The nature of the gallium - Iron bond in [Ar*GaFe(CO)₄]

The question of whether the gallium-iron bond in [Ar*GaFe(CO)₄] (Ar* = 2,6-(2,4,6-triisopropylphenyl)-phenyl) (1) should be considered as a single or a triple bond has been addressed by means of quantum chemical methods. Gradient-corrected density functional theory (BP86) calculations are reported for the gallium model complexes [(C₆H₅)GaFe(CO)₄] (1a) and [CpGaFe(CO)₄] (2a). Analysis of the bonding using the CDA partitioning schemes shows that there is a substantially higher degree of Ga ← Fe π backbonding in 1a than that of 2a. The Ga-Fe bond dissociation energy (D_e) of 1a (54.9 kcal mol^-1) is also clearly higher than in 2a (32.8 kcal mol^-1). This result lends some credence to the formulation of 1 with a Ga-Fe triple bond. However, the concepts applied in the discussion of the largely ionic Ga-Fe bonding situation in terms of single or multiple bonds -are not appropriate. Neither the formula with a Ga-Fe single bond nor the Lewis form with a Ga-Fe triple bond is a proper representation of the bonding interactions. The discussion for and against the triple-bond character of 1 is a pseudo-conflict caused by application of an ill-defined bonding model that is inappropriate.