Rates of formation of nickel(II) chelate complexes with bidentate substituted-pyridine ligands in dimethyl sulphoxide solution

Abstract

Rate data and activation parameters are reported for formation of mono-complexes of nickel(II) with the bidentate ligands 2-(aminomethyl)-, 2-(2-aminoethyl)-, 2-(aminomethyl)-6-methyl-, and 2-[(methylamino)methyl]-pyridine in anhydrous dimethyl sulphoxide (dmso) solution. At 298 K, all the rate constants are close to the value reported for solvent exchange between the [Ni(dmso)₆]²⁺ ion and bulk dmso, but because of the wide variation in the activation parameters (ΔH^‡ in the range 8·8 to 14·5 kcal mol^–1, ΔS^‡ between –14 and +7 cal K^–1 mol^–1) the agreement at elevated temperatures is not so good. The rate constants at 298 K (10^–3k_f between 2·1 and 5·4 l mol^–1 s^–1) confirm previous studies which indicate that the rates of reaction of the ion [Ni(dmso)₆]²⁺ with 2,2′-bipyridine (bipy) and 2,2′;6′,2″-terpyridine (terpy) in dmso are unusually slow (10^–3k_f 0·069 and 0·026 l mol^–1 s^–1 respectively). Steric factors are postulated to account for this rate difference. A plot of ΔH^‡ against ΔS is linear for the four ligands investigated, and, although values obtained previously for 4-phenylpyridine and for the solvent-exchange process agree with this correlation, values for bipy and terpy are not in agreement. The isokinetic temperature (at which the reactions would theoretically have the same rate) is 275 ± 10 K. ¹³C N.m.r. spectra of the ligands are also given.