''Paddle-wheel'' tris(cyclopentadienyl)tin(II) and -lead(II) complexes: Syntheses, structures, and model MO calculations

Nucleophilic addition reactions of cyclopentadienide anions (Cp-) to bis(cyclopentadienyl)-tin(II) and -lead(II) produces complexes containing ''paddle-wheel'' [(eta(5)-Cp)(3)E](-)(CP = C5H5; E = Sn, Pb) anions. Thus, the heavy main-group metals act as Lewis acids rather than as bases. The syntheses and X-ray structures of four such complexes, [(eta(5)-Cp)(2)Sn(mu-Cp)-NaPMDETA] (1;PMDETA = [(CH3)(2)NCH2CH2](2)NCH3),[Mg(THF)(6)](2+)[(eta(3)-Cp)(3)Sn-](2) (2), [(eta(5)-Cp)(2)Pb(mu-Cp)NaPMDETA] (3), and [Mg(THF)(6)](2+)[(eta(3)-Cp)(3)Pb-](2) (4), are reported. The monomeric complexes 1 and 3 are essentially isostructural and contain trigonal-planar (eta(5)-Cp)(3)E units linked, through bridging mu-eta(5)-Cp ligands, to NaPMDETA(+) cations in their molecular structures. Ab initio MO calculations have been used to probe the structures and charge distributions of the [(eta(5)-Cp)(3)E](-) anions in these complexes. These show that the natures of the species are highly dependent on the extent of alkali- or alkaline-earth-metal cation solvation. Thus, whereas unsolvated [(eta(5)-Cp)(2)E(mu-Cp)Na](E = Sn, Pb) are loose-contact complexes of [Cp2E] and [CpNa], [(eta(3)-Cp)(3)E](-) species are best formulated as triorganometal anions, Solid-state magic angle spinning (MAS) Sn-119 and C-13 NMR spectroscopic studies are in broad agreement with the model ab initio calculations and illustrate that in [(eta(5)-Cp)(2)Sn(mu-Cp)NaPMDETA], in which partial solvation of the Na+ coordination sphere occurs, the electronic situation approaches that in [Cp2Sn].