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

Abstract

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 d s^-1 for [Ni(bipy-N,N)₂]²⁺+bipy→[Ni(bipy-N,N)₃]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⁺ 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)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.