Structural control in the formation of multidecker sandwich anions of plumbocene: The effects of encapsulating the alkali metal counterions

Addition of CPM (Cp = C5H5; M = Li, Na, K, Cs) to Cp2E (Cp = C5H5; E = Sn, Pb-) produces pi-anions of general formula [Cp2x+1Ex](-). Multidecker anion s (with x > 1) can be prepared for Pb if crown or cryptand ligands coordina te the alkali metal cations. The syntheses and structures of the new comple xes [Cp3Sn](-.)[Li(12-crown-4)(2)](+) (2), [Cp2Pb(mu-Cp)Na .(15-crown-5)] ( 3), [Cp5Pb2]-[K(2,2,2-crypt)](+.) THF (4), [Cp2Pb(mu-Cp)Pb(mu-Cp)Cs(18-crow n-6)] (5), and [Cp-5-Pb-2](-)[Li(12-crown-4)(2)](+). 2THF (6) are reported. This study, together with that on [Cp-9-Pb-4](-)[Cp5Pb2](-)[{Li(12-crown-4 )(2)(+)}(2)] (1), which we have communicated previously, indicates that cha rge separation and lattice energy considerations subtly control the aggrega tion of the multidecker Pb(II) anions involved. These factors allow the pot ential control of the anion homologues formed by changing the solvation sph ere of the alkali metal cations. However, as is illustrated by the structur es of 1 and 6; which contain the same [Li(12-crown-4)(2)](+) countercation, in certain cases structural modification can be controlled by the reaction employed.