Aquation Rate Research Articles

A quantum chemical consideration of ligand exchange in palladium(ii) aqueous and chloride complexes

The behavior of potassium tetrachloropalladate(II) in media simulating biological fluids has been studied. In aqueous solutions of NaCl, the aquation rate is higher than the rate of chloro ligand introduction into the internal coordination sphere of palladium. In HCl solutions, on the contrary, the process of palladium chloro complex formation predominates. The latter is apparently due to protonation of water molecules composing aqua complexes. By means of the ZINDO/1 method, the substitution of ligands – water molecules and hydronium ion – in planar complexes of palladium(II) by chloride ion has been investigated. All complexes containing H2O and H3O+ ligands, other than [Pd(H2O)4]2+, have intramolecular hydrogen bonds. In [Pd(H2O)3(H3O)]3+ and trans-[Pd(H2O)2(H3O)Cl]2+, a “non-classic” symmetric hydrogen bond O ··· H ··· O is established (ZINDO/1, RHF/STO-6G*). By the first three steps the substitution of hydronium ion in the internal sphere of palladium atom is more favorable thermodynamically, compared to water molecules. Logarithms of stepwise stability constants of palladium(II) chloride complexes correlate linearly to enthalpies (ZINDO/1, PM3) of water substitution by chloride ion.

Structural and reactivity changes in ruthenium ammines induced by the NO ligand

The crystal structure of [Ru(NH 3) 5(OH 2)] 2(S 2O 6) 3 · 2H 2O has been solved by X-ray diffraction techniques. The relevant average interatomic distances are Ru–(NH 3) eq=2.09 Å, Ru–(NH 3) ax=2.08 Å and Ru–(OH 2) ax=2.11 Å. The aquation rate ( k H 2O ) and the activation parameters for the reaction trans -[ RuNO( NH 3) 4 Cl] 2++ H 2 O → k H 2 O trans -[ RuNO( NH 3) 4( H 2 O)] 3++ Cl − have been calculated as 8.4 × 10 −6 s −1 (0.10 mol L −1 CF 3COOH, μ=0.10 mol L −1, 40 °C), Δ H ≠ −1=2.3 kcal mol −1, Δ S ≠ −1=24.0 cal deg −1 mol and Δ G ≠ −1=8.6 kcal mol −1. A comparison between the structural data, kinetic and thermodynamic reactivities of [Ru(NH 3) 5(OH 2)] 3+ and trans-[RuNO(NH 3) 4(OH 2)] 3+ ions strongly suggests that the substitution of the trans NH 3 ligand in the coordination sphere of [Ru(NH 3) 5(OH 2)] 2+ by NO confers a character of Ru(III) to the formally Ru(II) center.

Kinetics of aquation and anation of ruthenium(II) arene anticancer complexes, acidity and X-ray structures of aqua adducts.

The aqua adducts of the anticancer complexes [(eta(6)-X)Ru(en)Cl][PF(6)] (X=biphenyl (Bip) 1, X=5,8,9,10-tetrahydroanthracene (THA) 2, X=9,10-dihydroanthracene (DHA) 3; en=ethylenediamime) were separated by HPLC and characterised by mass spectrometry as the products of hydrolysis in water. The X-ray structures of the aqua complexes [(eta(6)-X)Ru(en)Y][PF(6)](n), X=Bip, Y=0.5 H(2)O/0.5 OH, n=1.5 (4), X=THA, Y=0.5 H(2)O/0.5 OH, n=1.5 (5 A), X=THA, Y=H(2)O, n=2 (5 B), and X=DHA, Y=H(2)O, n=2 (6), are reported. In complex 4 there is a large propeller twist of 45 degrees of the pendant phenyl ring with respect to the coordinated phenyl ring. Although the THA ligand in 5 A and 5 B is relatively flat, the DHA ring system in 6 is markedly bent (hinge bend ca. 35 degrees ) as in the chloro complex 3 (41 degrees ). The rates of aquation of 1-3 determined by UV/Vis spectroscopy at various ionic strengths and temperatures (1.23-2.59x10(-3) s(-1) at 298 K, I=0.1 M) are >20x faster than that of cisplatin. The reverse, anation reactions were very rapid on addition of 100 mM NaCl (a similar concentration to that in blood plasma). The aquation and anation reactions were about two times faster for the DHA and THA complexes compared to the biphenyl complex. The hydrolysis reactions appear to occur by an associative pathway. The pK(a) values of the aqua adducts were determined by (1)H NMR spectroscopy as 7.71 for 4, 8.01 for 5 and 7.89 for 6. At physiologically-relevant concentrations (0.5-5 microM) and temperature (310 K), the complexes will exist in blood plasma as >89 % chloro complex, whereas in the cell nucleus significant amounts (45-65 %) of the more reactive aqua adducts would be formed together with smaller amounts of the hydroxo complexes (9-25 %, pH 7.4, [Cl(-)]=4 mM).

Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen) 2(NCMe) 2] 3+ in water, Me 2SO and MeCN: kinetics and stereochemistry

The synthesis and characterisation of cis- and trans-[Co(tmen) 2(NCCH 3) 2](ClO 4) 3 are described. Solvolysis rates have been measured by both 1H NMR spectroscopy and UV–Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k 1=5.61 × 10 −4 and k 2=5.35 × 10 −4 s −1, each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k=1.54 × 10 −5 s −1. The solvent exchange rates have been measured by 1H NMR spectroscopy in CD 3CN at 298.2 K: k ex( cis)= k ct + k cc=2.0 × 10 −5 and k ex( trans)= k tc + k tt=4.56 × 10 −6 s −1. From these data, the measured cis– trans isomerisation rate (1.71 × 10 −6 s −1) and equilibrium position in CH 3CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant – 69% trans product; cis reactant – 99% cis product. Aquation rates for cis- and trans-[Co(tmen) 2(NCCH 3) 2](ClO 4) 3 have also been determined spectrophotometrically and by NMR; k cis =1.3 × 10 −4 and k trans =2.7 × 10 −5 s −1. In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen) 2X 2] n+ complexes are found to be consistently much more reactive than their [Co(en) 2X 2] n+ analogues.

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles. Chemistry and excited-state behavior of 1,4-C2-cyclam complexes.

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported. The ligand appears to form only trans complexes, and the structure of trans-[Cr(1,4-C(2)-cyclam)Cl(2)]PF(6) is presented. The constraint imposed by the additional C(2) linkage distorts the bond angles significantly away from the ideal values of 90 and 180 degrees. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) (k(obs) for trans-[Cr(1,4-C(2)-cyclam)(H(2)O)(2)](3+) formation = 6.5 x 10(-)(2) s(-)(1), 0.01M HNO(3), 25 degrees C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl(2)](+). The complexes trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) and trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) are found to have extinction coefficients four to five times higher than their cyclam analogues, owed to the lack of centrosymmetry caused by the steric constraint. The trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) complex is a very weak emitter in aqueous solution with a broad room-temperature emission centered at 735 nm (tau = 0.24 micros). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C(2)-cyclam)(CN)(2)](+) in aqueous solution results in CN(-) ligand loss. This is in stark contrast to its unconstrained cyclam analogue, which is photoinert and has a room-temperature emission lifetime of 335 micros.

The influence of solvent structure on the solvolysis of a ruthenium(II) complex in water–acetonitrile mixtures

The solvent effect on the rate of aquation of trans-[Ru(py)4Cl2] has been investigated by u.v. spectroscopy in the 40–55 °C range in MeCN–H2O media. The thermodynamic activation parameters are calculated and discussed in terms of solvation effects. Log k was correlated with the reciprocal of the dielectric constant. A non-linear correlation was obtained and is discussed on the basis of specific solvation.

Kinetic control of the aquation of the Hexathiocyanatochromate(Iii) Ion by Associative interaction of Poly(Acrylamide) in Aqueous Medium

Cooperative interaction of poly(acrylamide) is found to decelerate significantly the aquation rate of [Cr(NCS)6]3- in acidic, neutral, and basic media, which is an indication of kinetic control of macromolecules on reactions.

The nature of the DNA template (single- versus double-stranded) affects the rate of aquation of a dinuclear Pt anticancer drugElectronic supplementary information (ESI) available: experimental conditions for the NMR reactions, the models used for the kinetic fits and [1H,15N] HSQC NMR spectra of the final products from reactions of 1 with the single strand (I) (before and after addition of

The rate of aquation of a dinuclear platinum anticancer agent is altered in the presence of template DNA with enhancement of hydrolysis in the presence of single-stranded over double-stranded DNA, emphasising how the alteration of chemical properties of small molecules in the presence of large host interactions is also dependent on the conformation and nature of that host.

Geometrical configuration of monomethyl–platinum(II) complexes driven by the size of entering nitrogen ligands

The reaction of the monoalkyl complex trans-[Pt(DMSO) 2Cl(CH 3)] with a large variety of heterocyclic nitrogen bases L, in chloroform solution, leads to the formation of uncharged complexes of the type [Pt(DMSO)(L)Cl(CH 3)], containing four different groups coordinated to the metal center. Only two out of the three different possible isomers were detected in solution. These two trans(C,N) and cis(C,N) species can be unambiguously identified through 1H NMR spectroscopy. For the trans(C,N) isomers, average values of 2 J PtH=75±4 Hz and 3 J PtH=36±4 Hz have been observed for the coordinated methyl and DMSO ligands, respectively. In the case of the cis(C,N) isomers, these values increase to 2 J PtH=83±2 Hz, and decrease to 3 J PtH=26±3 Hz due to the mutual exchange of ligands in trans position to CH 3 and DMSO. In the case of bulky asymmetric ligands, such as quinoline, 2-quinolinecarboxaldehyde, 2-methylquinoline, 5-aminoquinoline, 2-phenylpyridine and 2-chloropyridine, slow rotation of the hindered group around the PtN bond makes the coordinated DMSO ligand prochiral. NMR experiments have shown that the first reaction product is the trans(C,N) isomer as a consequence of the very fast removal of one DMSO ligand by the nitrogen bases from the starting complex trans-[Pt(DMSO) 2Cl(CH 3)]. This trans kinetic product undergoes a geometrical conversion into the more stable cis(C,N) isomer through the intermediacy of fast exchanging aqua-species. The rate of isomerization and the relative stability of the two isomers depends essentially on the rate of aquation and on the steric congestion imposed by the new L ligand on the metal.

Solvation of iron and chromium complexes in alcohol–water mixtures

Solubilities are reported for the perchlorates of five iron(II)-diimine complexes in t-BuOH–H2O and one in MeOH–H2O mixtures, for three iron(III)-3-hydroxy-4-pyranonate and three iron(III)-3-hydroxy-4-pyridinonate complexes in MeOH–H2O and t-BuOH–H2O, and for two chromium(III)-3-hydroxy-4-pyranonate complexes in MeOH–H2O. Transfer chemical potentials are thence derived for the various iron(II), iron(III) and chromium(III) complexes, for transfer from H2O into the respective mixed solvents (at 298.2 K). These results are combined with values reported earlier for related complexes, and for other alcohol–H2O mixtures, to give an overall picture of solvation, expressed in the thermodynamic format of transfer chemical potentials, for iron(II)-diimine, iron(III)-3-hydroxy-4-pyridinonate and chromium(III)-3-hydroxy-4-pyranonate complexes in H2O-rich aqueous-alcohol mixtures. Some spectroscopic (1H-n.m.r.; i.r.) and kinetic (aquation rate constants at 298.2 K) data are reported for the chromium(III) complexes.

DNA polymerase versus DNA binding to the anticancer drug, cis-platin

The activity of the Klenow fragment of E. coli DNA polymerase-I was inhibited in the presence of cis-diamminedichloroplatinum(II) at neutral pH in 5 mM chloride. Pre-incubations of cis-DDP with the polymerase and DNA revealed that the inhibition is primarily due to irreversible binding of the platinum complex to the enzyme. To understand the chemistry behind the inhibition, reactions of a model peptide, ERFKCPCPT and nucleotide, 5′-GMP with cis-DDP in mixtures were examined. The peptide, was selected from the DNA binding domain of human DNA polymerase-α while the mono-nucleotide serves as a model for the DNA binding. Reactions of cis-DDP with a mixture of the peptide and 5′-GMP at various concentrations ranging from equimolar to excess of nucleotide over the peptide revealed that the nucleotide can not effectively compete with the peptide. An appreciable nucleotide coordination was observed only when the nucleotide concentrations were exceeded by fourfold. At pH 6.5, the peptide complexation proceeds through the formation of an intermediate through a second order process (k=0.2 M−1 s−1) due to direct reaction between the starting dichloro-complex as well as through the aquated complex (k=>10 M−1 s−1). Platinum-195 NMR revealed that the product contains a coordination environment composed of two nitrogen and two sulfur donors consistent with the formulation that both cysteines are coordinated to and ammine ligands are retained by the metal center. Platinum(II) also readily replaced Zn(II) from the Zn-peptide complex, the latter metal ion is known to coordinate with four cysteine residues in the human DNA polymerase-α. Furthermore, the kinetics of reactions of cis-DDP and its hydrolyzed products with GpG and ApG were investigated and compared with that of peptide binding. These two dinucleotides represent the abundant binding sites in DNA reaction selected as model nucleotides. The reactions of the dichloro-platinum(II) complex with nucleotides, on the other hand, were largely controlled by the rate of aquations. The rate of first aquation process, k=1.3±0.1×10−4 s−1 evaluated from the kinetic profiles was invariant regardless of the ligands used. However, the second aquation rate constants lie in the narrow range 3–6×10−5 s−1, with GpG being favored over ApG.

Solvent Effects on the Micelle - Influenced Aquation Reactions of Some Iron (II) Phenanthroline Complexes in Various Solvent Systems

The relative rates of the micelle-catalyzed/inhibited aquation reactions of the complexes: Fe(Ph2Phen), Fe(Me2Phen) and Fe(MePhen were investigated in ethylene glycol, water and aqueous acetone. The pseudo first oder rate constant, K vs (Triton X-100) profiles reveal that at all the (TX-100) concentration ranges considered for ethylene glycol and water, and for aqueous aceton at (Triton X-100) >/minus 15.00(%v/v) the pseudo first order rate constant, k values follow the order: Fe(Me4 Phen)> Fe(ph2Phen) , suggesting a substrate binding strength of the order: Fe(Me2Phen) rates of the complexes in ethylene glycol and water fall within the same range of 0.0 - 100.0 x 10 S while the aquation rates in aqueous acetone are generally about 10 cubic faster. These observations have been attributed to evolution of different micellar types and aquation mechanisms.UNISWA Research Journal of Agriculture, Science and Technology Vol. 4 (1) 2000: pp 113-118

Aquation and Base Hydrolysis of trans-Tetraammine(methylamine)sulfatocobalt(III) Complex Ion

The kinetics of aquation and base hydrolysis reactions of fmns-[Co(NH 3 ) 4 (NH 2 CH 3 )(OSO 3 )] + have been studied. In acid solution the aquation rate, R aq , follows the equation R aq /[complex] = k s + k c [H + ], at constant ionic strenght μ = 1.0Μ. The activation parameters are ΔΗ* = 88.8 kj mol -1 , ΔS* = - 50.5 JK -1 mol -1 , ΔΗ* = 96.7 kj mol -1 and ΔS* =-23 JK -1 mol -1 . The rate constants at 25°C are k s = 4.15 χ 10 -6 s -1 and k c = 4.52M -1 s -1 . The rate of base hydrolysis, R OH , follows the equation R OH/ complex] = k 0H [OH - ]. The activation parameters are ΔΗ* OH =74.9kJmol -1 and ΔS* OH = 2JK -1 mol -1 and the rate constant is k OH = 0.58M 1 s -1 at 25°C and μ, = 0.15 Μ. The stereochemistry of the hidroxo product has been determined (cis-[Co(NH 3 ) 4 (NH 2 CH 3 )- (OH)] 2+ =9%). The results are discussed in the light of the reaction mechanisms proposed so far.

The synthesis and structure of some, rac–trans-, meso–trans- and (S,S)-trans- Co(III) complexes with Me6-[14]-diene(5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)

The reaction of cis-[Co(CO3)(Me6-[14]-diene)]ClO4 with S-(+)-mandelic acid gives first (S,S)-trans-[Co(S-mandelato)2(Me6-[14]-diene)]ClO4·2H2O (1) and then (R,S)-trans-[Co(S-mandelato)2(Me6-[14]-diene)]ClO4·H2O (2). With HCl, the cis-carbonato gives first rac–trans-[CoCl2(Me6-[14]-diene)]ClO4 (3) and then increasing amounts of meso–trans-[CoCl2(Me6-[14]-diene)]ClO4 (4). The rac–trans-perchlorate (3) can be isomerised partially to the meso-form (4) in refluxing methanol. Treatment of 1 with HCl/HClO4 gives (+)-(S,S)-trans-[CoCl2(Me6-[14]-diene)]ClO4 which racemises in methanol to give a rac-/meso-mixture. Attempts to resolve rac–trans-[CoCl2(Me6-[14]-diene)]+ directly with the antimonyl(III) tartrate anion were not successful, but work-up of the solution with HCl gave X-ray quality crystals of rac–trans-[CoCl2(Me6-[14]-diene)]2Sb2Cl8 (5) containing dimers of the seesaw shaped SbCl4− anion. trans-[CoBr2(Me6-[14]-diene)]ClO4 reacts with dilute aqueous ammonia to give trans-[Co(OH)(Me6-[14]-diene)(NH3)](ClO4)2. Acidification of this allows the isolation of synrac–trans-[Co(Me6-[14]-diene)(NH3)(OH2)](ClO4)3, which slowly isomerises in acidic solution [kisom=2.92×10−5 s−1 at 25°C] to the meso-form. Two isomers of [CoCl(Me6-[14]-dien)(NH3)]2+ have also been isolated, the synrac–trans-perchlorate and the meso–trans-tetrachlorozincate (6) and their aquation rates (chloride release) have been measured. The crystal structures of compounds 1–6 have been determined.

Quantitative comparison of the effect of methylD-glycopyranosides as cosolutes on the rates of base hydrolysis and aquation of some iron(II)-diimine Complexes

The kinetics of base hydrolysis and of aquation of some iron(II)–diimine complexes in the presence of stereoisomeric carbohydrates were monitored spectrophotometrically at 25.0°C. In basic aqueous solution dissociation of both Fe(1,10-phenanthroline)32+ and Fe(2,2\'-bipyridine)32+ is accelerated when methyl D-glycopyranosides are present. The aquation reaction of Fe(1,10-phenanthroline)32+ in an aqueous EDTA medium is also accelerated in the presence of carbohydrates, but that of Fe(5-nitro-1,10-phenanthroline)32+ is retarded. An equation obtained from a modification of the Savage–Wood additivity of groups principle applied to kinetics is used to quantify the kinetic medium effects observed. The magnitudes of these effects can be explained in terms of the hydration characteristics of the carbohydrates, which depend on their stereochemistry, and the change in the hydration environment of the iron(II)–diimine complexes during activation. Results and their interpretation for the aquation in acidic medium of the Fe(5-bromo-1,10-phenanthroline)32+ and Fe(4,7-dimethyl-1,10-phenanthroline)32+ complexes in highly aqueous methanol and ethanol solutions are also presented. The kinetic medium effects of the carbohydrates are consistent with those of simple alcohols.

1H,15N] N.M.R. Kinetic Studies of Reactions of cis- and trans-[PtCl2(15NH3)(c-C6H1115NH2)] with Guanosine 5'-Monophosphate

cis-[PtCl2(15NH3)(c-C6H11NH2)] is an active metabolite of the oral platinum(IV) anticancer drug cis,trans,cis-[PtCl2(CH3CO2)2(NH2)(c-C6H11NH2)]. Since it is likely that guanine bases on DNA are targets for this drug, we have analysed the kinetics of reaction of this platinum(II) metabolite with guanosine 5′-monophosphate (5′-GMP) at 310 K, pH 7, using [1H, 15N] n.m.r. methods. Reactions of the trans isomer are reported for comparison. The reactions proceed via aquated intermediates, and, for the cis isomer, the rates of aquation and substitution of H2O by 5′-GMP are 2-5 times faster trans to the amine ligand (c-C6H11NH2) compared to trans to NH3 for both the first and second steps. For the trans complex, the first aquation step is c. 3 times faster than for the cis complex, as expected from the higher trans influence of Cl¯, whereas the rate of the second aquation step (trans to N7 of 5′-GMP) is comparable to that trans to NH3. These findings have implications for the courses of reactions with DNA.

SOLVENT EFFECTS ON THE AQUATION OF CATIONIC, ANIONIC AND NONIONIC CHLOROCOBALT(III) COMPLEXES

The kinetics of aquation of α- and β- cis[Co(edda)Cl2]−, α- cis[Co(edda)(H2O)Cl] and α- cis[Co(trien)Cl2]+ were investigated in aqueous mixtures of t-BuOH, MeOH, DMSO, ethylene carbonate and urea. The transfer Gibbs energy values for the reactants, evaluated from the solubility of the complexes, confirm the decisive role of the reactant solvation on the aquation rate.

Kinetics and mechanism of H+(aq)-assisted aquation of [Cr(N3)(S-pdtra)]− and [Cr(N3(edtrp)]−ions

Two new complex anions, [Cr(N3)(S-pdtra)]− and [Cr(N3)(edtrp)]−, were obtained in solution by N3−/HN3 anation of the aqua analogues (S-pdtra = S-propane-1,2-diamine-N,N,N′-triacetate, edtrp = ethylenediamine-N,N,N′-tripropionate). Aquation of these species in acidic media leads to the same geometrical isomers as those used for the synthesis. The aquation rate is strongly dependent upon [H] and is substantially higher in D2O than in H2O. Protonation of the coordinated azide was not observed spectrophotometrically. The rate law and activation parameters have been determined and discussed.

Solvent/substrate effects on the micelle-catalyzed aquation of some iron(II)-phenanthroline complexes, II. Influence of urea and sodium benzoate on the aquation of Fe(Ph2Phen) 3 2+ in acetone

The micelle-catalyzed aquation rates of iron(II) phenanthroline complex Fe(Ph2Phen)32+ have been measured in aqueous acetone as solvent in the presence of urea and sodium benzoate as substrates. The variation patterns manifested by the estimated maximum catalytic factors and kovs [substrate] profiles show that the aquation rates of the complex are inhibited by urea, while they are significantly enhanced by sodium benzoate. These observations have been radionalized by considering the relative micelle bond-breaking capacities of the substrates, the relative strengths of the H-bonds formed by the substrate/OH− headgroups of the micelle and the H2O/OH− headgroups of the micelle pairs.

KINETICS OF DISSOCIATION OF NICKEL COMPLEXES WITH PENTADENTATE LIGANDS

Abstract The synthesis of the pentadentate ligands 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane (BPTAN) and 1,4,7-triazacyclononane-N,N′-diacetate (TCDA) are reported. The nickel(II) complexes of BPTAN and TCDA were synthesized and characterized by their elemental analyses, electrochemistry, and uv-vis spectra. The rate of nickel ion aquation from the BPTAN complex in aqueous solution in the presence of chloride ion was found to follow the rate law, Rate = k1[H+]2[Cl][NiL]/{1 + k2[Cl]}. At 60°C and I = 2.0 M, values for k1 and k2 were determined to be (1.7 ± 0.2) × 10−6 M−3 s−1 and 0.15 ± 0.04 M−1 respectively. For NiTCDA(H2O), the rate law was found to be independent of the chloride ion concentration. The rate law was found to follow the form, Rate = k1[H+][NiL]/(1 + k2H+]). At 60°C and I = 2.0 M, k1 and k2 were found to be equal to 3 M−1 s−1 and 0.90±0.07 M−1, respectively. This work has shown NiBPTAN(H2O)2+ to be one of the most inert nickel complexes to ligand dissociation that has been reported.