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Unlike mercury, which has an extensive +1 oxidation state chemistry, zinc usually adopts the +2 oxidation state. Decamethyldizincocene, Zn^sub 2^([eta]^sup 5^-C^sub 5^Me^sub 5^)^sub 2^, an organometallic compound of Zn(I) formally derived from the dimetallic [Zn-Zn]^sup 2+^ unit, has been isolated from the low-temperature (-10°C) reaction of Zn(C^sub 5^Me^sub 5^)^sub 2^ and Zn(C^sub 2^H^sub 5^)^sub 2^ in diethyl ether. X-ray studies show that it contains two eclipsed Zn([eta]^sub 5^-C^sub 5^Me^sub 5^) fragments with a Zn-Zn distance (± standard deviation) of 2.305(±3) angstroms, indicative of a metal-metal bonding interaction.
Of the Group 12 elements (zinc, cadmium, and mercury), the formal oxidation state of +1 is important only for mercury. For example, the dication Hg^sub 2^^sup 2+^ is well known, whereas the oxidation state of the lighter metals, Zn and Cd, is almost invariably +2 (1, 2). Only a few exceptions to this rule are known. The dication Cd^sub 2^^sup 2+^ has been known for many years (1, 2), and it has been structurally characterized by x-ray methods in Cd^sub 2^(AlCl^sub 4^)^sub 2^ (3) and by ^sup 113^Cd nuclear magnetic resonance (NMR) (4) in Cd^sub 2^Tp^sup Me2^^sub 2^ [Tp^sup Me2^ is hydrotris(3,5-dimethylpyrazolyl)borate]. There is evidence for the formation of Zn^sub 2^^sup 2+^ ions in ZnCl^sub 2^/Zn glasses at high temper atures (5) and in zeolite matrices (6). Very recently, the formation of mononuclear, paramagnetic Zn^sup +^ in a microporous crystalline silicoaluminophosphate has been reported (7).
However, despite extensive research (1, 8), molecular complexes of the metal-metal-bonded Zn^sub 2^^sup 2+^ unit have not been reported. We unexpectedly succeeded in isolating a compound of this type, namely decamethyldizincocene, Zn^sub 2^([eta]^sup 5^-C^sub 5^Me^sub 5^)^sub 2^ (where Me is CH^sub 3^) (compound 1), and we have characterized this molecule by chemical, spectroscopic, and x-ray methods.
Zincocene, Zn(C^sub 5^H^sub 5^)^sub 2^, and diethylzinc, Zn(C^sub 2^H^sub 5^)^sub 2^, have been reported to react at 60°C, for 1 hour, to give the half-sandwich complex Zn([eta]^sup 5^-C^sub 5^H^sub 5^)(C^sub 2^H^sub 5^) in quantitative yield (9). We have found that the analogous reaction of Zn(C^sub 5^Me^sub 5^)^sub 2^ (10) and Zn(C^sub 2^H^sub 5^)^sub 2^ is fast at room temperature and produces a mixture of two compounds, 1 and 2 (Fig. 1), in a ratio that depends on the reaction conditions. In some instances, partial decomposition and...