Formation and dissociation of tetrahedral nickel(II) and cobalt(II) complexes of 'dithioimidodiphosphato' ligands, measurement of kinetically determined stability constants, and kinetic investigations of the rapid formation of tetrahedral bis(ligand)copper(II) complexes and their rates of reduction to trinuclear copper(I) species

The stopped-flow technique was used to measure the rates of formation and dissociation of tetrahedral [ML2] complexes (M2+ = Ni2+ or Co2+) of four bidentate S-2-donor 'dithioimidodiphosphato' ligands L- (HL = [(RR2)-R-1 P(= S)]NH[P(= S)(RR4)-R-3], R-1 to R-4 = alkyl) at 25.0 degrees in MeOH/H2O 95:5 (v/v) solution and in the presence of either MOPS (= 3-(morpholin-4-yl)propanc-1-sulfonic acid) or 2,6-lutidine (= 2,6-dimethylpyridine) buffers. ne kinetically determined equilibrium formation constants for [ML](+) ions (M = Ni or Co) are 10(-5) K = 0.50 +/- 0.01 or 1.64 +/- 0.071 mol(-1) for L = L-3 (R-1 = R-2 = Me(CH2)(2)CH(Me), R-3 = R-4 = Mc(2)CH), 1.27 +/- 0.02 or 7.93 +/- 0.091 mol(-1) for L = L-7 (R-1 to R-4 = Me2CHCH2), 0.88 +/- 0.04 or 3.84 +/- 0.131 mol(-1) for L = L-8 (R-1 to R-4 = Me2CH), and in case of Ni2+ 1.88 +/- 0.04 1 mol(-1) for L = L-6 (R-1 = R-3 = Bu, R-2 = R-4 = 'Bu) (see Table 3; for L-3 and L-6 - L-8, see Table 1). Whereas the tetrahedral Ni2+ complexes dissociate more slowly than the analogous Co2+ species, in all cases, the Co2+ complexes arc more stable than those of Ni2+ due to their larger formation rate constants (Table 3). Reactions of Cu2+ with eight ligands HL (R-1 to R-4 = alkyl, alkoxy, aryl, and aryloxy) show that formation of intensely colored tetrahedral [(CuL2)-L-II] species is too fast be measured with the available stopped-flow apparatus (t(1/2) < 2 ms), but the subsequent rates of reduction of [(CuL2)-L-II] to give trinuclear products [(CU3L3)-L-1] are measurable. An X-ray analysis establishes the structure of one of the [Cu3L3] complexes, where R-1 = R-2 = Me2CHO and R-3 = R-4 = 2-(tert-butyl)phenyl (L = L-5), and a multiwavelength stopped-flow kinetic experiment establishes the spectrum of a tetrahedral [(CuL2)-L-II] species prior to the reduction reactions. The redox reactions proceed at 25.0 degrees with first-order rate constants in the range 0.285 s(-1) (R-1 to R-4 = PhO; L = L-11) to 2.58 10(-4) s(-1) (R-1 to R-4 = Me2CHCH2; L = L-7) (Table 4).