Prospects for Three-Electron Donor Boronyl (BO) Ligands and Dioxodiborene (B₂O₂) Ligands as Bridging Groups in Binuclear Iron Carbonyl Derivatives

Recent experimental work (2010) on (Cy₃P)₂Pt­(BO)Br indicates that the oxygen atom of the boronyl (BO) ligand is more basic than that in the ubiquitous CO ligand. This suggests that bridging BO ligands in unsaturated binuclear metal carbonyl derivatives should readily function as three-electron donor bridging ligands involving both the oxygen and the boron atoms. In this connection, density functional theory shows that three of the four lowest energy singlet Fe₂(BO)₂(CO)₇ structures have such a bridging η²-μ-BO group as well as a formal Fe–Fe single bond. In addition, all four of the lowest energy singlet Fe₂(BO)₂(CO)₆ structures have two bridging η²-μ-BO groups and formal Fe–Fe single bonds. Other Fe₂(BO)₂(CO)_<i>n</i> (<i>n</i> = 7, 6) structures are found in which the two BO groups have coupled to form a bridging dioxodiborene (B₂O₂) ligand with B–B bonding distances of ∼1.84 Å. All of these Fe₂(μ-B₂O₂)­(CO)_<i>n</i> structures have long Fe···Fe distances indicating a lack of direct iron–iron bonding. One of the singlet Fe₂(BO)₂(CO)₇ structures has such a bridging dioxodiborene ligand with cis stereochemistry functioning as a six-electron donor to the pair of iron atoms. In addition, the lowest energy triplet structures for both Fe₂(BO)₂(CO)₇ and Fe₂(BO)₂(CO)₆ have bridging dioxodiborene ligands with trans stereochemistry functioning as a four-electron donor to the pair of iron atoms