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 [(η(5)-C(5)H(5))(L)(2)M(GaX)](+) (M = Fe, L = CO, PMe(3); X = Cl, Br, I, NMe(2), Mes; M = Ru, Os: L = CO, PMe(3); X = I, NMe(2), Mes) at the BP86/TZ2P/ZORA level of theory. Calculated geometric parameters for the model iron iodogallylene system [(η(5)-C(5)H(5))(Me(3)P)(2)Fe(GaI)](+) are in excellent agreement with the recently reported experimental values for [(η(5)-C(5)Me(5))(dppe)Fe(GaI)](+). The M-Ga bonds in these systems are shorter than expected for single bonds, an observation attributed not to significant M-Ga π 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, ΔE(σ) is found to be overwhelmingly the dominant contribution to the orbital interaction between [(η(5)-C(5)H(5))(L)(2)M](+) and [GaX] fragments (with ΔE(π) equating to only 8.0-18.6% of the total orbital contributions); GaX ligands thus behave as predominantly σ-donor ligands. Electrostatic contributions to the overall interaction energy ΔE(int) are also very important, being comparable in magnitude (or in some cases even larger than) the corresponding orbital interactions.