| Summary: | The structures and bonding of selected neutral M(CO)<sub>6</sub> complexes (M = Sc, Y, La, Lu, Ac and U) have been studied by density functional theory calculations. The calculations revealed the preference for C<sub>2v</sub> symmetry and low-spin electronic state for most of these complexes. The relative stability of the low-symmetry species increases gradually with the size of the metal atom. While the characteristic O<sub>h</sub> hexa-coordinated structure is favored in the high-spin electronic state of the smaller metals, for heavier metals, important advantages of the C<sub>2v</sub> vs. O<sub>h</sub> structures include larger charge transfer interactions in terms of transferred electrons as well as better steric conditions. Our joint experimental–theoretical analysis detected and confirmed the O<sub>h</sub> structure of the Sc(CO)<sub>6</sub> complex in cryogenic CO/Ar matrices.
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