Assessing the donor ability of boratabenzene and 9-borataphenanthrene anions through metal complexes with carbonyl ligands.

A series of anionic group 6 tricarbonyl and neutral rhodium dicarbonyl complexes featuring a boratabenzene (L1, with a phenyl on boron, a trimethylsilyl group on the adjacent carbon and methyl groups on the other carbons) and a borataphenanthrene ligand (L2, with a phenyl group on boron and a trimethylsilyl group on the adjacent carbon) are prepared. The donor ability of the boracyclic ligands is evaluated experimentally and theoretically by the stretching frequencies of the CO ancillary ligands. Overall, the donor ability of the ligands falls into the following trend: L1 > cyclopentadienyl > L2 > mesitylene.