Bonding and structure of Y₃C₂ and Y₃C⁺₂

Vibrational frequencies of yttrium normal modes of triyttrium dicarbide neutral and cation have been determined from pulsed field ionization-zero electron kinetic energy photoelectron spectra. The yttrium stretching mode has a frequency of 228 cm^-1 in the ion. The two deformation modes have frequencies of 82 and 24 cm^-1 in the neutral and a degenerate frequency of 86 cm^-1 in the ion. Geometries and ground electronic states have been determined by the comparison of the experiments and theoretical calculations. The cluster has a trigonal bipyramid structure with the ion symmetry of D_3h and the neutral C_2u. The ground state is identified as ²B₁ for the neutral and ¹A′₁ for the ion. While the Y-Y bonding has a covalent nature, the Y-C bonding has considerable ionic character. Both the Y-Y and Y-C bonds are stronger in the ion than in the neutral. The enhancement of the Y-Y bonding upon ionization is largely due to the removal of the antibonding 5s character from the yttrium atoms and the Y-C due to the increase of the electrostatic attractions between the yttrium and carbon atoms.