Alkali metal complexes of a phosphine-borane-stabilised carbanion: influence of co-ligands on structure

The adducts [[(Me3Si)(2){Me2P(BH3)}C]K(L)(n)](m) [L = THF, n = 0.5, m = infinity(2a); L = tmeda (2b), pmdeta (2c), n = 1, m = 2] may be synthesised by treatment of solvent- free [[(Me3Si)(2){Me2P(BH3)}C]K](infinity) (2) with the corresponding Lewis base (tmeda = N,N,N',N'- tetramethylethylenediamine; pmdeta = N,N,N',N'',N''- pentamethyldiethylenetriamine). X-Ray crystallography reveals that, whereas 2 crystallises with a complex 2-dimensional sheet structure, 2a crystallises as a ribbon-type one-dimensional polymer and both 2b and 2c crystallise as dimers. The corresponding complex with 12-crown-4,[K(12-crown-4)(2)][(Me3Si)(2){Me2P(BH3)}C](2d) crystallises as a separated ion pair. The complexes [[(Me3Si)(2){Me2P(BH3)} C]M(pmdeta)](n) [M = Na, n = 1 (6); M = Rb, n = 2 ( 7)] may be synthesised by treatment of [(Me3Si)(2){Me2P(BH3)}C]M with pmdeta. Whereas 6 crystallises as a discrete monomer, compound 7 crystallises as a dimer. Compounds 2, 2a-2d, 6, 7 and the corresponding caesium derivative [[(Me3Si)(2){Me2P(BH3)}C]Cs( pmdeta)](2) (5) provide an opportunity to consider the influence of the ionic radius of the metal and the nature of the co-ligands on the structures of alkali metal complexes of a phosphine-borane-stabilised carbanion.