Pentamethylcyclopentadienylgallium (Cp*Ga):  Alternative Synthesis and Application as a Terminal and Bridging Ligand in the Chemistry of Chromium, Iron, Cobalt, and Nickel

An alternative synthetic route to η5-pentamethylcyclopentadienylgallium (Me5C5Ga, Cp*Ga, 1) via reductive dehalogenation of (pentamethylcyclopentadienyl)diiodogallane (Cp*GaI2, 2) is reported. In addition, the behavior of compound 1 as a ligand in transition metal chemistry is examined. In the reaction of 1 with Cr(CO)5(C8H14) (C8H14 = cis-cyclooctene), Fe2(CO)9, Fe(CO)3CHT (CHT = cycloheptatriene), Co2(CO)8, and Ni(CO)4 the new complexes Cr(Cp*Ga)(CO)5 (3), Fe(Cp*Ga)(CO)4 (4), Fe2(μ-Cp*Ga)3(CO)6 (5), Co2(μ-Cp*Ga)2(CO)6 (6), and Ni4(μ-Cp*Ga)4(CO)6 (8), respectively, were obtained. They were characterized by analytical and spectroscopic methods; solid-state structures were determined by X-ray diffraction analysis. In the reaction of 1 with Ni(CO)4, the complex Ni(Cp*Ga)(CO)3 (7) is an intermediate, which was characterized by IR and NMR data. The η5-bonding mode within the Cp*Ga ligand is maintained, apart from one Cp*Ga unit in 5 which exhibits η3/η1-bonding. The cone angle of η5-Cp*Ga is 112°, as determined on the basis of structural data of 3 and 4. Concerning the σ-donor/π-acceptor properties, 1 is classified as a predominant electron donor.