Reaction of [NiBrRL 2] (R = 2,4,6-Me 3C 6H 2, 2,5-Cl 2C 6H 3, 3,5-Cl 2C 6H 3; L = PPh 3, dppe (1,2-diphenylphosphinoethane), PetPh 2) with alkynes PhCCPh, MeOOCCCCOOMe, PhCCMe and PhCCCOOEt in toluene gave alkenyl compounds only when R was 3,5-Cl 2C 6H 3. The rate depends on the nature of both the ligand L and the alkyne, and is strongly inhibited by free phosphine, in keeping with an associative mechanism for the substitution of L by alkyne. Reactions with MeOOCCCCOOMe gives cis-addition products, as indicated by the structure of [NiBr(C(COOMe)C(COOMe)(3,5-Cl 2C 6H 3))(PPh 3) 2], determined by X-ray diffraction. In the case of PhCCPh a mixture of cis-and trans-addition products is obtained. a separate isomer [NiBr(C(Ph)C(Ph)(3,5-Cl 2C 6H 3))dppe] was shown to undergo isomerisation to an equilibrium mixture of both isomers. The reaction in the case of unsymmetrical alkynes PhCCR′ (R′ = COOEt, Me) gives one cis-addition product when R′ is COOEt and a mixture of cis- and trans-addition complexes when R′ is Me. The regioisomers obtained are those with the sterically largest substituent to the nickel atom. The results suggest that the mechanism is a cis-addition process, with the possibility of isomerization depending on the lability of the phosphine ligands in the alkenyl complex. No intermediate in the insertion or isomerization processes was detected.