Multiple Bonding In Rhodium Monoboride. Quasi-Atomic Analyses Of The Ground And Low-Lying Excited States

The bonding structures of the ground state and the lowest five excited states of rhodium monoboride are identified by determining the quasi-atomic orbitals in full valence space MCSCF wave functions and the interactions between these orbitals. A quadruple bond, namely two pi-bonds and two sigma-bonds, is identified and characterized for the X-1 Sigma(+) ground state, in agreement with a previous report (Cheung et al. J. Phys. Chem. Lett. 2020, 11, 659-663). However, in all excited states, the bonding is predicted to be weaker because, in these states, one of the sigma-bonding interactions has a small magnitude. In the a(3)Delta and A(1)Delta states, the bond order is between a triple and quadruple bond. In the b(3)Sigma(+) state, the Rh-B linkage is a triple bond. In the c(3)Pi and B-1 Pi states, the atoms are linked by a double bond due to an additional weakening of the two pi-bonds. The decreases in the predicted bond strengths are reflected in the decreases of the predicted binding energies and in the increases of the predicted bond lengths from the X-1 Sigma(+) ground state to the c(3)Pi and the B-1 Pi excited states. Notably, the 5p sigma orbital of rhodium, which is vacant in the ground state of the atom, plays a significant role in the molecule.