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meso-{Ph 2 PCH(Ph)CH(Ph)PPh 2 }, meso-{Ph 2 PCH(pyr)CH(pyr)PPh 2 }, erythro-{Ph 2 PCH(Ph)CH(pyr)PPh 2 }, rac-{Ph 2 PCH(pyr)CH(pyr)PPh 2 }, threo-{Ph 2 PCH(Ph)CH(pyr)PPh 2 }, and threo-{Ph 2 PCH(Ph)CH(pym)PPh 2 }, [pyr = 2-pyridyl, pym = 2-pyrimidyl] reacted with [Rh(COD)Cl] 2 (COD = 1,5-cyclooctadiene) to give cationic rhodium() complexes [Rh(COD){L}] ϩ (L = diphosphine ligand), which were isolated as their PF 6Ϫ salts, 1-6 respectively. In 2 and in 3 the phosphine ligand adopts a P,PЈ,N-coordination mode whereas 1, and 4-6 exhibit simple P,PЈ-coordination for the parent ligands and no evidence for N-coordination is observed. In solution 2 undergoes a fluxional process involving interchange of the coordinated and non-coordinated pyridyl environments. Variable temperature NMR studies revealed an enthalpy of activation (∆H ‡ ) of 64.3 kJ mol Ϫ1 and an entropy of activation (∆S ‡ ) of 0.005 kJ K Ϫ1 mol Ϫ1 for this process in ortho-dichlorobenzene solution. Complex 3 exhibits no similar fluxional behaviour. A single-crystal X-ray analysis of 2 revealed a nitrogen-rhodium distance of 2.369(3) Å for the coordinated pyridyl group, which is slightly longer than each of the phosphorus-rhodium distances [2.2868(7) Å and 2.3649(8) Å]. This suggests a relatively weak nitrogen-rhodium bonding interaction.

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