B-B Coupling and B-B Catenation: Computational Study of the Structure and Reactions of Metal-Bis(borylene) Complexes.

A detailed molecular orbital analysis of the metal-bis(borylene) complex [Fe(CO) {B(Dur)B(N(SiMe ) )}] (Dur=2,3,5,6-tetramethylphenyl) (1 a) serves as a focal point of recent developments in this area of chemistry, such as B-B coupling and B-B catenation reactions. There is strong a π delocalization between the Fe(CO) and (B-Dur)(B-N(SiMe ) ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe ) on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B σ-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong σ-donating ligands, such as PMe or PCy , induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to [Fe(CO) {B(Dur)B(N(SiMe ) )}] (3 a) with a short Fe-Fe distance of 2.355 Å. A detailed mechanism for the reaction of 3 a with CO to give the B-B catenation product 2 f is presented. The bonding of all intermediates is compared to their isolobal main-group analogues.