Solvent-free synthesis and crystal structure of (Ph3PI)I5, the third member in the series Ph3P(I2)n (n = 1, 2 and 3).

Red crystals of iodotriphenylphosphonium pentaiodide, C18H15IP+.I5-, appear on cooling the black melt formed by heating a mixture of the commonplace reagents triphenylphosphine and molecular iodine. The compound has the highest I:P ratio hitherto established for a crystalline iodophosphonium polyiodide and constitutes the third member of the series Ph3P(I2)n (n = 1, 2 and 3). All atoms occupy general positions in the triclinic space group P-1. Comparison of the bond lengths within the above series reveals a pattern of primary and secondary bonding that is highly reminiscent of the much studied polyiodides, I(-)2n+1, where one of the I2 moieties has been replaced by a P-I group.