The relaxation spectra of nickel(II) and cobalt(II) arginine complexes

The temperature-jump method has been used to determine the nickel(II)- and cobalt(II)-arginine complexation kinetics. In the pH range studied, the neutral form of the ligand, HL, is the attacking, as well as the complexed, ligand species. The reactions reported on are of the type where n = 1, 2, 3 and M is Ni or Co. At 25° and ionic strength 0.1M the association rate constants are: for nickel(II) k₁ = 2.3 × 10³(±20%), k₂ = 2.4 × 10⁴(±20%), k₃ = 3.5 × 10⁴(±40%) M^?1 sec^?1; for cobalt(II) k₁ = 1.5 × 10⁵(±20%), k₂ = 8.7 × 10⁵(±20%), k₃ = 2.0 × 10⁵(±40%) M^?1 sec^?1. Arginine binds to metal ions less well than homologous chelating agents due to the electrostatic repulsion arising from the positively charged terminus of the zwitterion. Kinetically, the effect appears in the association rate constants with nickel reactions more strongly influenced than cobalt.     

Kinetic studies on the metal(II) tartarate–peroxomonosulfate reaction

The kinetics of oxidation of tartaric acid (TAR) by peroxomonosulfate (PMS) in the presence of Cu(II) and Ni(II) ions was studied in the pH range 4.05–5.20 and also in alkaline medium (pH ～12.7). The rate was calculated by measuring the [PMS] at various time intervals. The metal ions concentration range used in the kinetic studies was 2.50 × 10^?5 to 1.00 × 10^?4 M [Cu(II)], 2.50 × 10^?4 to 2.00 × 10^?3M [Ni(II)], 0.05 to 0.10 M [TAR], and µ = 0.15 M. The metal(II) tartarates, not TAR/tartarate, are oxidized by PMS. The oxidation of copper(II) tartarate at the acidic pH shows an appreciable induction period, usually 30–60 min, as in classical autocatalysis reaction. The induction period in nickel(II) tartarate is small. Analysis of the [PMS]–time profile shows that the reactions proceed through autocatalysis. In alkaline medium, the Cu(II) tartarate–PMS reaction involves autocatalysis whereas Ni(II) tartarate obeys simple first‐order kinetics with respect to [PMS]. The calculated rate constants for the initial oxidation (k₁) and catalyzed oxidation (k₂) at [TAR] = 0.05 M, pH 4.05, and 31°C are Cu(II) (1.00 × 10^?4 M): k₁ = 4.12 × 10^?6 s^?1, k₂ = 7.76 × 10^?1 M^?1s^?1 and Ni(II) (1.00 × 10^?3 M): k₁ = 5.80 × 10^?5 s^?1, k₂ = 8.11 × 10^?2 M^?1 s^?1. The results suggest that the initial reaction is the oxidative decarboxylation of the tartarate to an aldehyde. The aldehyde intermediate may react with the alpha hydroxyl group of the tartarate to give a hemi acetal, which may be responsible for the autocatalysis. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 620–630, 2011     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Kinetics and mechanisms of substitution of the axial and equatorial ligands in an iron(II) macrocyclic heme model complex

The complex [Fe(imox) (Nmim)₂], where imox is a planar bis(iminooxime) macrocyclic ligand and Nmim=N-methyl imidazole, undergoes substitution reactions in the presence of 2,2′bipyridine (bipy), leading to a series of intermediate mixed complexes. The forward and reverse rate constants for the substitution of the Nmim ligand by water are k₁ = 0.23 s^?1 and k_?1=62 M^?1s^?1, respectively. The binding of a bipy to the [Fe(imox) (Nmim) (H₂O)]⁺ complex proceeds according to k₂ = 4.7×10^?2 M^?1 s^?1 and k_?2 = 3.6 × 10^?4 s^?1, yielding [Fe(imox) (bipy) (Nmim)]⁺, which eliminates Nmim according to k₃ = 1.6 × 10^?4 s^?1. Further substitution in the [Fe(imox) (bipy)]⁺ complex with bipy takes place very slowly leading to the [Fe(imox) (bipy)₂]⁺ and [Fe(bipy)₃]²⁺ complexes. © 1993 John Wiley & Sons, Inc.     

Studies in nickel(IV) chemistry. Electron transfer kinetics of the reaction between tris(dimethylglyoximato)nickelate(IV) and hexacyanoferrate(II) in aqueous medium

The kinetics of electron transfer from hexacyanoferrate(II) to tris(dimethylglyoximato)-nickelate(IV), Ni(dmg)₃^2?, to produce Fe(CN)₆^3? and Ni(dmgH)₂, follows a pseudo-first-order disappearance in the Ni(IV). The pseudo-first-order rate constants k_obs are linearly dependent on [Fe(CN)₆^4?]₀ in a fiftyfold range of 2 × 10^?4?1 × 10^?2M, and the average values of k_obs/[Fe(CN)₆^4?]₀ range from 194M^?1·s^?1 at pH = 5.20 to 0.2M^?1·s^?1 at pH = 9.07 in aqueous medium at 35°C and μ = 0.57M. Results are interpreted in terms of a probable mechanism involving rate-determining outer sphere one-electron transfer steps from the reductant and one-protonated reductant species to the unprotonated and one-protonated Ni(IV) species present in solution. The more electrophilic one-protonated reductant species apparently reacts several orders of magnitude faster than the unprotonated one.     

Characterization of Anilinepentacyanoferrate (II) and (III)

The anilinepentacyanoferrate (II) complex has been characterized in aqueous solution. The complex exhibits a predominant ligand field transition at λ_max = 415 nm with ?_max = 494 M^?1 cm^?1. The corresponding Fe(III) complex displays a strong absorption at λ_max = 700nm(?_max = 1.61×10⁴ M^?1 sec^?1) which can be assigned as a ligand to metal charge transfer transition. The rate constants of formation and dissociation for the Fc(II) complex are (3.14±0.18)×10² M^?1W^?1 and 0.985±0.005 sec^?1, respectively, at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The cyclic voltammetry of the complex shows that a reversible redox process is observed with E_1/2 value of 0.51±0.01 V vs. NHE at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The kinetic study of the oxidation of the Fe(II) complex by ferricyanide ion yielded the rate constant of the reaction k_et = (1.43±0.04)x10 M sec^?1 at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C.     

Kinetic studies of the reactions of pentacyanoferrate(III) complexes with L‐ascorbic acid

The reduction of Fe(CN)₅L^2? (L = pyridine, isonicotinamide, 4,4′‐bipyridine) complexes by ascorbic acid has been subjected to a detailed kinetic study in the range of pH 1–7.5. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the ascorbic acid in the form of H₂A(k₀), HA^?(k₁), and A^2? (k₂), depending on the pH of the solution, followed by a rapid scavenge of the ascorbic acid radicals by Fe(III) complex. With given Ka₁ and Ka₂, the rate constants are k₀ = 1.8, 7.0, and 4.4 M^?1 s^?1; k₁ = 2.4 × 10³, 5.8 × 10³, and 5.3 × 10³ M^?1 s^?1; k₂ = 6.5 × 10⁸, 8.8 × 10⁸, and 7.9 × 10⁸ M^?1 s^?1 for L = py, isn, and bpy, respectively, at μ = 0.10 M HClO₄/LiClO₄, T = 25°C. The kinetic results are compatible with the Marcus prediction. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 126–133, 2005     

The kinetics of cobalt(II) complexation with o-phthalate in aqueous solution

The rate constants for the formation and dissociation of cobalt(II) complexes with o-phthalate in aqueous solution have been determined by a pressure-jump technique. The forward and reverse rate constants obtained are k_f = 1.1 x 10⁶ M⁻¹ s⁻¹ and k_r = 1.6 x 1O³ s⁻ at 25°C, respectively. The activation parameters of the reaction were also obtained from the temperature variation study. The results indicate that chelate ring closure and rupture are affected by the rigidity of the benzene ring of the ligand, and thus the rate determining step of the reaction is the chelate ring closure process.     

Linear free energy relationships between reaction rate constants and equilibrium constants of complex compounds: III. Kinetics and mechanisms of ternary complex formation between (5-X-1, 10-phenanthroline)copper(II) and threonine

The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO₂, Cl, H, CH₃) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, k_f(M^?1·s^?1), for the complexation reaction, CuA + L CuAL, are as follows; X=NO₂, 8.68×10⁸; X = Cl, 7.13×10⁸; X=H, 6.12×10⁸; X=CH₃, 5.42×10⁸. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (logK^CuA_CuAL) of the complexes CuAL and log k_f as follows: logK^CuA_CuAL = 0.13+0.83 logk_f, r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(k^R_f/K^O_f = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.     

The tris(picolinate)vanadate(II) ion: Redox properties and electron transfer kinetics with cobalt(III) amines

Vanadium(II) ions form with the pyridine-2-carboxylate ligand a deep blue, tris-substituted complex absorbing at 660 nm (ε = 7.2 × 10³ M^?1) cm^?1) with a shoulder at 450 nm. Reversible spectroelectrochemistry and cyclic voltammetry were observed for this complex, with E_{$\text{1}\text{2}$} = ?0.448 V vs NHE, and ΔS_rc^θ = ?6 cal · mol^?1 · deg^?1. Electron transfer kinetics with [CO(en)₃]³⁺ led to k₁₂ = 3100 M^?1 s^?, ΔH^≠ = 12.4 kcal · mol^?1 and ΔS^≠ = ?0.9 cal · mol^?1 · deg^?1 (I = 0.10 M). For the related [Co(NH₃)₆]³⁺ complex, k₁₃ = 1.9 × 10⁴ M^?1 s^?1. The self-exchange rate constant and activation parameters were analysed in terms of relative Marcus theory.     

Synthesis and base hydrolysis of chloro(N-(2-pyridinylmethylene)-N′-[3-[(2-pyridinylmethylene)amino]-propyl]-1,3-propanediamine)cobalt(III) perchlora

Synthesis of [Co(pyDPT)Cl](ClO₄)₂ from pentadentate ligand and cobalt(II) yields one cis isomer. Hydrolysis in base is extremely rapid (k_OH 1.8 × 10⁶ M^?1 s^?1 at 250°) in this complex, where the geometry and ligand character fix the single site for conjugate base formation as cis to the leaving group.     

THE INTERACTION OF 2-AMINO-2-HYDROXYMETHYL-1,3-PROPANEDIOL WITH COBALT(II) AND MANGANESE(II) IONS

Abstract

The stepwise complex formation between 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) with Co(II) and Mn(II) was studied by potentiometry at constant ionic strength 2.0 M (NaClO₄) and T = (25.0 ± 0.1)°C, from pH measurements. Data of average ligand number (Bjerrum's function) were obtained from such measurements followed by integration to obtain Leden's function, F ₀(L). Graphical treatment and matrix solution of simultaneous equations have shown two overall stability constants of mononuclear stepwise complexes for the Mn(II)/TRIS system (β₁ = (5.04 ± 0.02) M^?1 and β₂ = (5.4 ± 0.5) M^?2) and three for the Co(II)/TRIS system (β₁ = (1.67 ± 0.02) × 10² M^?1, β₂ = (7.01 ± 0.05) × 10³ M^?2 and β₃ = (2.4 ± 0.4) × 10⁴ M^?3). Slow spontaneous oxidation of Co(II) solutions by dissolved oxygen, accelerated by S(IV), occurs in a buffer solution TRIS/HTRIS⁺ 0.010/0.030 M, with a synergistic effect of Mn(II).     

OXYGEN COMPLEX FORMATION BY COBALT(II)-TRIS (2-AMINOETHYL) AMINE+

Abstract

The reversible oxygenation of the Co(II) complex of tris(2-aminoethyl)amine (TREN, L) has been studied in some detail. The equilibrium constant K _O2 =10^26.92 M^?2 atm^?1, corresponding to the quotient [H⁺] [L₂Co₂(O₂) (OH)³⁺]/[Co²⁺]² [L]² P_O2 was determined by potentiometric equilibrium measurements of hydrogen ion concentration. Values for the thermodynamic constants, ΔH° =–63 ± 9 kcal/mole and ΔS° =–100 ± 15 cal/deg. mol, were calculated from the temperature dependence of the equilibrium constant. Oxygen stoichiometry, measured with a polarographic sensor, indicated the formation of a binuclear (peroxo bridged) complex, and the potentiometric equilibrium data indicated the presence of a second, μ-hydroxo, bridge. Measurement of the kinetics of the fast reaction between the cobalt(II)-TREN complex and dioxygen gave the value of the second order rate constant for the formation of the dioxygen complex as k ₁ =2.8 × 10⁺³ sec^?1 mol^?1. The first order rate constant for the decomposition of the dioxygen complex measured by stopped-flow was found to be k _?2 =0.7 sec^?1. Kinetic and equilibrium data are discussed with respect to the probable structure and mechanism of formation of the dioxygen complex, and are compared with similar data previously reported for analogous complexes. The oxygen complex reported is unique with respect to its extremely slow rate of conversion to inert cobalt(III) complexes.     

Oxidation of manganese(II) by ozone and reduction of manganese(III) by hydrogen peroxide in acidic solution

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×10³ M⁻¹ s⁻¹ in HClO₄ and k=(1.8±0.2)×10³M⁻¹ s⁻¹ in H₂SO₄. The plausible mechanism is an oxygen atom transfer from O₃ to Mn²⁺ producing the manganyl ion MnO²⁺, which subsequently reacts rapidly with Mn²⁺ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H₂O₂)=2.8×10³M⁻¹ s⁻¹ at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H₂O₂ with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 207–214, 1998.     

Nanostructured Cobalt(II) Tetracarboxyphthalocyanine Complex Supported Within the MWCNT Frameworks: Electron Transport and Charge Storage Capabilities

The electrochemical redox properties of a surface‐confined thin solid film of nanostructured cobalt(II) tetracarboxyphthalocyanine integrated with multiwalled carbon nanotube (nanoCoTCPc/MWCNT) have been investigated. This novel nanoCoTCPc/MWCNT material was characterized using SEM, TEM, zeta analysis and electrochemical methods. The nanoCoTCPc/MWCNT nanohybrid material exhibited an extra‐ordinarily high conductivity (15 mS cm^?1), which is more than an order of magnitude greater than that of the MWCNT‐SO₃H (527 µS cm^?1) and three orders of a magnitude greater than the nanoCoTCPc (4.33 µS cm^?1). The heterogeneous electron transfer rate constant decreases as follows: nanoCoTCPc/MWCNT (k_app≈19.73×10^?3 cm s^?1)>MWCNT‐SO₃H (k_app≈11.63×10^?3 cm s^?1)>nanoCoTCPc (k_app≈1.09×10^?3 cm s^?1). The energy‐storage capability was typical of pseudocapacitive behaviour; at a current density of 10 µA cm^?2, the pseudocapacitance decreases as nanoCoTCPc/MWCNT (3.71×10^?4 F cm^?2)>nanoCoTCPc (2.57×10^?4 F cm^?2)>MWCNT‐SO₃H (2.28×10^?4 F cm^?2). The new nanoCoTCPc/MWCNT nanohybrid material promises to serve as a potential material for the fabrication of thin film electrocatalysts or energy‐storage devices.     

Copolymerization of methyl vinyl ketone with styrene in the presence of cobalt(II) nitrate

Copolymerization of methyl vinyl ketone (MVK) with styrene was carried out at 50°C in the presence of cobalt(II) nitrate. The resulting monomer reactivity ratios decreased with an increasing concentration of the cobalt salt. This finding suggests that the metal salt participates in the propagation step of the copolymerization. Absolute copolymerization parameters were determined by assuming a three-component system as free MVK (M₁), MVK complexed with cobalt(II) nitrate (M₂), and styrene (M₃): k₁₁/k₁₂ = 0.184, k₁₁/k₁₃ = 0.235, k₂₂/k₂₁ = 7.18 × 10⁻², k₂₂/k₂₃ = 6.79 × 10⁻⁴, k₃₃/k₃₁ = 0.380, and k₃₃/k₃₂ = 2.77 × 10⁻³, and Q₂ = 19.65 and e₂ = 2.83. The complexed MVK monomer is more reactive to the polymer radical with the terminal styrene unit than the free MVK. Very small values for the monomer reactivity ratios, k₂₂/k₂₃ and k₃₃/k₃₂, show the marked alternating tendency for the copolymerization of the complexed monomer with styrene. In practice, however, alternating copolymer could not be obtained because of the poor solubility of cobalt(II) nitrate.     

THE KINETICS OF ELECTRON TRANSFER FOR A SERIES OF IRON(II) CYCLIDENE COMPLEXES

Abstract

The kinetics of oxidation of a series of iron(II) cyclidene complexes by tris(1,10-phenanthroline)cobalt(III) in methanol have been measured by stopped flow spectrophotometry. The reactions obey a first order rate law when the cobalt(III) complex is present in large excess. The corresponding second order rate constants fall in the range 5.0–130 × 10⁵ M^?1s^?1 (25°C). A linear correlation between the logarithm of the rate constant and the iron(III)/(II) redox potential indicates that the reactions behave as simple outer-sphere electron transfer processes. The self-exchange rate constants for the iron cyclidene complexes have been estimated from the Marcus equation and found to vary between 0.7 × 10⁷ M¹s¹ and 9 × 10⁷ M¹s¹. The dependence of the self-exchange rate constant on ligand structure is discussed.     

Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, k_u = (3.9 ± 0.1) × 10³ L mol^?1 s^?1 and k_c = (6 ± 1) × 10⁴ L mol^?1 s^?1. The formation constant for the acetatoiron(II) complex, K_f, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10^?2 L mol^?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol^?1 and + 40 ± 10 J K^?1 mol^?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004     

A laser flash photolysis/IR diode laser absorption study of the reaction of chlorine atoms with selected alkanes

A high‐resolution IR diode laser in conjunction with a Herriot multiple reflection flow‐cell has been used to directly determine the rate coefficients for simple alkanes with Cl atoms at room temperature (298 K). The following results were obtained: k(Cl + n‐butane) = (1.91 ± 0.10) × 10^?10 cm³ molecule^?1 s^?1, k(Cl + n‐pentane) = (2.46 ± 0.12) × 10^?10 cm³ molecule^?1 s^?1, k(Cl + iso‐pentane) = (1.94 ± 0.10) × 10^?10 cm³ molecule^?1 s^?1, k(Cl + neopentane) = (1.01 ± 0.05) × 10^?10 cm³ molecule^?1 s^?1, k(Cl + n‐hexane) = (3.44 ± 0.17) × 10^?10 cm³ molecule^?1 s^?1 where the error limits are ±1σ. These values have been used in conjunction with our own previous measurements on Cl + ethane and literature values on Cl + propane and Cl + iso‐butane to generate a structure activity relationship (SAR) for Cl atom abstraction reactions based on direct measurements. The resulting best fit parameters are k_p = (2.61 ± 0.12) × 10^?11 cm³ molecule^?1 s^?1, k_s = (8.40 ± 0.60) × 10^?11 cm³ molecule^?1 s^?1, k_t = (5.90 ± 0.30) × 10^?11 cm³ molecule^?1 s^?1, with f( ? CH₂^? ) = f (? CH₂^? ) = f (?C?) = f = 0.85 ± 0.06. Tests were carried out to investigate the potential interference from production of excited state HCl(v = 1) in the Cl + alkane reactions. There is some evidence for HCl(v = 1) production in the reaction of Cl with shape n‐hexane. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 86–94, 2002     

Kinetics of the reaction of nitric oxide with polypyridylamine iron(II) complexes

The reactions of three polypyridylamine ferrous complexes, [Fe(TPEN)]²⁺, [Fe(TPPN)]²⁺, and [Fe(TPTN)]²⁺, with nitric oxide (NO) (where TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, TPPN = N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-propylenediamine, and TPTN = N,N,N′,N′-tetrakis(2-pyridylmethyl)trimethylenediamine) were investigated. The first two complexes, which are spin-crossover systems, presented second-order rate constants for complex formation reactions (k_f) of 8.4 × 10³ and 9.3 × 10³ M^?1 s^?1, respectively (pH 5.0, 25 °C, I = 0.1 M). In contrast, the [Fe(TPTN)]²⁺ complex, which is in low-spin ground state, did not show any detectable reaction with NO. k_f values are lower than those of high-spin Fe(II) complexes, such as [Fe(EDTA)]^2? (EDTA = ethylenediaminetetraacetate) and [Fe(H₂O)]²⁺, but higher than low-spin Fe(II) complexes, such as [Fe(CN)₅(H₂O)]^3? and [Fe(bipyridine)₃]²⁺. The release of NO from the [Fe(TPEN)NO]²⁺ and [Fe(TPPN)NO]²⁺ complexes were also studied, showing the values 15.6 and 17.7 s^?1, respectively, comparable to the high-spin aminocarboxylate analogs. A mechanism is proposed based on the spin-crossover behavior and the geometry of these complexes and is discussed in the context of previous publications.