THEORETICAL-STUDY OF THE DISSOCIATION OF A SINGLE CARBONYL FROM CHROMIUM HEXACARBONYL
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Extended-basis-set Hartree-Fock-Roothaan calculations are reported for the dissociation of a single carbonyl ligand from chromium hexacarbonyl. As expected, the theoretical dissociation energy for the process Cr(CO)6 Cr(CO)5 + CO (49.8 kcal mol-1) is larger than the average dissociation energy (29.5 kcal mol-1). The theoretical dissociation energy for the ion, Cr(CO)6+ Cr(CO)5+ + CO (30.8 kcal mol-1), agrees well with the recent mass spectral value (33.0 kcal mol-1) of Michels, Flesch, and Svec (Inorg. Chem. 1980,19, 479). Energy partitioning and analysis suggests that bonding accounts for about 25% of the dissociation energy. Plots of the change in electron density as the carbonyl dissociates are reported. Comparison of these plots with those produced from various population analyses suggest that the bond involves primarily a rehybridization of the CO with little net transfer of electron density to the metal. The bond, however, does involve a net transfer of charge from the metal to the carbonyl. Thus, the chromium is found to bear a net positive charge in agreement with the recent X-ray diffraction study of Rees and Mitschler (J. Am. Chem. Soc. 1976, 98, 7918). At very long chromium-carbon distances the carbonyl moiety may act like a -only ligand. Calculations of the change in the force between the C and the O of a free carbonyl as it approaches the Cr(CO)5 fragment show that at moderately long Cr-C distances the force is such that the C-O bond distance should be shorter than that of free CO. A well-documented example of this is provided by the low-temperature X-ray structure of Rh2(O2CCH3)4(CO)2 by Koh and Christoph (J. Am. Chem. Soc. 1979, 101, 1422). This molecule has a long Rh-C distance of 2.092 (4) and a remarkably short C-0 distance of 1.120 (4) . Recently, Gagn et al. (Inorg. Chem. 1982, 21, 254) reported the structure of a Cu dimer that had C-0 distances of 1.10 (1) and 1.11 (1) . 1983, American Chemical Society. All rights reserved.
