The mechanism of formation of cyclic triphosphenium ions [-(CH(2))(n)P(R(2))PP(R(2))-](+) 3 from diphosphanes R(2)P(CH(2))(n)PR(2) and phosphorus(III) halides PX(3)(X = Cl or Br) has been unequivocally established for the six-membered heterocycles with R = Et, (i)Pr or c-Hex, n= 3, and for five-membered rings with R = Et, n= 2. The initial stage is the formation of an acyclic species, [R(2)P(CH(2))(n)P(R(2))PX(2)](+)X(-) 1. The cation of this species cyclises to a symmetrical dication [-R(2)P(CH(2))(n)P(R(2))P(X)-](2+) 2 by loss of halide, where the middle P atom has an X group attached and is still formally P(iii). The rate-determining step is then a redox reaction to form the final cyclic monocationic product 3, with a 'bare' middle P atom. Several transient intermediate species, including the precursor cyclic dication 2 in each case, have been identified by means of (31)P NMR solution-state spectroscopy.

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Direct alkylation of cyclic triphosphenium ions by triflates to give di-ium dications is only possible for small organic substituents on the attacking reagent. The dicationic products are not intrinsically unstable, however, and in many instances they may be synthesised by an alternative route, pioneered by Schmidpeter and co-workers. These species may be readily identified in solution by (31)P NMR spectroscopy. The crystal and molecular structures of five such derivatives have been ascertained for the first time by single crystal X-ray diffraction at 120 K. The results confirm that normal single P-P bond lengths are present in the dications, in contrast with the monocationic parent cyclic triphosphenium ions, where structural determinations have shown that the P-P bond lengths are intermediate between single and double bonds.

In the presence of SnCl(2), a novel cycloaddition of two moles of R'PCl(2) (R' = Et or Ph) to a diphosphane has been achieved, forming a cyclic dication with four linked phosphorus atoms and an organic backbone; two of these species have been characterised by single-crystal X-ray diffraction, and the mechanism of the reaction has been established.

The first four-membered ring cyclic triphosphenium ions with carbon substituents (methyl or cyclohexyl) on the outer phosphorus atoms have been identified in solution by31 P NMR spectroscopy. The cyclohexyl derivative in the presence of [SnCl 6 ] 2− as counterion was stable enough for methylation by methyl triflate to be carried out.