Zinc-zinc bonded zincocene structures.: Synthesis and characterization of Zn2(η5-C5Me5)2 and Zn2(η5-C5Me4Et)2

While, in general, decamethylzincocene, Zn(C5Me5)(2), and other zincocenes, Zn(C5Me4R)(2) (R = H, Bu-t, SiMe3), react with dialkyl and diaryl derivatives, ZnR'(2), to give the half-sandwich compounds (eta(5)-C5Me4R)ZnR', under certain conditions the reactions of Zn(C5Me5)(2) with ZnEt2 or ZnPh2 produce unexpectedly the dizincocene Zn-2(eta(5)-C5Me5)(2) (1) in low yields, most likely as a result of the coupling of two (eta(5)-C5Me5)Zn-center dot radicals. An improved, large scale (ca. 2 g) synthesis of 1 has been achieved by reduction of equimolar mixtures of Zn(C5Me5)(2) and ZnCl2 with KH in tetrahydrofuran. The analogous reduction of Zn(C5Me4R)(2) (R = H, SiMe3, Bu-t) yields only decomposition products, but the isotopically labeled dimetallocene Zn-68(2)(eta(5)-C5Me5)(2) and the related compound Zn-2(eta(5)-C5Me4Et)(2) (2) have been obtained by this procedure. Compound 2 has lower thermal stability than 1, but it has been unequivocally characterized by low-temperature X-ray diffraction studies. As for 1 a combination of structural characterization techniques has provided unambiguous evidence for its formulation as the Zn-Zn bonded dimer Zn-2(eta(5)-C5Me4Et)(2), with a short Zn-Zn bond of 2.295(3) A indicative of a strong Zn-Zn bonding interaction. The electronic structure and the bonding properties of 1 and those of related dizincocenes Zn-2(eta(5)-Cp')(2) have been studied by DFT methods (B3LYP level), with computed bond distances and angles for dizincocene 1 very similar to the experimental values. The Zn-Zn bond is strong (ca. 62 kcal center dot mol(-1) for 1) and resides in the HOMO-4, that has a contribution of Zn orbitals close to 60%, consisting mostly of the Zn 4s orbitals (more than 96%).