Nature of M-Ga Bonds in Cationic Metal-Gallylene Complexes of Iron, Ruthenium, and Osmium, [(η5-C5H5)(L)2M(GaX)]+: A Theoretical Study

Density Functional Theory calculations have been performed for the cationic half-sandwich gallylene complexes of iron, ruthenium, and osmium [(eta(5)-C5H5)(L)(2)M(GaX)](+) (M = Fe, L = CO, PMe3; X = Cl, Br, I, NMe2, Mes; M = Ru, Os: L = CO, PMe3; X = I, NMe2, Mes) at the BP86/TZ2P/ZORA level of theory. Calculated geometric parameters for the model iron iodogallylene system [(eta(5)-C5H5)(Me3P)(2)Fe(Gal)](+) are in excellent agreement with the recently reported experimental values for [(eta(5)-C5Me5)(dppe)Fe(Gal)](+). The M-Ga bonds in these systems are shorter than expected for single bonds, an observation attributed not to significant M-Ga pi orbital contributions, but due instead primarily to high gallium s-orbital contributions to the M-Ga bonding orbitals. Such a finding is in line with the tenets of Bent's Rule insofar as correspondingly greater gallium p-orbital character is found in the bonds to the (more electronegative) gallylene substituent X. Consistent with this, Delta E-sigma is found to be overwhelmingly the dominant contribution to the orbital interaction between [(eta(5)-C5H5)(L)(2)M](+) and [GaX] fragments (with Delta E-pi equating to only 8.0-18.6% of the total orbital contributions); GaX ligands thus behave as predominantly sigma-donor ligands. Electrostatic contributions to the overall interaction energy Delta E-int are also very important, being comparable in magnitude (or in some cases even larger than) the corresponding orbital interactions.