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

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