SIGMA AND PI BINDING EFFECTS IN COORDINATION OF CARBON MONOXIDE AND COMPARISON WITH CYANIDE ION

Semiempirical SCF calculations have been performed for a series of XCO and, for comparison, XCN molecules according to the structure of the CNDO/2 scheme. Computed CO bond orders and binding energies are found to correlate well with the CO stretching force constants. Examination of the results for HCO+ and COH+ reveals a role of the parent “lone-pair” orbitals for effecting an increase in CO bond order and binding energy upon adduct formation. Comparison of HCO+ and COH+ also reveals a fundamental difference in the role of the very stable 3óMO of the parent in the two cases. For substituents with occupied π symmetry orbitals, charge transfer to CO via the π orbitals is found to be large, and the resultant CO bond weakening generally dominates ó stabilization. The computed “three-center” π MO's of XCO can be transformed to an equivalent basis of substituent π and parent π and π* orbitals, and, in this basis set, back-donation to the parent by the substituent is easily accounted for as well as some πtransfer in the direction parent π → X. This analysis indicates considerable BH3 and CH3 hyperconjugation with CO in BH3CO and CH3CO+. The results for BH3CO, CH3CO+, HCO+, and HCO* indicate the operation of a synergic charge flow in the CO moiety. The operation of synergism in XCO is also seen by comparison of XCO with XCN. Both CO and CN are found to respond to adduct formation in similar fashion. Cyanide is computed to be the better donor and poorer acceptor although ü donation by CO is enhanced by back-bonding. Variation of the total bond orders and binding energies across both series of substituted molecules appears to be mainly due to changes in CN and CO ü bonding. Further, while ü stabilization of CN- is one consequence of carbon addition, an even more important source of increased CN stabilization appears to arise from a decrease in C-N repulsion as a result of charge withdrawal from the carbon atom. This mechanism is not so important for the isoelectronic but neutral CO. © 1969, American Chemical Society. All rights reserved.