Phosphonium salts bond lengths

The trithiodeltate dianion (185) in its phosphonium salt, (Ph3MeP)2 (c-C3S3) 3H20, is very close to symmetry and is aromatic (see 185a) The mean C—C bond length of 1.405 A is longer than in cyclopropenylium ions the C—S bond length, 1.676 A, indicates partial double-bond character" ... 

Reaction of the bromotrifluoromethane/tris(diethylamino)phosphane system with benzalde-hyde leads to the formation of the phosphonium salt 19, which decomposes at 140 C, affording the bromophenylethanc 20 and not the alcohol. Due to the ease of formation of a phosphorus compound with a P = 0 bond, in this case the intermediate carbenium ion reacts with the bromide anion, giving rise to the final reaction product. The reaction is general in character pcrfluoroalkyl bromides with different carbon-chain lengths (from 2 to 6) have been used as reactants. 

It is worth noting that a higher anionic reactivity is produced by increasing steric hindrance around the cation. This can be achieved by increasing the length of alkyl chains or by shortening the heteroatom-carbon bond using ammonium instead of phosphonium salts. 

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