Organic anions. Part 11. Hard sphere electrostatic calculations on group 1 organometallic compounds: ion pairs of monoanions

Abstract

Structures for the lithium salts of allyl anion; W-, S-, and U-pentadienyl anion; benzyl anion; benzyhydryl anion; trityl anion; trans,trans-, cis,trans-, and cis,cis-1,3-diphenylallyl anion; cyclo-pentadienyl anion; indenyl anion; and fluorenyl anion have each been calculated by the HSE method and the results of these calculations have been compared with known X-ray crystal structures and the results of MO calculations. In almost all cases there is good agreement. In most cases HSE calculations, like MO calculations, find more than one potential bonding site for the cation and in such cases it may be that several species are in equilibrium in solution. Since the HSE method works satisfactorily for the lithium salts, R^–, Li⁺, it is then used to predict the structures of ion pairs, for this same series of anions, where the counterion is a caesium or there is a solvent-separated ion pair. R^–, Cs⁺; R^–, SS⁺, and for the related triple ions, R^–, 2Li⁺; R^–, 2Cs⁺; R^–, 2SS⁺; R^–, Li+, Cs⁺; and R^–, Li⁺, SS⁺(where SS⁺ is the abbreviation used for the counterion in the solvent-separated ion pair and was modelled on Li+, 4THF, where THF = tetrahydrofuran). For the 1 : 1 ion pairs the predicted structure for the ‘best’ energy minimum is usually independent of the size of the counterion (Li⁺, Cs⁺, or SS ⁺) but the number of local minima decreases as the size of the counterion increases. The structures of triple ions, R^–, 2M⁺, cannot be inferred in any simple way from those of the 1 : 1 ion pairs, R^–, M ⁺. The energy surfaces for these triple ions are rather flat, cation/cation repulsion is a very significant factor, and the structures vary widely according to the sizes of the counterions.