Acceptor properties of several phosphorus(v) chloro compounds have been studied by solution and solid state (^31)P n.m.r. techniques, using pyridine, 1,10-phenanthroline, 2,2'-dipyridyl and chloride ions as ligands. Six co-ordinate adduct formation has been detected in most systems. As reported previously(^1), phosphorus pentachloride forms a molecular 1:1 adduct with pyridine. Bidentate pyridines produce PC1(_4)(L-L)(^+) PC1(_6)(^-) (L-L = 2,2'-dipyridyl or 1,10-phenanthroline). Non-stoichiometric adducts PC1(_4)phen(+)(PC1(_6)(^-))(_1-x)C1(^-)(_x) (x<l) disproportionate on dissolution to the 2:1 complex. PC1(_4)(^+) SbC1(_6)(^-) reacts with pyridine in nitrobenzene to give the equilibrium PC1(_4) (pyridine)(_2)(^+) SbC1(_6)(^-) = PC1(_5).pyridine + SbC1(_5).pyridine Solid PC1(_4)(pyridine)(_2)(^+) SbC1(_6)(^-) has been successfully isolated, however. Solution-stable adducts PC1(_4)(L-L)(^+) SbC1(_6)(^-) are formed with bidentate pyridines. Phenyltetrachlorophosphorane PhPC1(_4), catechyl phosphorus trichloride (C(_6)H(_4)0(_2))PC1(_3) and bis-catechyl phosphorus monochloride (C(_6)H(_4)0(_2))(_2) PC1 yield chloride ion adducts which are partially dissociated in solution. Each has been isolated as a solid. These phosphoranes also form molecular 1:1 adducts with pyridine, of which only PhPC1(_4) pyridine dissociates in solution. In the presence of excess pyridine, (C(_6)H(_4)0(_2))PCl(_2) (pyridine)(_2)(^+)C1(^-) and (C(_6)H(_4)0(_2))(_2)P (pyridine)(_2)(^+)C1(^-) equilibrate with the 1:1 adducts. The acceptors slowly produce cationic adducts with bidentate pyridines viz. PhPC1(_3)(L_L)(^+) C1(^-), (C(_6)H(_4)0(_2))PC1(_2)(L-L)(^+) (C(_6)H(_4)0(_2))PC1(_4)(^-) and (C(_6)H(_4)0(_2))(_2)P(dipyridyl)(^+) C1(^-). Similar cationic adducts Z(_4)P(L-L)(^+) MC1(_6)(^-) are rapidly formed by addition of bidentate ligands to PhPC1(_3)(^+) SbC1(_6)(^-), PhPC1(_3)(^+) PC1(_6_(^-), (C(_6)H(_4)0(_2))PC1(_2) (^+) SbC1(_6)(^-) and (C(_6)H(_4)0(_2))(_2)P(^+) SbC1(_6)(^-). The solid hexachloroantimonate adducts possess unexpected stability to water and moist air. Pyridine adducts Z(_4)P(pyridine)(_2)(^+)SbC1(_6)(^-) are formed with (C(_6)H(_4)0(_2))PC1(_2)(^+) SbC1(_6)(^-) and (C(_6)H(_4)0(_2))(_2)P(^+) SbC1(_6)(^-) but not with PhPC1(_3)(^+) SbC1(_6)(^-). Preliminary experiments with methyltetrachlorophosphorane (MePC1(_4)) show the formation of MePC1(_5)(^-) on addition of chloride ions. The addition of substituted pyridines to PCl(_5) and PCl(_4)(^+) SbC1(_6)(^-) has been investigated. 3- and 4-substituted non- methylated pyridines yield complexes, but 2-substituted pyridines show a much lower tendency to co-ordinate. Methyl pyridines are attacked by the phosphorus species in solution. Reactions of the type R(_3)P + PC1(_5) → R(_3)PC1(-2) + PC1(_3) R(_3)PC1(_2) + PC1(_5) → R(_3)PC1(^+) PC1(_6)(^-) have also been studied. By variation of the reaction stoichiometry, either R(_3)PC1(_2) or R3PC1(^+) PC1(_6)(^-) may be prepared. With PhPC1(_2), however, only PhPC1(_3)(+)PC1(_6)(^-) has been isolated
