The base-catalysed addition of diphenylphosphine to the substituted diphenylethynes RC᎐ ᎐ ᎐ CRЈ (R = Ph, RЈ = Ph, o-tolyl, m-tolyl or 2-biphenyl; R = m-tolyl, RЈ = o-tolyl or m-tolyl) yielded Ph 2 PC(R)᎐ ᎐ CHRЈ and/or Ph 2 PCH(R)CH(RЈ)PPh 2 . Proton, 13 C, 13 P and two-dimensional rotating frame Overhauser enhancement 1 H NMR spectra have been used to determine the stereochemical pathways of the reactions and the stereochemistry of the products. In general the more hindered alkynes undergo monoaddition ultimately to yield phosphinoalkenes with the Ph 2 P attached to the carbon bearing the least bulky substituent and cis to the olefinic proton, while for the less hindered alkynes the trans isomer is formed initially and this then reacts further to give meso/erythro-diphosphinoalkanes. Bis(o-tolyl)ethyne does not react with Ph 2 PH under the same conditions. Crystal structures were determined for E-and Z-Ph 2 P(Ph)C᎐ ᎐ CHPh and show distortions of interbond angles consistent with the pattern of strain implied by the foregoing reactions. The sulfides of the phosphinoalkenes and the Mo(CO) 4 complexes of the diphosphinoalkanes were also prepared and their 1 H, 13 C and 31 P NMR spectra recorded. In several cases the pattern of 13 CO NMR signals for the complexes was used unambiguously to determine the stereochemistry of the parent diphosphines.

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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.