Synthesis and spectral studies of Cu(II), Ni(II), Co(II) macrocyclic complexes by template reaction of (±)- trans -1,2-diaminocyclohexane,metal(II) nitrate and salicylaldehyde derivatives

Six new macrocyclic complexes were synthesized by template reactions of salicylaldehyde derivatives with (±)-trans-1,2-diaminocyclohexane and metal(II) nitrates. The metal to ligand ratio was 1 : 1. The Cu(II) complexes are proposed to be square planar and the Ni(II) and Co(II) complexes are proposed to be tetrahedral. The complexes are 1 : 2 electrolytes as shown by their molar conductivities (Λ_M) in DMF. The metal complexes are characterized by elemental analyses, FT-IR, UV–Vis, magnetic susceptibility measurements, molar conductivity measurements and mass spectra.     

Interactions of metal ions with the intermediate of thiamine catalysis: Crystal structures of Cd(II), Hg(II) and Pt(II) complexes of 2-(α-hydroxybenzyl)thiamine

Five complexes of formulae Cd(HBT)X₃·H₂O, Hg₂X₅(HBT) (X=Cl, Br), and Pt(HBT)(NO₂)₃ were prepared by reacting CdX₂, HgX₂ and K₂Pt(NO₂)₄ with 2-(α-hydroxybenzyl)thiamine (HBT), an active intermediate of thiamine catalysis, and their crystal structures were determined by X-ray diffraction. The metal ion binds to the N(1′) site of the pyrimidine ring in each case, despite the different shapes and sizes of metal coordination units; a tetrahedral unit in the cadmium complexes, a double-metal unit consisting of two tetrahedral Hg(II) ions in the mercury complexes and a square-planar unit in the platinum complex. The HBT ligands in these complexes adopt the S conformation, as usually observed in C(2)-substituted derivatives of thiamine, with average torsion angles ϕ_T being ±99° and ϕ_P being ±175°. A ‘two-point’ anion-bridge between the amino group of the pyrimidine ring and the cationic thiazolium ring of the same molecule is found in all the structures, being of the form N(4′α)–H…X₁–M–X₂…thiazolium-ring (M=metal ion), which is one of the factors that affect the S conformation. Stacking interactions between the pyrimidine and phenyl rings play an important role in the molecular conformation and crystal packing. The intramolecular close contact between the oxygen of the C(2)-substituent and the sulfur of the thiazolium ring is also a common feature to these complexes, which gives the mechanistic implications.     

Ruthenium(II)/(III) and oxovanadium(IV) complexes of highly conjugated β-diketones and their macrocyclic Schiff base binuclear copper(II) complexes

Ruthenium(II) complexes of the general formula [Ru(PPh3)2(L)(L)]ClO4 [L=2,2-bipyridine or 1,10-phenanthroline;L=2-hydroxy--4-X-phenylcinnamoylacetophenone] have been prepared by reacting L and L with Ru(PPh3)3Cl2 in CH2Cl2. The complexes are diamagnetic and absorb intensely in the visible region, owing to the MLCT transition. Hexacoordinated ruthenium(III) complexes, [RuCl2(PPh3)2(L)], have also been prepared by reacting Ru(PPh3)3Cl3 with -diketones. Solutions of ruthenium(III) complexes show rhombic e.s.r. spectra at 77 K, and distortion from the octahedral symmetry has been identified from the line spacings. The conjugation in diketones favours reversibility in RuII/III and RuIII/IV and stabilize ruthenium in different oxidation states owing to d–* interaction. Oxovanadyl(IV) complexes of the -diketones with a metal-to-ligand ratio of 1:2 and square pyramidal geometry were also prepared. The e.s.r spectra of these complexes show the presence of an unpaired electron in the dxy orbital and the hyperfine splitting constants are sensitive to solvent change. ¶ A new class of highly conjugated Schiff bases obtained from the above diketones and 2-aminothiophenol behave as dibasic, tridentate ligands in their copper(II) complexes. The subnormal magnetic moments and hyperfine splittings of these complexes are ascribed to an antiferromagnetic exchange interaction arising from dimerization. Cyclic voltammograms show that the electron transfer occurs in two steps corresponding to CuII–CuI and CuI–CuI redox states.     

Variations of structures on changing the ratios of metal ions in rare Ca(II)–Zn(II) hetero-metallic self-assembled coordination polymers of hexamethylenetetramine and benzoate

Two rare hetero-metallic calcium(II)-zinc(II) complexes [CaZn₄(OBz)₁₀(μ₂-hmt)₂]_n (1) and [Ca₂Zn₄(OBz)₁₂(μ₂-hmt)₂]_n (2) have been synthesized using basic zinc carbonate, benzoic acid (HOBz), hydrated calcium chloride and hexamethylenetetramine (hmt) by varying the molar ratio of the reactants. Both the complexes have been analyzed by elemental analysis, IR spectroscopy and X-ray crystallography. The complex 1 is a 1D polymer which contains one calcium ion and four zinc atoms in the asymmetric unit together with ten benzoates and two hmts. The polymer has been constructed by the alternate joining of paddle-wheel Zn₂(OBz)₄ units and Zn₂Ca trinuclear species by μ₂-hmt bridging molecules connecting Zn²⁺ ions. Zinc atoms have five coordinate square pyramidal geometries and four coordinate tetrahedral geometries in Zn₂(OBz)₄ and Zn₂Ca moieties, respectively, whereas calcium atoms have six-coordinate distorted octahedral geometry. Complex 2 is also a 1D polymer but unlike complex 1, it contains four independent zinc and two independent calcium atoms in the asymmetric unit together with twelve benzoates and two hmts. By contrast, the polymeric structure of complex 2 has been formed by the connection of Zn₂Ca trinuclear species via μ₂ hmt bridging molecules at Zn centers. Complex 2 is also a 1D polymer but unlike complex 1, it contains four independent zinc and three independent calcium atoms in the asymmetric unit together with twelve benzoates and two hmts. All four zinc atoms are four coordinate with tetrahedral environments and the calcium atoms are six coordinated (two are located on a center of symmetry) exhibiting a distorted octahedral geometry.     

Mössbauer analysis of substituted diiron(II) tetraiminediphenolate macrocyclic complexes

A new series of substituted diiron(II) complexes [Fe(2)(tidf)(L)(2)(MeOH)(2)](n+) (tidf=a two compartment tetraiminediphenolate macrocycle; n=0 or 2+; L=NCS(-), CN(-), N(3)(-), pyrazine (pz), 4-cyanopyridine (4-cnpy) and 4-mercaptopyridine (4-shpy)) and one tetranuclear complex, {[Fe(2)(tidf)(CH(3)OH)(2)](2)(mu-4-cnpy)(2)}(ClO(4))(4) were isolated and characterized by elemental analysis, conductivity measurements, M?ssbauer and FTIR.     

Dependence of the chemical properties of macrocyclic [Ni(II)(2)L(μ-O(2)CR)](+) complexes on the basicity of the carboxylato coligands (L(2-) = macrocyclic N(6)S(2) ligand)

The dependence of the properties of mixed ligand [Ni(II)(2)L(μ-O(2)CR)](+) complexes (where L(2-) represents a 24-membered macrocyclic hexaamine-dithiophenolato ligand) on the basicity of the carboxylato coligands has been examined. For this purpose 19 different [Ni(II)(2)L(μ-O(2)CR)](+) complexes (2-20) incorporating carboxylates with pK(b) values in the range 9 to 14 have been prepared by the reaction of [Ni(II)(2)L(μ-Cl)](+) (1) and the respective sodium or triethylammonium carboxylates. The resulting carboxylato complexes, isolated as ClO(4)(-) or BPh(4)(-) salts, have been fully characterized by elemental analyses, IR, UV/vis spectroscopy, and X-ray crystallography. The possibility of accessing the [Ni(II)(2)L(μ-O(2)CR)](+) complexes by carboxylate exchange reactions has also been examined. The main findings are as follows: (i) Substitution reactions between 1 and NaO(2)CR are not affected by the basicity or the steric hindrance of the carboxylate. (ii) Complexes 2-20 form an isostructural series of bisoctahedral [Ni(II)(2)L(μ-O(2)CR)](+) compounds with a N(3)Ni(μ-SR)(2)(μ-O(2)CR)NiN(3) core. (iii) They are readily identified by their ν(as)(CO) and ν(s)(CO) stretching vibration bands in the ranges 1684-1576 cm(-1) and 1428-1348 cm(-1), respectively. (iv) The spin-allowed (3)A(2g) → (3)T(2g) (ν(1)) transition of the NiOS(2)N(3) chromophore is steadily red-shifted by about 7.5 nm per pK(b) unit with increasing pK(b) of the carboxylate ion. (v) The less basic the carboxylate ion, the more stable the complex. The stability difference across the series, estimated from the difference of the individual ligand field stabilization energies (LFSE), amounts to about 4.2 kJ/mol [Δ(LFSE)(2,18)]. (vi) The "second-sphere stabilization" of the nickel complexes is not reflected in the electronic absorption spectra, as these forces are aligned perpendicularly to the Ni-O bonds. (vii) Coordination of a basic carboxylate donor to the [Ni(II)(2)L](2+) fragment weakens its Ni-N and Ni-S bonds. This bond weakening is reflected in small but significant bond length changes. (viii) The [Ni(II)(2)L(μ-O(2)CR)](+) complexes are relatively inert to carboxylate exchange reactions, except for the formato complex [Ni(II)(2)L(μ-O(2)CH)](+) (8), which reacts with both more and less basic carboxylato ligands.     

N-Ferrocenylcarbonyl-N′-benzoylhydrazine and its transition metal(II) complexes

Summary A new ferrocene derivative, N-ferrocenylcarbonyl-N-benzoylhydrazine (H₂FB) and its transition metal complexes, [M(FB)]₂·H₂O (M = Mn^II, Co^II, Cu^II, Zn^II, Cd^II or Hg^II) and M(HFB)₂·nH₂O (M = Mn^II or Cd^II) were prepared by reacting H₂FB with the metal(II) acetates and characterized by elemental analyses, i.r. and u.v. spectroscopy and t.g.a. H₂FB appears to act as a tetradentate ligand, coordinating to the metal through the nitrogen enolic oxygen atoms.     

The infrared spectra of ethylenediamine complexes—II. Tris-, bis- and mono(ethylenediamine) complexes of metal(II) halides

The i.r. spectra of the complexes M(en)₃X₂ (M = Mn, Fe, Co, Ni, Cu, Zn), trans-Cu(en)₂X₂, Ni(en)₂X₂ and M(en)X₂ (M = Ni, Cu, Zn; X = Cl, Br, I) have been studied. Assignments are proposed for the tris(ethylenediamine) complexes on the basis of ¹⁵N-, N₂D₄- and C₂D₄-labelling of en and the effects of metal ion substitution in relation to our earlier study of [M(en)₃]SO₄ complexes. Assignments for the bis(ethylenediamine) complexes are based on our observations of halogen-sensitivity and earlier studies on metal isotope labelling and ligand deuteration of the halide complexes and a normal coordinate analysis of the [Cu(en)₂]²⁺ species. The spectra of the halide complexes have been extended below 200 cm⁻¹ for the first time. Finally, the spectra of the mono(ethylenediamine) complexes are discussed in relation to their known or probable structures.     

Structural effects of the lone pair on lead(II), and parallels with the coordination geometry of mercury(II). Does the lone pair on lead(II) form H-bonds? Structures of the lead(II) and mercury(II) complexes of the pendant-donor macrocycle DOTAM (1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane)

The synthesis and structures of [Pb(DOTAM)](ClO4)2.4.5H2O (1) and [Hg(DOTAM)](ClO4)2.0.5CH3OH.1.5H2O (2) are reported, where DOTAM is 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane. Compound 1 is triclinic, space group P, a = 12.767(3) A, b = 13.528(2) A, c = 18.385(3) A, alpha = 101.45(2) degrees, beta = 93.32(2) degrees, gamma = 90.53(2) degrees, Z = 4, R = 0.0500. Compound 2 is monoclinic, space group Cc, a = 12.767(3) A, b = 13.528(2) A, c = 18.385(3) A, beta = 101.91(2) degrees, Z = 4, R = 0.0381. The Pb(II) ion in 1 has an average Pb-N = 2.63 A to four N-donors from the macrocyclic ring, and four O-donors (average Pb-O = 2.77 A) from the amide pendant donors of the macrocycle, with a water molecule placed with Pb-O = 3.52 A above the proposed site of the lone pair (Lp) on Pb. The Hg(II) in 2 appears to be only six-coordinate, with four Hg-N bond lengths averaging 2.44 A, and two Hg-O from pendant amide donors at 2.41 A. The other two amide donors appear to be noncoordinating, with Hg-O distances of 2.74 and 2.82 A. A water situated 3.52 A above the proposed site of the lone pair on Pb(II) in 1 is oriented in such a way that it might be thought to be forming a Pb-Lp.H-O-H hydrogen bond. It is concluded that that this is not an H-bond, but that the presence of the lone pair allows a closer approach of the hydrogens to Pb than would be true otherwise. The structural analogy in the VSEPR sense between Pb(II), which has the 5d(10)6s(2) outer electron structure, and the Hg(II) ion, which has the 5d10 structure, is examined. The tendency of Hg(II) toward linear coordination, with two short Hg-L bonds (L = ligand) at 180 degrees to each other, and other donor groups at roughly 90 degrees to this and at much longer bond distances, is paralleled by Pb(II). One of the short Hg-L bonds is replaced in the Pb(II) structures by the lone pair (Lp), which is opposite the short Pb-L bond, or in some cases 2-4 shorter Pb-L bonds.     

Synthesis and spectral studies of macrocyclic Pb(Ⅱ), Zn(Ⅱ), Cd(II)and La(Ⅲ) complexes derived from 1,4-bis(3-aminopropoxy)butane with metal nitrate and salicylaldehyde derivatives

Eight new macrocyclic complexes were synthesized by template reaction of 1,4-bis(3-aminopropoxy)butane with metal nitrate and 1,3-bis(2-forrnylphenyl)propane or 1,4-bis(2-formylphenyl)butane and their structures were proposed on the basis of elemental analysis, FTIR, UV-vis, molar conductivity measurements, 1H NMR and mass spectra. The metals to ligand molar ratios of the complexes were found to be 1:1. The complexes are 1:2 electrolytes for Pb(II), Zn(II) and Cd(II) complexes and 1:3 electrolytes for La(lIl) as shown by their molar conductivities (Am) in DMSO at 10-3 tool L-l. Due to the existence of free ions in these complexes,such complexes are electrically conductive. The configurations of La(Ⅲ) and Pb(U) were proposed to probably octahedral and Zn(II) and Cd(II) complexes were proposed to probably tetrahedral.     

The properties of the new Mn(II)–Co(II)–Mn(II)-type hetero-trinuclear oxime metal complexes with N4 and N4O2 ligands

1,3- and 1,2-calix[4]crown-7 and calix[4]crown-9 cone conformers were synthesized in acceptable yields by sequential introduction of two distal or proximal polyethylene glycolic chains with terminal hydroxyls at the lower rim, monotosylation, and intramolecular ring closure reaction. According to the two-phase extraction experiment, the title compounds showed mediocre affinity for alkali and alkaline earth metal picrates. The 1,2-calix[4]crown-9 extracted Sr²⁺ selectively among other alkaline earth metal cations.     

Structure of the catena-bis(trimethylamine)cadmium(II)-tetra-μ-cyanonickelate(II) complex and contact self-stabilization of molecules

The [Cd(N(CH₃)₃)₂Ni(CN)₄] complex crystallizes in a tetragonal system, space group 14/mmm with two formula units per unit cell (XRD, Rigaku AFC-6A diffractometer, λ MoK_α, ω/2θ scan mode, θ_max = 38?, 635 observed unique reflections, 53 parameters, R = 0.027). The structure consists of parallel polymer layers made up of coordinated metal atoms and bridging cyanides. The octahedral environment of Cd(II) involves six nitrogen atoms of the four cyanide groups in the layer plane (2.323(4) Å) and the two trimethylamine ligands in the transposition (2.42(1) Å). The square-planar environment of Ni(II) consists of four carbon atoms of the cyanide ligands (1.857(3) Å). The layers are packed according to van der Waals type; the “hollows” near the nickel atoms are filled by the “hills” of the trimethylamino groups from the neighboring layer (the interlayer distance is 7 Å). The spatial complementarity of the layers leads to close packing of the complex and explains the lack of a clathrate-forming ability in the latter. The trimethylamine ligands here play the same role as guest molecules in Hofmann clathrates, stabilizing the planar polymer structure of the complex. This phenomenon is called contact self-stabilization.

     

Synthesis,characterization, and biological activity studies of copper(II)–metal(II) binuclear complexes of dipyridylglyoxal bis(2-hydroxybenzoyl hydrazone)

A new series of copper(II) mononuclear and copper(II)–metal(II) binuclear complexes [(H₂L)Cu] ? H₂O, [CuLM] ? nH₂O, and [Cu(H₂L)M(OAc)₂] ? nH₂O, n = 1–2, M = Co(II), Ni(II), Cu(II), or Zn(II), and L is the anion of dipyridylglyoxal bis(2-hydroxybenzoyl hydrazone), H₄L, were synthesized and characterized. Elemental analyses, molar conductivities, and FT-IR spectra support the formulation of these complexes. IR data suggest that H₄L is dibasic tetradentate in [(H₂L)Cu] ? H₂O and [Cu(H₂L)M(OAc)₂] ? nH₂O but tetrabasic hexadentate in [CuLM] ? nH₂O (n = 1–2). Thermal studies indicate that waters are of crystallization and the complexes are thermally stable to 347–402°C depending upon the nature of the complex. Magnetic moment values indicate magnetic exchange interaction between Cu(II) and M(II) centers in binuclear complexes. The electronic spectral data show that d–d transitions of CuN₂O₂ in the mononuclear complex are blue shifted in binuclear complexes in the sequences: Cu–Cu > Cu–Ni > Cu–Co > Cu–Zn, suggesting that the binuclear complexes [CuLM] ? nH₂O are more planar than the mononuclear complex. The structures of complexes were optimized through molecular mechanics applying MM ⁺force field coupled with molecular dynamics simulation. [(H₂L)Cu] ? nH₂O, [CuLM] ? nH₂O, and the free ligand were screened for antimicrobial activities on some Gram-positive and Gram-negative bacterial species. The free ligand is inactive against all studied bacteria. The screening data showed that [CuLCu] ? H₂O > [(H₂L)Cu] ? H₂O > [CuLZn] ? H₂O > [CuLNi] ? 2H₂O ≈ [CuLCo] ? H₂O in order of biological activity. The data are discussed in terms of their compositions and structures.     

Synthesis,structure and magnetism of a hydrogen-bond-linked three-dimensional network of a novel nickel(II)–cobalt(II)–nickel(II) complex containing a new macrocyclic mononuclear complex ligand

A novel trinuclear complex, [Co(NiL)₂(H₂O)₂](ClO₄)₂ · 2C₂H₅OH, was prepared by self-assembly using [NiL] as a new complex ligand; L is the dianion of dimethyl 5,6,7,8,15,16-hexahydro-6,7-dioxodibenzo[1,4,8,11]tetraazacyclotetradecine-13,18-dicarboxylate. The structure of the trinuclear complex was determined by X-ray crystallography. The Co^II ion is at the center of the trinuclear complex cation and occupies a distorted octahedral O₆ environment, approximating to O _h with a ⁴ T _1g ground state for Co^II that has an unquenched spin–orbit coupling reflected in the magnetic properties. Two Ni^II ions reside in completely same and slightly distorted square-planar N₄ coordination geometries. Co^II and each Ni^II are bridged by an oxamido group from one of the two macrocyclic ligands (L). O—H...O and ... interactions link the trinuclear fragments, perchlorate ions and C₂H₅OH molecules to form a three-dimensional supramolecular architecture.     

Radical scavenging and in vitro antifungal activities of Cu(II) and Co(II) complexes of the t-butylphenyl derivative of porphyrazine

Co(II) and Cu(II) complexes and metal-free t-butylphenyl peripherally substituted porphyrazine (Pz) have been screened for in vitro antifungal (Aspergillus niger) and antioxidant (free radical scavenging, superoxide radical scavenging, and reducing power) activities. The results were compared with synthetic antioxidants, e.g., butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), trolox, or α-tocopherol. The free radical scavenging activity of H₂Pz was higher than the CuPz complex. However, CuPz complex showed higher superoxide radical scavenging activity than BHA, BHT, and trolox while H₂Pz and CoPz showed weaker activity than BHA, BHT, and trolox. The reducing power of all complexes was similar to that of BHT and α-tocopherol on a per molar basis. The ligand and complexes have antifungal activity against A. niger. The compounds have significant superoxide radical scavenging activity against various antioxidant systems in vitro.     

Synthesis and characterization of M(II) (M = Cd,Hg and Pb) complexes with naphthodiaza-crown macrocyclic ligand and study of metal ion recognition by fluorescence, 1H NMR spectroscopy,and DFT calculation

To find metal ion recognition by L (L = O₂N₂-donor naphthodiaza-crown macrocyclic ligand), the complexes [ML]²⁺ (M = Cd, Hg and Pb) were synthesized and characterized by IR, ¹H, ¹³C NMR, and mass spectrometry, as well as elemental microanalysis. Hg(II) showed perceptible enhancement of the fluorescence of L in which ultra-low limit of detection for Hg(II) by L was determined as 1 nM in ethanol and DMSO. L reserved selectivity of Hg(II) in its binary mixtures with metal cations in solution. A 1 : 1 stoichiometry was found for the interaction of Hg(II) with L while Benesi–Hildebrand method was applied to calculate its complexation binding constant (K_BH) employing fluorescence spectrophotometry. The monitoring of the chemical shifts in ¹H NMR spectra of these complexes demonstrated that the central macrocycle of L was tailored for the size of Hg(II). Density functional theory calculations using B₃LYP/6–31G* basis set demonstrated that the macrocycle cavity of L was properly fitted for complex formation with Hg(II) cation, while both Cd(II) and Pb(II) cations did not form strong bonds with L from inadequate cation size. The present study shows detection method of Hg(II) and also possible application of naphthodiaza as an appropriate fluorophore macrocyclic ligand for detecting other metal ions.     

Characterization and spectroscopic study of new complexes of Cd(II), Hg(II) and Pb(II) with the sodium salt of morin-5′-sulfonic acid

Solid compounds of Cd(II), Hg(II) and Pb(II) with the sodium salt of morin-5′-sulfonic acid (NaMSA) were obtained. The molecular formula of the complexes are: Cd(C₁₅H₈O₁₀SNa)₂?·?6H₂O, CdOH(C₁₅H₈O₁₀SNa)?·?4H₂O, Hg(C₁₅H₈O₁₀S)?·?4H₂O and Pb(C₁₅H₈O₁₀S)?·?3H₂O. Some of their physicochemical properties such as UV-Vis, infrared, ¹³C NMR and mass spectra, thermogravimetric analysis, and solubility were studied. On the basis of spectroscopic data NaMSA was bound to Cd²⁺ via 4C=O and 3C?–?oxygen and the Hg²⁺ and Pb²⁺ ions by 5C–OH, 4C=O and 3C–OH.     

Copper(II), zinc(II) and mixed metal copper(II)-zinc(II) peroxocomplexes containing 2,2′,6′,2″-terpyridine

Summary Facile reaction of 2,2,6,2-terpyridine (L; terpy) with copper or zinc powders or their mixtures, in the presence of an excess of H₂O₂, leads to novel complexes [Cu(L)-(O₂ ^2–)]·3H₂O, [Zn(L)(O ₂ ^2– )]·H₂O and [Cu,Zn(L)₂(O ₂ ^2– )₂]· 4H₂O, respectively, which were isolated and characterized by elemental and micro- analysis, e.s.r., electronic, i.r. and thermogravimetric analysis in air and argon.     

Complexation of the zinc(II) ion with 2,2′-bipyridine and 1,10-phenanthroline in 4-methylpyridine and solvation structure of the manganese(II), cobalt(II), nickel(II), and zinc(II) ions in 4-methylpyridine and 3-methylpyridine

Complexation of the zinc(II) ion with 2,2-bipyridine (bpy) and 1,10-phenanthroline (phen) has been calorimetrically studied in 4-methylpyridine (4Me-py) containing 0.1 mol dm^–3 (n-C₄H₉)₄NClO₄ as a constant ionic medium at 25°C. The formation of [ZnL]²⁺, [ZnL₂]²⁺, and [ZnL₃]²⁺ (L=bpy, phen), and their formation constants, reaction enthalpies and entropies were determined. Our EXAFS (extended X-ray absorption fine structure) measurements showed that the solvation structure of the manganese(II), cobalt(II), and nickel(II) ions is six-coordinate octahedral in 4Me-py and 3-methylpyridine (3Me-py), while that of the zinc(II) ion is four-coordinate tetrahedral in 4Me-py. Since [ZnL₃]²⁺ is expected to have an octahedral structure, a tetrahedral-to-octahedral structural change should take place at a certain step of complexation. The thermodynamic parameters, especially reaction entropies, indicate that the structural change occurs at the formation of [Zn(bpy)₂]²⁺ and [Zn(phen)]²⁺.