Preference of C2v Symmetry in Low-Spin Hexacarbonyls of Rare-Earth and f Elements

The structures and bonding of selected neutral M(CO)6 complexes (M = Sc, Y, La, Lu, Ac and U) have been studied by density functional theory calculations. The calculations revealed the preference for C2v 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 Oh hexa-coordinated structure is favored in the high-spin electronic state of the smaller metals, for heavier metals, important advantages of the C2v vs. Oh 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 Oh structure of the Sc(CO)6 complex in cryogenic CO/Ar matrices.