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

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 I-3(-) 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 I-3(-) 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 I-3(-)), with a similar to 10 kcal/mol barrier for isomerization. A developing asymmetry in the I-I bonding of "end-on" coordinated I-3(-) led us to consider in some detail the obvious fragmentation to a coordinated I- and free I-2. While the signs of incipient fragmentation in that direction are there, these is a definite advantage to maintaining some I- to I-2 bonding in triiodide complexes.