Kinetically hindered reaction of bis(2,2′-bipyridine) chloro and thiocyanato-N nickel(II) complexes with 2,2′-bipyridine in N,N-dimethylformamide

Reaction kinetics of bis(2,2'-bipyridine) chloro and thiocyanato-N nickel(II) complexes with 2,2'-bipyridine (bipy) have been studied in N,N-dimethylformamide (DMF) by spectrophotometry at 298 K. The rate constant k(f0) of 37.2 mol(-1) dm(3) s(-1) for [Ni(bipy-N,N)(2)](2+) + bipy --> [Ni(bipy-N,N)(2)](2+) is obtained. Interestingly, it is found that the corresponding apparent rate of formation k(f) of [Ni(bipy-N,N)(3)](2+) in solution containing chloride or thiocyanate ions is significantly smaller than k(f0). In solution containing these anions, the [NiX(bipy-N,N)(2)](+) and [NiX2(bipy-N,N)(2)](+) (X = Cl or NCS) complexes are extensively formed,and their formation constants and individual electronic spectra have been determined. The k(f) of [Ni(bipy-N,N)(3)](2+) C was thus analysed by considering the formation of these ternary complexes in equilibrium, and the intrinsic rate constants k(f1) and k(f2) for the reactions [NiX(bipy-N,N)(3)](2+) bipy --> [Ni(bipy-N,N)(3)](2+) + X- and [NiX2(bipy-N,N)(2)] + bipy --> [Ni(bipy-N,N)(3)](2+) + 2X(-), respectively, have been obtained. The k(f1) and k(f2) values thus obtained are significantly smaller than k(f0), implying that these reaction pathways are strongly hindered. The formation of [Ni(bipy-N,N)(3)](2+) may proceed through either [Ni(bipy-N,N)(2)(bipy-N)S](2+) or [NiX(bipy-N,N)(2)(bipy-N)](+) as the intermediate, where bipy-N denotes a unidentately coordinating 2,2'-bipyridine and S the solvent. The kinetic results suggest that the pathway through the intermediate [NiX(bipy-N,N)(2)(bipy-N)](+) is unusually suppressed, probably due to strong intramolecular ligand-ligand interaction between a unidentately coordinating 2,2'-bipyridine and the anion.