Hypervalent Compounds as Ligands: I3-Anion Adducts with Transition Metal Pentacarbonyls

Abstract

Just a couple of transition metal complexes of the familiar triiodide anion are known. To investigate the bonding in these, as well as isomeric possibilities, we examined theoretically adducts of I3(-) with model organometallic fragments, [Cr(CO)5] and [Mn(CO)5](+). Bonding energy computations were augmented by a Natural Bond Orbital (NBO) perturbation theory analysis and Energy Decomposition Analysis (EDA). The bonding between I3(-) and the organometallic fragment is substantial, especially for the electrostatically driven anion-cation case. "End-on" coordination is favored by 5-13 kcal/mol over "side-on" (to the central I of I3(-)), with a ∼10 kcal/mol barrier for isomerization. A developing asymmetry in the I-I bonding of "end-on" coordinated I3(-) led us to consider in some detail the obvious fragmentation to a coordinated I(-) and free I2. While the signs of incipient fragmentation in that direction are there, these is a definite advantage to maintaining some I(-) to I2 bonding in triiodide complexes.