Triphenylphosphonium and triethylphosphonium halides

Reaction of triethylphosphine with HI, HBr or HCl in ethereal solution yields the solid triethylphosphonium salts Et3PHI, Et3PHBr and Et3PHCl, respectively. Infrared spectroscopy indicates the presence of hydrogen bonds Et3PHX, the strengths of these bonds increasing in the sequence X = I < Br < Cl. By reaction of triethylphosphine with HF liquid products are obtained, consisting of (mainly) Et3PHF or Et3P · 2 HF (depending on the ratio of the reactants). While Et3PHF shows very strong PHF bonding and thus resembles a (polar) molecular compound, Et3P · 2 HF is salt-like, consisting of the ions [Et3PH]⊕ and [HF2]⊖. Solutions of water in triethylphosphine contain a considerable proportion of Et3PHOH, which has a PHOH bond of moderate strength. Triphenylphosphine in ether reacts with HI or HBr to form the solid triphenylphosphonium salts Ph3PHI and Ph3PHBr; with HCl the solid salt Ph3P · 2 HCl is formed. This salt consists of the ions [Ph3PH]⊕ and [HCl2]⊖; it is the first reported [HCl2]⊖ salt of a phosphonium base. Ph3P · 2 HCl slowly decomposes into triphenylphosphine and hydrogen chloride; Ph3PHCl may be an unstable intermediate in this reaction, but no evidence was found for the existence of Ph3PHCl as a stable compound. The alleged compound Ph3P · 1.33 HCl1 is a mixture of Ph3P · 2 HCl and Ph3P. Triphenylphosphine reacts with HF to form liquid products consisting of Ph3PHF and Ph3P · 2 HF, the latter compound consisting of [Ph3PH]⊕ and [HF2]⊖ ions. Very strong PHX bonds are found in Ph3PHF; in Ph3PHBr and Ph3PHI the PHX bonds are considerably less strong, while such bonds are absent in Ph3P · 2 HCl and Ph3P · 2HF. The differences between triphenylphosphine and triethylphosphine in their reactions with hydrogen halides (and water) can be explained by their difference in basicity.