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

The structures and energetics of the neutral Bn–1Hn–1Fe(CO)x (x = 4, 3) and the dianions [Bn–1Hn–1Fe(CO)3]2– (n = 6–14) have been investigated by density functional theory. The low-energy structures of the tricarbonyl dianions [Bn–1Hn–1Fe(CO)3]2– are all found to have closo deltahedral structures in accordance with their 2n+2 skeletal electrons. The low-energy structures of the neutral tricarbonyls Bn–1Hn–1Fe(CO)3 (n = 6–14) with only 2n 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 Bn–1Hn–1Fe(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 Bn–1Hn–1Fe(CO)4. Thus, migration of a carbonyl group from an iron atom to a boron atom gives closo Bn–2Hn–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 Bn–1Hn–1Fe(CO)4 structures, a carbonyl group is inserted into the central n-vertex FeBn–1 deltahedron to give a Bn–1Hn–1(CO)Fe(CO)3 structure with a central (n+1)-vertex FeCBn–1 deltahedron that can be an isocloso deltahedron or a μ3–BH face-capped n-vertex FeCBn–2 closo deltahedron. Other low-energy Bn–1Hn–1Fe(CO)4 structures include Bn–1Hn–1Fe(CO)2(μ-CO)2 structures with two of the carbonyl groups bridging FeB2 faces (n = 6, 7, 10) or Fe–B edges (n = 12) or structures in which a closo Bn–1Hn–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.