Binuclear iron boronyl carbonyls isoelectronic with the well-known decacarbonyldimanganese

Abstract

The recent (2010) synthesis of the boronylplatinum halide (R₃P)₂Pt(BO)Br by Braunschweig and coworkers makes interesting the chemistry of metal carbonyl boronyls isoelectronic with the binary metal carbonyls M_x(CO)_y. In this connection, theoretical studies on Fe₂(BO)₂(CO)₈ isoelectronic with the known Mn₂(CO)₁₀ predict symmetrical unbridged structures consisting of Fe(BO)(CO)₄ units linked by an iron–iron single bond of ∼2.9 Å length. These Fe₂(BO)₂(CO)₈ structures with a staggered arrangement of the BO and CO ligands are similar to the experimental structure of Mn₂(CO)₁₀. Three such structures are found within ∼3 kcal mol⁻¹ of energy differing only in the locations of the terminal BO groups. Related higher energy unbridged eclipsed Fe₂(BO)₂(CO)₈ structures are found with small imaginary vibrational frequencies of around 30i cm⁻¹. Following the corresponding normal modes leads to the corresponding staggered structures. Even higher energy symmetrical Fe₂(μ-CO)₂(BO)₂(CO)₈ structures are found with two bridging carbonyl groups lying 15 to 21 kcal mol⁻¹ above the global minimum, again similar to the isoelectronic Mn₂(CO)₁₀ system. In addition to the above structures higher energy unsymmetrical Fe₂(BO)₂(CO)₈ structures are found, which have no analogues in Mn₂(CO)₁₀ chemistry. These include structures in which an Fe(CO)₄ unit is connected to an Fe(BO)(CO)₄ unit through a bridging BO group without an iron–iron bond as well as a structure in which an Fe(CO)₅ unit is connected to an Fe(BO)₂(CO)₃ unit by an Fe–Fe dative bond. The lowest energy Fe₂(BO)₂(CO)₈ structure is viable towards symmetric dissociation into two Fe(BO)(CO)₄ radicals as well as unsymmetrical dissociation into Fe(CO)₅ + Fe(BO)₂(CO)₃ with dissociation energies in excess of 15 kcal mol⁻¹.