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The reactions of R 2 P-PR 2 with R'E-ER', (where E = Se, S, O, Te) to give R 2 P-ER' have been explored experimentally and computationally. The reaction of Ph 2 P-PPh 2 with PhSe-SePh gives Ph 2 P-SePh (1) rapidly and quantitatively. The P-P/Se-Se reaction is inhibited by the addition of the radical scavenger TEMPO which is consistent with a radical mechanism for the heterometathesis reaction. Compound 1 has been fully characterised, including by X-ray crystallography. A range of other Ar 2 P-SeR (R = Ph, n Bu or CH 2 CH 2 CO 2 H) have also been prepared and characterised. The reaction of 1 with [Mo(CO) 4 (nbd)] (nbd = norbornadiene) gives two products which, from their characteristic 31 P NMR data, have been identified as cis -[Mo(CO) 4 (Ph 2 PSePh- P ) 2 ] (8) and the mixed-donor complex cis -[Mo(CO) 4 (Ph 2 P-SePh- P )(Ph 2 P-SePh- Se )] (9). It is deduced that the P and Se atoms in ligand 1 have comparable capacity to coordinate to Mo(0). The reaction of Ph 2 P-PPh 2 with PhS-SPh gives Ph 2 P-SPh (2) quantitatively but no reaction was observed between Ph 2 P-PPh 2 and PhTe-TePh. Heterometathesis between Ph 2 P-PPh 2 and t BuO-O t Bu does not occur thermally but has been observed under UV irradiation to give Ph 2 P-O t Bu along with P(V) oxidation by-products. DFT calculations have been carried out to illuminate why heterometatheses with dichalcogenides R'E-ER' occur readily when E = S and Se but not when E = O and Te. The calculations show that heterometathesis is predicted to be thermodynamically favourable for E = O, S and Se and unfavourable for E = Te. The fact that a metathesis reaction between Ph 2 P-PPh 2 with t BuO-O t Bu is not observed in the absence of UV radiation, is therefore due to kinetics.

By combining the diphosphanes Ar2P-PAr2, where Ar = C6H5, 4-C6H4Me, 4-C6H4OMe, 3,5-C6H3(CF3)2, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z2P-PZ'2 is a function of the difference between the Z and Z' substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar2P˙ radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)2P-P(o-Tol)2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl3 and CH2Cl2. The role of the chlorinated solvent is ascribed to the formation of Ar2PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)2P-P(o-Tol)2 is tentatively attributed to the o-CH3 groups quenching the (o-Tol)2P˙ radicals or inhibiting the metathesis reaction sterically.