ELECTRONIC STRUCTURE AND CHEMICAL BONDING IN THE METAL-CARBONYL SERIES $Ni(CO)_{4}, [Co(CO)_{4}]^{-}, [Fe(CO)_{4}]^{-}$

Author Institution: Philips Research Laboratories, EindhovenThe problem of bonding in metalcarbonyls is of interest not only because of these compounds themselves but also because its understanding would throw light upon the metal-carbon bond in general. The usual chemical approaches to this problem are theoretically somewhat unsatisfactory. Valencebond considerations, never easy to make quantitative, are not very attractive qualitatively, either, since a number of resonating structures have to be taken into account whose relative importance is difficult to assess. A qualitative M.O.-treatment, certainly a much better way to discuss these compounds also demands some subjective empirical assumptions to carry it through. Taking the more physical point of view that the essential features of chemical bonding are determined by the energetically most favorable electronic charge distribution (in normal space), in particular that of the valence-electrons (3d, 4s and 4p for the metal, 2s and 2p for C and O), one is led to Roothaan's LCAO-SCF approach as the presently best available method to approximate this distribution. The assumptions and approximations underlying our application of this method to these rather complicated molecules will be discussed briefly. The results show that even when not too much stress is laid upon numerical accuracy, as we were forced to do here, a satisfactory outcome can be obtained. Some energetic comparisons with available experimental data for $Ni(CO)_{4}$ and a comparison with SCF-MO's of free CO will be presented to show why we have reasonable confidence in the charge density obtained. The population-analysis of the m.o.'s and its implications for the bonding characteristics of each individual m.o. as well as for the overall picture of the chemical bonds in these molecules will be discussed. The salient features showing up when going from Ni to Co and Fe will be pointed out and related to characteristic changes in the CO stretching force-constants observed