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)(3){B(Dur)B(N(SiMe3)(2))}] (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 pi delocalization between the Fe(CO)(3) and (B-Dur)(B-N(SiMe3)(2)) units; the short B-B distance in 1 a is due to this pi delocalization. The pi-donor ligand N(SiMe3)(2) on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B sigma-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong sigma-donating ligands, such as PMe3 or PCy3, induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to Fe(CO)(2){B(Dur)B(N(SiMe3)(2))}](2) (3 a) with a short Fe-Fe distance of 2.355 angstrom. 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