Pentagonal five-center four-electron π bond in ternary B3N2H5 cluster: an extension of the concept of three-center four-electron ω bond.

ABSTRACT

Boron-based heteroatomic rings can have exotic chemical bonding, in which the p lone-pairs of heteroatoms manage to participate in delocalized π bonding, compensating for boron's electron-deficiency. We explore herein the bonding properties of ternary B-N-H systems with a pentagonal ring, using the B3N2H50/-/2- clusters as examples. Computational structural searches lead to perfectly planar C2v B3N2H5 (1, 1A1) and C2v B3N2H5- (2, 2B1) as global minima for the neutral species and monoanion, which feature a pentagonal B3N2 ring. The corresponding dianion C2v B3N2H52- (3, 1A1) is a local minimum, whose global minimum adopts a chain-like open structure. Bonding analyses reveal a five-center four-electron (5c-4e) π system in 1, dubbed the 5c-4e o-bond. It is a 4π system in the bonding/nonbonding combination, originating from two N 2p lone-pairs, which can be considered as an extension of the concept of 3c-4e ω-bond. The extra electrons in 2 and 3 occupy a markedly destabilized π orbital. Thus, a 4π configuration, rather than a π sextet according to the (4n + 2) Hückel rule, is electronically robust for the B3N2H50/-/2- system. Infrared and photoelectron spectra are predicted for 1 and 2, respectively. Structural evolution of ring-like and chain-like isomers with charge-state in B3N2H50/-/2- is elucidated. B3N2H5- (2) is used as ligand for sandwich-type complexes C2h [(B3N2H5)2Fe]2- and C2h [(B3N2H5)2Fe]Li2.