Polyhedral Ferraboranes with Iron Carbonyl Vertices: Carbonyl Migration Processes in the Iron Tetracarbonyl Derivatives.

The structures and energetics of the neutral BHFe() ( = 4, 3) and the dianions [BHFe(CO)] ( = 6-14) have been investigated by density functional theory. The low-energy structures of the tricarbonyl dianions [BHFe(CO)] are all found to have deltahedral structures in accordance with their 2+2 skeletal electrons. The low-energy structures of the neutral tricarbonyls BHFe(CO) ( = 6-14) with only 2 skeletal electrons are based on capped (-1)-vertex deltahedra ( = 6, 7, 8) or deltahedra with a degree 6 vertex for the iron atom. The 8- and 9-vertex deltahedra are also found in low-energy BHFe(CO) structures relating to the nondegeneracy of their frontier molecular orbitals. Carbonyl migration occurs in most of the low-energy structures of the tetracarbonyls BHFe(CO). Thus, migration of a carbonyl group from an iron atom to a boron atom gives BH(BCO)(μ-H)Fe(CO) structures with a BCO vertex and a hydrogen atom bridging a B-B deltahedral edge. In other low-energy BHFe(CO) structures, a carbonyl group is inserted into the central -vertex FeB deltahedron to give a BH(CO)Fe(CO) structure with a central (+1)-vertex FeCB deltahedron that can be an deltahedron or a μ-BH face-capped -vertex FeCB deltahedron. Other low-energy BHFe(CO) structures include BHFe(CO)(μ-CO) structures with two of the carbonyl groups bridging FeB faces ( = 6, 7, 10) or Fe-B edges ( = 12) or structures in which a BH ligand ( = 6, 7, 10, 12) is bonded to an Fe(CO) unit with exclusively terminal carbonyl groups through B-H-Fe bridges.