“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” [(η 5 -Cp) 3 E] - (Cp = C 5 H 5; 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, [(η 5 -Cp) 2 Sn(μ-Cp)NaPMDETA] ( 1; PMDETA = [(CH 3 ) 2 NCH 2 CH 2 ] 2 NCH 3 ), [Mg(THF) 6 ] 2+ [(η 3 -Cp) 3 Sn - ] 2 ( 2 ), [(η 5 -Cp) 2 Pb(μ-Cp)NaPMDETA] ( 3 ), and [Mg(THF) 6 ] 2+ [(η 3 -Cp) 3 Pb - ] 2 ( 4 ), are reported. The monomeric complexes 1 and 3 are essentially isostructural and contain trigonal-planar (η 5 -Cp) 3 E units linked, through bridging μ-η 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 [(η 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 [(η 5 -Cp) 2 E(μ-Cp)Na] (E = Sn, Pb) are loose-contact complexes of [Cp 2 E] and [CpNa], [(η 3 -Cp) 3 E] - species are best formulated as triorganometal anions. Solid-state magic angle spinning (MAS) 119 Sn and 13 C NMR spectroscopic studies are in broad agreement with the model ab initio calculations and illustrate that in [(η 5 -Cp) 2 Sn(μ-Cp)NaPMDETA], in which partial solvation of the Na + coordination sphere occurs, the electronic situation approaches that in [Cp 2 Sn].