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

[Image: see text] 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 2n+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 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 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.