Octacoordinate metal carbonyls of scandium and yttrium: theoretical calculations and experimental observation

The transition metal carbonyls are among the most important complexes in coordination chemistry. The maximum coordination number in these complexes is seven. Because the cations Sc(+) and Y(+) have empty second outermost d orbital subshells, they can possibly bond eight CO ligands, forming the 18-electron d(10)s(2)p(6) noble gas configuration. The aim of this study is to determine whether the octacoordinate metal carbonyls of Sc(+) and Y(+) exist.

The structures and bonding of M(CO)n(+) (M = Sc and Y, n = 7-9) were studied using Density Functional Theory (DFT) calculations with the functionals of B3LYP and BP86. The cationic complexes from laser ablation of Sc and Y in CO gas were analyzed by time-of-flight mass spectrometry.

The structures of M(CO)n(+) (M = Sc and Y, n = 7-9) and the bond dissociation energies for the last CO ligand in M(CO)n(+) (M = Sc and Y, n = 8 and 9) were obtained using DFT calculations. The products in the experiment for both metals include the series MO(CO)n(+), MO(H2O)(CO)n(+) and M(CO)n(+) (M = Sc or Y). The intensities of the MO(CO)n(+) and MO(H2O)(CO)n(+) ions change gradually with the number of CO ligands, while most M(CO)n(+) ions are very weak except for three intense ones, Sc(CO)7(+), Sc(CO)8(+) and Y(CO)8(+).

Comparisons between the theoretical calculations and the experimental observations indicate that eight CO ligands are chemically bonded on the central atom in the singlet state of Sc(CO)8(+) ((1)A1 state of D(4d) symmetry) and the singlet and triplet states of Y(CO)8(+) ((1)A1 state of D(4d) symmetry and (3)A(1g) state of O(h) symmetry). The (1)A1 states of both Sc(CO)8(+) and Y(CO)8(+) have the 18-electron d(10)s(2)p(6) noble gas configuration. In M(CO)9(+) (M = Sc or Y), the ninth CO is weakly adsorbed on the external shell.