Binary and ternary complexes of nickel(II) with 2-aminomethylbenzimidazole and salicylaldehyde: kinetic and equilibrium studies

The complexation of NiII with 2-aminomethylbenzimidazole (L) has been investigated at 20–40 °C, I= 0.30 mol dm-3. Both monoprotonated and unprotonated ligands bind the metal ion to form [NiL]2+. and the rate and activation parameters for the formation and acid-catalysed dissociation of this chelate are calculated. In the presence of Salicylaldehyde (Hsal), a mixed-ligand complex. [NiL(sal)]+, is also formed as an intermediate which further condenses to the Schiff-base complex, [NiL']+[HL'=N-(benzimidazol-2-ylmethyl)salicylideneimine]. The kinetics of the fast ternary complex formation and its slow intramolecular transformation to the Schifi base complex have been investigated at 25 °C. The presence of L in the co-ordination sphere of Ni2+ enhances the dissociation of [NiL(sal)]+ to [NiL]2+ and sal– with respect to [Ni(Sal)]+, as evidenced by the stability constants of [NiL(sal)]+ and [Ni(sal)]+. Calculations based on the values of ΔS° for the ionisation of H2L2+ and the formation of [NiL]2+combined with S [> with combining macron]aq°(H+) and S[> with combining macron]aq°(Ni2+) data yielded the values S[> with combining macron]aq°(H2L2+)–S[> with combining macron]aq°(L)= 110, S[> with combining macron]aq°(L)–S[> with combining macron]aq°(HL+)=–116 and S[> with combining macron]aq°([NiL]2+)–S[> with combining macron]aq°(L)=–294 J K−1 mol−1, which presumably reflect the varying solvent-ordering effects of L, HL+, H2L2+ and [NiL]2+

Complex formation between nickel(II) and some pentamine (substituted salicylato)cobalt(III) ions

The kinetics of reversible complexation of NiII with pentamine(substituted salicylato)cobalt(III) ions, [Co(N5){O2CC6H3(X)OH}]2+[N5= 5NH3, (en)2(NH3)(cis isomer, en = ethane-1,2-diamine) or tetren (tetraethylenepentamine), X = 3-NO2; N5= 5NH3, X = 5-NO2], was investigated by the stopped-flow technique at 15–35 °C, pH 5.70–6.90 and I= 0.30 mol dm−3(ClO4−). The formation of [(Co(N5){O2CC6H3(X)O}Ni]3+ occurs via the reaction of [Ni(OH2)6]2+ with the phenoxide form of the cobalt(III) substrates. The rate and activation parameters have been determined for the formation and dissociation of the binuclear species in which nickel(II) is chelated by the salicylate moiety. The data are consistent with and Id mechanism. The rate constant for spontaneous dissociation of the binuclear species to the reacting partners is sensitive to the nature of the pentamine moiety and decreases in the sequence tetren > (en)2(NH3)− 5NH3. The acid-catalysed dissociation of cis-[(en)2(NH3)Co{O2CC6H3(NO2−3)O}Ni]3+ conforms to a two-step process

Kinetics and mechanism of complex formation between Co(II)and (3-nitrosalicylato)(tetren)cobalt(III) ion in aqueous medium

668-672The kinetics of reversible complexation of Co(II) with (3-nitrosalicylato)(tetren)cobalt(III) ion has been investigated by the stopped flow technique at 15-35oC, pH=5.80-6.90 and I=0.30 mol dm-3 (ClO). The formation of the binuclear species, (tetren) CoO2CC6H3(3NO2)OCo3+, occurs via the reaction of CO(OH2) with the phenoxide form of the cobalt(III) substrate. At 25oC, kf = (8.29 ± 0.07) X 104 dm3 mol-1 S-1, DH# = 30.6 ± 1.9 kJ mol1-, DS# = -48 ± 6 JK-1 mol-1, and kr = (35.9 ± 0.4) S-1, DH# = 52.7 ± 1.9 kJ mol-1, DS#= -39 ± 6 JK-1 mol- I. where kf and kr denote the rate constants for the formation and dissociation of the binuclear species, (tetren)CoO2CC6H3(3NO2)OCo3+, respectively. Data are consistent with Id mechanism. The first bond formation between Co (HO2) and the coordinated salicylate of the cobalt(III) substrate in, the encounter complex is rate limiting. The binuclear species exists in dynamic equilibrium between its monodentate and chelate form

Base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine) cobalt(III) ion: a comparative study of the role of ion pairs and micelles

The kinetics of base hydrolysis of (αβ S)-(o-methoxy benzoato) (tetraethylenepentamine)cobalt(III) obeyed the rate law: kobs = kOH[OH-], in the range 0.05 □ [OH-]T, mol dm-3 □ 1.0, I = 1.0 mol dm-3, and 20.0-40.0°C. At 25°C, kOH = 13.4 ±0.4 dm3 mol-1 s-1, ΔH≠ = 93 ±2 kJ mol-1 and ΔS≠ = 90 ±5 JK-1 mol-1. Several anions of varying charge and basicity, CH3CO2-, SO32-, SO42-, CO32-, C2O42-, CH2(CO2)22-, PO43-, and citrate3- had no effect on the rate while phthalate2-, NTA3-, EDTA4-, and DTPA5- accelerated the process via formation of the reactive ion pairs. The anionic (SDS), cationic (CTAB), and neutral (Triton X-100) micelles, however, retarded the reaction, the effect being in the order SDS> CTAB > Triton X-100. The importance of electrostatic and hydrophobic effects of the micelles on the selective partitioning of the reactants between the micellar and bulk aqueous pseudo-phases which control the rate are discussed

Kinetics and mechanism of complex formation between beryllium(II) and the 3-nitrosalicylatopentaamminecobalt(III) ion

The kinetics of formation and dissociation of the binuclear complex of Be<SUP>2+</SUP> with 3-nitrosalicylatopentaamminecobalt(III) have been investigated in the 20-40 and 25-40 &#176;C ranges (I = 0.3 mol dm <SUP>-3</SUP>), respectively. At 25 &#176;C the rate and activation parameters for the formation of the binuclear species are: k<SUB>f</SUB> = 26.9 &#215; 102 dm3mol<SUP>-1</SUP>s<SUP>-1</SUP>, H = 104 &#177; 7kJ mol<SUP>-1</SUP> S = 91 &#177; 22JK<SUP>-1</SUP>mor<SUP>-1</SUP>.The rate constant, activation enthalpy and activation entropy for the acid-catalysed dissociation of the binuclear species are: 1.25 &#177; 0.08dm3mol <SUP>&#8722;1</SUP> at 25 &#176;C, 53 &#177; 3kJ mol<SUP>-1</SUP> and - 67 &#177; 9 J K<SUP>-1</SUP> mol<SUP>-1</SUP>, respectively. The formation of the binuclear species is chelation controlled while the dechelation is acid catalysed

Steric effects on complex formation between nickel(II) and (2-imidazoleazo) benzene, 2,2'-biimidazole and 2,2'-bibenzimidazole

The kinetics and mechanism of reversible complexation of Ni<SUP>II</SUP> with (2-imidazoleazo)benzene (IAB), 2,2&#39;-biimidazole (Biim) and 2,2&#39;-bibenzimidazole (Bibzm) have been investigated at 15-35 &#176;C, I = 0.30 mol dm<SUP>&#8722;3</SUP>. The stability constants, K<SUB>M</SUB>, of the [NiL]<SUP>2+</SUP> species vary in the sequence: [Ni(IAB)]<SUP>2+</SUP> &lt;[Ni(Bibzm)]<SUP>2+</SUP> &lt;[Ni(Biim)]<SUP>2+</SUP>. The values of the spontaneous dissociation rate constant (k<SUB>r</SUB>) at 25 &#176;C decrease in the sequence: [Ni(IAB)]<SUP>2+</SUP> &gt; [Ni(Biim)]<SUP>2+</SUP>> [Ni(Bibzm)]<SUP>2+</SUP>. The aquation of [Ni(IAB)]<SUP>2+</SUP> is insensitive to acid catalysis, whilst [Ni(Biim)]<SUP>2+</SUP> is relatively more susceptible towards acid-catalysed aquation than [Ni(Bibzm)]<SUP>2+</SUP>. The chelate ring in [NiL]<SUP>2+</SUP> (L = IAB, Biim or Bibzm) is sterically strained. The formation of [Ni(IAB)]<SUP>2+</SUP> and [Ni(Bibzm)]<SUP>2+</SUP> may be chelation controlled while the normal I<SUB>d</SUB> mechanism is supported by our data for [Ni(Biim)]<SUP>2+</SUP>

Base catalysed hydrolysis of β2-cis-(chloro)(benzimidazole)(trienthylenetetramine)cobalt(III)cation: A comparison of the reactivities of the benzimidazole and benzimidazolato species of the complex

747-751The base hydrolysis of β2-cis-(chloro)(benzimidazole)(triethylenetetramine)cobalt(III) obeys the rate law: -dln[complex]total/dt=(k [OH-] + k kOH[OH-]2)/(1 + KOH[OH-]) in the range of pH = 7.40-12, for which k, k and KOH denote the second order rate constants of the benzimidazole, benzimidazolato form of the complex, and the equilibrium constant for the OH- promoted formation of the benzimidazolato species from the parent complex respectively. At 25°C and 1= 0.05 mol dm-3, k = (4.79±0.15) x 106 dm3 mol-1 S-1, k = (9.2±0.6) x 104 dm3 mol-1 S-1 and KOH = (2.64 ± 0.07) x 105 dm3 mol-1. At [OH-] = (2.6-11.6) x 10-3 mol dm-3 (l = 1.0 mol dm-3) and 10-30°C, kobs,= k [OH-] is valid with k (25°C) = (4.42 ± 0.04) x 104 dm3 mol-1 S-1, H≠ = 67.6 ± 1.0 kJ mol-1 and S≠ = 71 ± 3 JK-1 mol-1. There is no general base catalysis discounting the possibility of the rate limiting NH-deprotonation mechanism. Comparison of the rate data shows that the benzimidazole form of the complex ((trien)Co(bzmH)Cl2+) base hydrolyses ~ 40 times faster than its benzimidazolato analogue despite the fact that the conjugate base of the latter (net charge zero) is likely to be more reactive than the same derived from the former (net charge + 1). This reactivity trend (i.e. k > k) reflects the perturbation of the pK of the NH proton of the coordinated trien forming the reactive conjugate base. In the conventional SN1CB mechanism the reactive conjugate base is believed to be generated by the deprotonation of the planar secondary N - H of the trien ligand trans to the coordinated benzimidazole or its conjugate base (bzm)

Acid-base equilibria of some salicylatopentaminecobalt (III) ions in mixed solvent media: An attempt to study molecular recognition

1058-1062The study of the acid-base equilibria of cis-[Co(en)2(RNH2)O2CC6H4OH]2+] (R=H, CH3, CH3CH2) in methanol-water, propan-2-ol-water, terr-butanol-water, acetone-water, and ethylene glycol-water (0 vol% organic solvent 70) are reported at 25°C. The transfer free energy of the phenol fonn relative to that of phenoxide form of the complexes, where transfer occurs from water to the- mixed solvents, have been evaluated. Solubility studies for the sparingly soluble dithionate salt of the phenol form of these complexes have been made in methanol-water media at 25°C and the transfer free. energy of the phenol and phenoxide species (relative to water) have been assessed. Results highlight theelectrostatic and nonelectrostatic medium effects on the solvation of the phenol and phenoxide species of the complexes. The transfer free energy data suggest that the complex ions recognise the cosolvent molecules in their Gurney cosphere via both coulombic and hydrophobic interactions

Effect of solvent on the reactions of coordination complexes. Part 21. Base hydrolysis of some cis-(halogeno)(amine)-bis(ethylenediamine)cobalt(III) complexes in methanol-water media: the role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of a series of (halogeno)(amine)cobalt(III) complexes, cis-[(en)<SUB>2</SUB>Co(B)X]<SUP>2+</SUP>(B = NH<SUB>3</SUB>, CH<SUB>3</SUB>NH<SUB>2</SUB>, CH<SUB>3</SUB>CH<SUB>2</SUB>NH<SUB>2</SUB>, C<SUB>6</SUB>H<SUB>5</SUB>CH<SUB>2</SUB>NH<SUB>2</SUB>, C<SUB>6</SUB>H<SUB>1</SUB>1NH<SUB>2</SUB>, imidazole, N-methylimidazole, benzimidazole, CH<SUB>3</SUB>CH(OH)CH<SUB>2</SUB>NH<SUB>2</SUB>, X = Cl, Br) have been investigated in methanol&#8211;water media (0&#8211;90 vol.% MeOH) at 15.0 t/&#176;C 40.0 (0.02 mol dm<SUP>&#8722;3</SUP> NaOH). The relative second-order rate constants at ionic strength, I= 0, [(kOH<SUB>2</SUB>)0 s/k0 w<SUB>2</SUB>], generally increased non-linearly with increasing mole fraction (X<SUB>MeOH</SUB>) of MeOH; the effect was sensitive to the bulkiness and hydrophobicity of the non-labile amine ligands. The solvent effects on rate have been interpreted in terms of the role of solvent structure and preferential solvation of the initial state and transition state, presumably due to the hydrophobic interaction. This is evident from the linear plot of log(kOH<SUB>2</SUB>)0 sversusV&#176;1/3 at 25 &#176;C where V&#176; is the standard partial molar volume of the complex ion in aqueous medium at 25 &#176;C. The activation enthalpies and entropies also varied non-linearly with X<SUB>MeOH</SUB> and the observed extrema in the plots of &#916;H<SUP>&#x0023;</SUP>(&#916;S<SUP>&#x0023;</SUP>)vs. X<SUB>MeOH</SUB> presumably reflected the solvent structural effect on the solvation component of these thermodynamic parameters

Effects of solvent on the reactions of coordination complexes. Part 24. Kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylene-pentamine)cobalt(III) complexes in methanol+water media: The role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes, [(tetren)CoO2CR]2+ (R=-NH2CH2, pyridine-2&#8212;, &#8212;NH2CH2CH2, &#8212; NH2CH(CH3) (&#945; &#946;S isomer); R=&#8212; NH2CH(CH3) (&#945; &#946;R isomer)), have been investigated in methanol-water media (0-80 vol % MeOH) at 15.0&#8804;t&#176;C&#8804;40.0 (0.02 mol dm-3 NaOH). The second-order rate constant at zero ionic strength, k2&#176;, increases nonlinearly with XMeOH. The transfer free energy of the initial state and the transition state of the amido conjugate base ([&#916;tG (i)](s&#8592;w)) for the glycinato- and pyridine-2-carboxylato complexes have been calculated using the solubility data of their picrate salts, pKNH date of their N-protonated forms, and the k2&#176; values in mixed solvent media. The kinetic solvent effects have been interpreted in terms of preferential solvation of the initial state, transition state, and the solvent structure. The activation enthalpies and entropies varied nonlinearly with XMeOH displaying extrema, which is attributable to the solvent structural effects on these thermodynamic parameters. It is also evident that the mutation process, &#945; &#946;R&#8594;&#945; &#946;S isomer for the &#945;-alaninato complex, where this isomerisation refers to the arrangement of the tetren skeleton around the planar secondary NH is sensitive to the nature of the cosolvent molecules and solvent structure. The mutation process is generally more favorable for the five coordinate amido conjugate bases than the initial state