Methyllithium clusters

MO modeling (HF/3-21G) of the reaction of organolithium compounds with carbonyl groups has examined the interaction of formaldehyde with the dimer of methyllithium. The reaction is predicted to proceed by initial complexation of the carbonyl group at lithium, followed by a rate-determining formation of the new carbon-carbon bond. The cluster then reorganizes to incorporate the newly formed alkoxide ion. The TS is reached very early in the second step with only slight formation of the C—C bond. This feature is consistent with the fast and exothermic nature of the addition step. 

Ethylidynetrinickel cluster 1 (NiCp)3CCH3 was prepared for the first time from the reaction of nickelocene with methyllithium, in diethyl ether and THF. 1 sublimes at 60 °C (0.001 Torr), melts without decomposition at 158-160 °C, oxidizes easily in air but does not hydrolyze in water. The mass spectrum shows a parent peak at m/z 396 ( Ni). The C NMR of 1 established the presence of the ethylidyne carbon atom at S 289.3 (s). The molecular structure of 1 is shown on Fig. 1. The nickel framework is best described as an isoceles triangle. The C2 atom of the apical methylidyne group is tilted slightly towards the Ni 1 atom. 

It has been shown above that cluster 1 is formed in the reactions of nikelocene with methyllithium, vinyllithium, phenyllithium in the presence of THF and with ethyllithium. 

The mechanism of the reaction of nickelocene with methyllithium is complicated and not, as yet, fully explained. The new C-C bond and a number of higher cyclopentadienylnickel clusters are formed in this reaction. The clusters are often unstable and difficult to isolate in a pure state. There is, however, no doubt, that the first step of the reaction is the formation of unstable CpNiCH3. This complex... 

The first trinickel cluster with an open structure is Ni3Cp2(Ph2C2)3 18, which was obtained from the reaction of nickelocene with methyllithium in the presence of diphenylacetylene, with a molar ratio of the reactants of 1 1 2 (Eq. (18)). It had been postulated earlier from spectroscopic evidence that the cluster Ni3Cp2-(CF3C2CF3)3 has a similar structure. 

The formation of a hexanickel cluster has also been postulated in the reaction of nickelocene with methyllithium (Scheme 2). If the intermediate species (NiCp)4C, formed in the reaction (20), reacts with (NiCp)2), a (cyclopentadienyl)carbido-hexanickel cluster 25 would be formed (Eq. (22))... 

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