Electronic Structure and Chemical Bonding in the Ground and Low‐lying Electronic States of Ta2

The electronic structure and chemical bonding of the ground and low-lying Lambda - S and Omega states of Ta-2 were investigated at the multiconfiguration second-order perturbation theory (CASSCF//CASPT2) level. The ground state of Ta-2 is computed to be a X-3 Sigma(-)(g) state (R-e = 2.120 angstrom, omega(e) = 323 cm(-1), and D-e = 4.65 eV), with two low-lying singlet states close to it (a(1) Sigma(+)(g) : T-e = 409 cm(-1), R-e = 2.131 angstrom, and omega(e) = 313 cm(-1); b(1) Gamma(g): T-e = 1, 038 cm(-1), R-e = 2.127 angstrom, and omega(e) = 316 cm(-1)). These electronic states are derived from the same electronic configuration: vertical bar 13 sigma(2)(g)14 sigma(2)(g)7 delta(2)(g)13 pi(4)(u)>. The effective bond order of the X-3 Sigma(-)(g) state is 4.52, which indicates that the Ta atoms are bound by a quintuple chemical bond. The a(1) Sigma(+)(g) state interacts strongly with the X-3 Sigma(-)(g) g ground state by a second-order spin-orbit interaction, giving rise to the (1)0(g)(+) (ground state) (dominated by the X-3 Sigma(-)(g) Lambda - S ground state) and (9)0(g)(+) (dominated by the a(1) Sigma(+)(g) Lambda - S state) Omega states. These results are in line with those reported for the group 5B homonuclear transition metal diatomics. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 1306-1315, 2011