IR Studies of Coordinatively Unsaturated Surface Compounds on Silica Gel. IV. CO Complexes of Chromium(II) and Chromium(III)

Three different chromium(II) surface compounds have been identified by their IR spectra of adsorbed CO. Especially important in the interpretation of the experiments are the bands between 2050 and 2035 cm−1 and the relative intensities of the CO bands at 2100 and 2120 cm−1. The bands between 2192 and 2179 cm−1 show also a different pattern for each of the different chromium(II) surface compounds. Two chromium(III) surface compounds could be identified by the presence of either one CO band near 2200 cm−1 or two bands near 2206 and 2196 cm−1. Three models are proposed for the chromium(II) title compounds, all related to dinuclear chromium(II) surface complexes: Models one and two have chromium(II) ions with trans and cis configuration in different combinations, respectively, and the third model has a bridging oxygen ion from a siloxan group as a distorting ligand. The three models explain the CO absorptions sufficiently and one is nearly quantitatively in agreement with previous volumetric adsorption studies of CO on chromium(II), which showed the surprising effect of decreasing CO adsorption with decreasing temperature. Two models for the chromium(III) surface compounds are proposed with either an oxygen double bridge or only one bridging oxygen.