room or low temperatures was investigated and a reaction mechanism is proposed. Both reactions (conversion to a cis surface complex and the reduction) are explained on the basis of a CO insertion into a Ni(II)O‐ bond, by which a relatively unstable nickel(I) compound is produced (vibration at 1 758 (1 804) cm −1 ).