Fluxional bonds in quasi‐planar B 18 2 − and half‐sandwich MB 18 − (M = K, Rb, and Cs)

Detailed molecular orbital and bonding analyses reveal the existence of both fluxional σ‐ and π‐bonds in the global minima C sB 18 2 −( 1 ) and C s MB 18 ( 3 ) and transition states C sB 18 2 −( 2 ) and C sMB 18 − ( 4 ) of B 18 2 −dianion and MB 18 − monoanions (M = K, Rb, and Cs). It is the fluxional bonds that facilitate the fluxional behaviors of the quasi‐planar B 18 2 −and half‐sandwich MB 18 − which possess energy barriers smaller than the difference of the corresponding zero‐point corrections. © 2019 Wiley Periodicals, Inc.