On 3d bonding in the transition metal trimers: The electronic structure of equilateral triangle Ca3, Sc3, Sc+3, and Ti+3

Abstract

CASSCF/CCI calculations are presented for the low-lying states of Sc3 and Sc+3 and SCF/CI calculations are presented for the 1A′1 state of Ca3 arising from three ground state (4s2) Ca atoms, all for equilateral triangle geometries. The calculations use effective core potentials, developed by Hay and Wadt, which replace the Ne core but include the 3s and 3p core levels along with the valence electrons in the calculations. The bonding in Ca3 arises by 4s→4p promotion and leads to a well depth of about 0.5 eV for R(Ca–Ca)=7.5a0. For Sc3 the 4s bonding is similar to that in Ca3, but the 3d electrons are also strongly bonding leading to a 2A′2 ground state with a well depth of about 1.0 eV and R(Sc–Sc)=5.75a0. The good 3d bonding orbitals (bonding between all three atoms) are 3da″2 derived from atomic 3dπ″ and 3da1 derived from atomic 3dσ, while 3dπ′ atomic orbitals lead to 3de′ orbitals which are bonding between pairs of atoms, and the 3dδ′ and 3dδ″ derived levels are nonbonding. (Here the atomic symmetry is given with respect to an axis connecting the atom to the center of the molecule.) Based on the Sc3 calculations and preliminary calculations on Ti+3 , the bonding in V+3 and Cr+3 is also discussed.