Theoretical Study of Bimolecular Nucleophilic Substitution at Four-, Five-, and Six-Coordinate Metal Carbonyl Radicals

Walsh diagrams, contour maps, and the atomic character and energies of the frontier molecular orbitals derived from SCF-Xα-DV calculations have been employed to deduce the most favorable mode of nucleophilic attack at homoleptic metal carbonyl radicals constrained to ideal octahedral, trigonal-bipyramidal, square-pyramidal, and tetrahedral geometries. These studies show that two-center three-elelctron bonding may stabilize a hypervalent 19-electron (19e) transition state or intermediate formed during nucleophilic attack. Spin-polarized calculations suggest a difference between these 19e species and those formed via reduction of saturated, 18e organometallics. For example, little spin density is calculated to delocalize onto the CO ligands when a nucleophile attacks Mn(CO)5 to form a 19e complex. Nucleophilic attack at a face, rather than at an edge of an octahedron, is predicted for the 17e complex V(CO)6. Nucleophilic attack at the open face of the square pyramid is the only mode of attack (of six available) that can be stabilized by two-center three-electron bonding for the square-pyramidal 17e complex Mn(CO)5. Preferred attack at an edge in the equatorial plane is predicted for the trigonalbipyramidal 17e radical Mn(CO)5. Attack at a tetrahedral face, analogous to SN2 substitution at carbon, is predicted for the tetrahedral 17e species Co(CO)4. © 1988, American Chemical Society. All rights reserved.

Duke Scholars

Author Michael J. Therien Chemistry