Treatment of solutions of the halophosphines TrtP(H)F (Trt ס trityl, Ph 3 C) 1a and TrtP(H)Cl 1b with equimolar amounts of TOB (tetrachloro-orthobenzoquinone) led to the formation of mixtures of products. They contained the phosphoranes 2a and 2b, which were formed by oxidation with TOB and are in equilibrium with the phosphines 3a and 3b. Moreover, the trityl phosphonite 4, which was formed by dehydrohalogenation of 2a, 2b and 3a, 3b, was observed in both mixtures. The dehydrohalogenation was found to be reversible in the case of HF. The pure compounds 4 and 5 were obtained from the reaction of TrtPCl 2 with tetrachlorocatechol 4 and by the oxidation of 1a and 1b with two equivalents of TOB. Because of its importance in this reaction sequence, an X-ray crystal structure determination was carried out on 4. The P-O bond lengths of 168.4(2) and 167.7(2) pm are probably to be attributed to a bond-lengthening effect of the chlorine atoms of the quinone. As a comparison with analogous systems reveals, the phosphorane 5 is an example of a r 5 k 5 (P) species in which the phosphorus atom exhibits square-pyramidal co-