Analysis of the effect of spin-orbit coupling on the electronic structure and excitation spectrum of the Bi2(2-) anion in (K-crypt)2Bi2 on the basis of relativistic electronic structure calculations.

The Bi 2 2 - anions that have been characterized in (K-crypt) 2 Bi 2 are isoelectronic with O 2 but are diamagnetic and EPR-silent, unlike O 2 . The UV-vis spectrum measured for (K-crypt) 2 Bi 2 shows two broad absorption peaks located at 2.05 and 2.85 eV, but no absorption at lower energies down to 0.62 eV. To account for these observations, the electronic structures of the isoelectronic diatomic dianions Q 2 2 - (Q = N, P, As, Sb, Bi) were compared on the basis of relativistic density functional theory calculations, and the electronic excitations of Bi 2 2 - were analyzed on the basis of relativistic configuration interaction calculations. The extent of spin-orbit coupling, brought about by the relativistic effect, increases steadily in the order N < P < As < Sb < Bi such that the "closed-shell" state is more stable than the "open-shell" state for Bi 2 2 - , while the opposite is the case for N 2 2 - , P 2 2 - , As 2 2 - , and Sb 2 2 - . The nature of the electronic excitations of Bi 2 2 - was assigned and discussed from the viewpoint of molecular orbitals in the absence of spin-orbit coupling.