Metal based drugs: design,synthesis and in-vitro antimicrobial screening of Co(II), Ni(II), Cu(II) and Zn(II) complexes with some new carboxamide derived compounds: crystal structures of N-[ethyl(propan-2-yl)carbamothioyl]thiophene-2-carboxamide and its copper(II) complex

A new series of compounds derived from thiophene-2-carboxamide were synthesized and characterized by IR, ¹H-NMR and ¹³C-NMR, mass spectrometry and elemental analysis. These compounds were further used to prepare their Co(II), Ni(II), Cu(II) and Zn(II) metal complexes. All metal(II) complexes were air and moisture stable. Physical, spectral and analytical data have shown the Ni(II) and Cu(II) complexes to exhibit distorted square-planar and Co(II) and Zn(II) complexes tetrahedral geometries. The ligand (L¹) and its Cu(II) complex were characterized by the single-crystal X-ray diffraction method. All the ligands and their metal(II) complexes were screened for their in-vitro antimicrobial activity. The antibacterial and antifungal bioactivity data showed that the metal(II) complexes were found to be more potent than the parent ligands against one or more bacterial and fungal strains.     

A chelate complex‐enhanced luminol system for selective determination of Co(II), Fe(II) and Cr(III)

A determination method for Co(II), Fe(II) and Cr(III) ions by luminol‐H₂O₂ system using chelating reagents is presented. A metal ion‐chelating ligand complex with a Co(II) ion and a chelating reagent like ethylenediaminetetraacetic acid (EDTA) produced highly enhanced chemiluminescence (CL) intensity as well as longer lifetime in the luminol‐H₂O₂ system compared to metals that exist as free ions. Whereas free Cu(II) and Pb(II) ions had a strong catalytic effect on the luminol‐H₂O₂ system, significantly, the complexes of Cu(II) and Pb(II) with chelating reagents lost their catalytic activity due to the chelating reagents acting as masking agents. Based on the observed phenomenon, it was possible to determine Co(II), Fe(II) and Cr(III) ions with enhanced sensitivity and selectivity using the chelating reagents of the luminol‐H₂O₂ system. The effects of ligand, H₂O₂ concentration, pH, buffer solution and concentrations of chelating reagents on CL intensity of the luminol‐H₂O₂ system were investigated and optimized for the determination of Co(II), Fe(II) and Cr(III) ions. Under optimized conditions, the calibration curve of metal ions was linear over the range of 2.0 × 10^‐8 to 2.0 × 10^‐5 M for Co(II), 1.0 × 10^‐7 to 2.0 × 10^‐5 M for Fe (II) and 2.0 × 10^‐7 to 1.0 × 10^‐4 M for Cr(III). Limits of detection (3σ/s) were 1.2 × 10^‐8, 4.0 × 10^‐8 and 1.2 × 10^‐7 M for Co(II), Fe(II) and Cr(III), respectively. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis,physico-chemical characterization and antimicrobial activity of cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes with some acyldihydrazones

Complexes of the type [M(bssdh)]Cl and [M(dspdh)]Cl, where M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II); Hbssdh = benzil salicylaldehyde succinic acid dihydrazone, Hdspdh = diacetyl salicylaldehyde phthalic acid dihydrazone have been synthesized and characterized with the help of elemental analyses, electrical conductance, magnetic susceptibility measurements, electronic, ESR and IR spectra and X–ray diffraction studies. Magnetic moment values and electronic spectral transitions indicate a spin free octahedral structure for Co(II), Ni(II) and Cu(II) complexes. IR spectral studies suggest that both the ligands behave as monobasic hexadentate ligands coordinating through three > C = O, two > C = N– and a phenolate group to the metal. ESR spectra of Cu(II) complexes are axial type and suggest as the ground state. X–ray powder diffraction parameters for [Co(bssdh)]Cl and [Co(dspdh)]Cl complexes correspond to an orthorhombic crystal lattice. The ligands as well as their metal complexes show a significant antifungal and antibacterial activity against various fungi and bacteria. The metal complexes are more active than the parent ligands.     

Sheet and chain polymeric transition metal complexes formed by N,N-o-phenylenedimethylenebis(pyridin-4-one)

The preparations are reported of the ‘extended reach’ ligand N,N^′-o-phenylene-dimethylenebis(pyridin-4-one) (o-XBP4) and of a range of its metal complexes with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II), two of which have been shown by X-ray studies to have polymeric structures. In the compound [Mn(o-XBP4)(H₂O)₂(NO₃)](NO₃) the o-XBP4 ligands link ‘Mn(H₂O)₂(NO₃)’ units into chains which are then cross-linked into sheets by the bridging action of the coordinated nitrate. In [Cu(o-XBP4)(NO₃)₂] chains are also formed by the bridging action of the o-XBP4 ligands but here they simply pack trough-in-trough with no nitrate cross-linking. X-band EPR spectra are reported for these and the other Mn and Cu compounds as are relevant spectroscopic results for the other complexes.     

Mononuclear Ni(II) and Zn(II) complexes of some potentially binucleating Schiff base ligands

Mononuclear Zn(II) and Ni(II) complexes have been prepared from two new Schiff base ligands in which two alternative co-ordination sites (N₂O₂ or O₂O₂) occur. The first ligands is the Schiff base derived from 1,2-diaminobenzene and 2-hydroxy-3-carboxyl-1-napthaldehyde (bopaH₄). The complexes of this ligand contain the metal ions in the N₂O₂ coordination site as a result of the steric requirements of the co-ordinated ligand. The second ligand series are derivatives of X-substituted 1,2-diaminobenzenes, 2-hydroxy-3-carboxy-1-naphthaldehyde and 2-hydroxy-5-methyl isophthaldehyde (X-bolaH₃). In this case Ni(II) occupies the N₂O₂ site in its complexes with the X-bolaH₃ ligands, whereas the Zn(II) complexes are co-ordinate through the O₂O₂ site since the steric restrictions are less severe.     

Synthesis, structure, and properties of monomeric Fe(II), Co(II), and Ni(II) complexes of neutral N-(aryl)-2-pyridinecarboxamides

We have prepared and structurally characterized six-coordinate Fe(II), Co(II), and Ni(II) complexes of types [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Fe, 1; Co, 3; and Ni, 5) and [M^II(HL²)₃][ClO₄]₂ · MeCN (M = Fe, 2 and Co, 4) of bidentate pyridine amide ligands, N-(phenyl)-2-pyridinecarboxamide (HL¹) and N-(4-methylphenyl)-2-pyridinecarboxamide (HL²). The metal centers in bis(ligand)-diaqua complexes 1, 3 and 5 are coordinated by two pyridyl N and two amide O atoms from two HL¹ ligands and six-coordination is completed by coordination of two water molecules. The complexes are isomorphous and possess trans-octahedral geometry. The metal centers in isomorphous tris(ligand) complexes 2 and 4 are coordinated by three pyridyl N and three amide O atoms from three HL² ligands. The relative dispositions of the pyridine N and amide O atoms reveal that the pseudo-octahedral geometry have the meridional stereochemistry. To the best of our knowledge, this work provides the first examples of structurally characterized six-coordinate iron(II) complexes in which the coordination is solely by neutral pyridine amide ligands providing pyridine N and amide O donor atoms, with or without water coordination. Careful analyses of structural parameters of 1-5 along with that reported in the literature [M^II(HL¹)₂(H₂O)₂][ClO₄]₂ (M = Cu and Zn) and [Co^III(L²)₃] have allowed us to arrive at a number of structural correlations/generalizations. The complexes are uniformly high-spin. Spectroscopic (IR and UV/Vis) and redox properties of the complexes have also been investigated.     

Synthesis, structural and corrosion inhibition studies on cobalt(II), nickel(II), copper(II) and zinc(II) complexes with 2-acetylthiophene benzoylhydrazone

Cobalt(II), nickel(II), copper(II) and zinc(II) complexes with 2-acetylthiophene benzoylhydrazone have been synthesized and characterized by elemental analyses, magnetic susceptibility measurements, electronic, IR, NMR and ESR spectral techniques. The molecular structures of ligand and its copper(II) complex have been determined by single crystal X-ray diffraction technique. The Cu(II) complex possesses a CuN₂O₂ chromophore with a considerable delocalization of charge. The structure of the complex is stabilized by intermolecular π–π stacking and C–H?π interactions. Hatbh acts as a monobasic bidentate ligand in all the complexes bonding through a deprotonated C–O⁻ and >CN groups. Electronic spectral studies indicate an octahedral geometry for the Ni(II) complex while square planar geometry for the Co(II) and Cu(II) complexes. ESR spectrum of the Cu(II) complex exhibits a square planar geometry in solid and in DMSO solution. The trend g_|| > g_⊥ > 2.0023 indicates the presence of an unpaired electron in the d_x²-y² orbital of Cu(II). The electro-chemical study of Cu(II) complex reveals a metal based reversible redox behavior. The Ni(II) complex shows exothermic multi-step decomposition pattern of the bonded ligand. The ligand and its most of the metal complexes show appreciable corrosion inhibition properties for mild steel in 1 M HCl medium. [Co(atbh)₂] complex exhibited the greatest impact on corrosion inhibition among the other compounds.     

Chiral bimetallic complexes from chiral salen metal complexes and mercury (II) halides and acetates: the anionic groups interact with Cu(II) in apical position

A series of chiral bimetallic complexes have been prepared containing both Cu(II) and Hg(II) metal centers. The complexes possess chiral salen ligands which host Cu(II) in the center of the cis-N₂O₂ chromophore and Hg(II) via two oxygen atoms of the chromophore. Halogen and acetate groups from mercury salts interact with the Cu(II) center. The X-ray crystallographic data of 11 reveals a short distance of Cl?Cu (3.22-3.26 Å). EPR study also discloses a strong interaction, in particular, of acetate group with Cu.     

Synthesis, physico-chemical characterization and antimicrobial activity of cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes with some acyldihydrazones

Complexes of the type [M(bssdh)]Cl and [M(dspdh)]Cl, where M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II); Hbssdh = benzil salicylaldehyde succinic acid dihydrazone, Hdspdh = diacetyl salicylaldehyde phthalic acid dihydrazone have been synthesized and characterized with the help of elemental analyses, electrical conductance, magnetic susceptibility measurements, electronic, ESR and IR spectra and X-ray diffraction studies. Magnetic moment values and electronic spectral transitions indicate a spin free octahedral structure for Co(II), Ni(II) and Cu(II) complexes. IR spectral studies suggest that both the ligands behave as monobasic hexadentate ligands coordinating through three > C = O, two > C = N- and a phenolate group to the metal. ESR spectra of Cu(II) complexes are axial type and suggest d(x(2)-y(2)) as the ground state. X-ray powder diffraction parameters for [Co(bssdh)]Cl and [Co(dspdh)]Cl complexes correspond to an orthorhombic crystal lattice. The ligands as well as their metal complexes show a significant antifungal and antibacterial activity against various fungi and bacteria. The metal complexes are more active than the parent ligands.     

Free metal ion depletion by "Good's" buffers. I. N-(2-acetamido)iminodiacetic acid 1:1 complexes with calcium(ii). magnesium(II), zinc(II), manganese(II), cobalt(II), nickel(II), and copper(II).

Formation (affinity) constants for 1:1 complexes of N-(2-acetamido)iminodiacetic acid (ADAH₂) with Ca(II), Mg(II), Mn(II), Zn(II), Co(II), Ni(II), and Cu(II) have been determined. Probable structures of the various metal chelates existing in solution are discussed. Values for the deprotonation of the amide group in [Cu(ADA)] and subsequent hydroxo complex formation are also reported. The use of ADA as a buffer is considered in terms of metal buffers complexes which can be formed at physiological pH, i.e., at pH 7.0 there is essentially no free metal ion in 1:1 M²⁺ to ADA solutions.     

Synthesis,physico-chemical studies of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) complexes with some p-substituted acetophenone benzoylhydrazones and their antimicrobial activity

Complexes of the type [M(pabh)(H₂O)Cl], [M(pcbh)(H₂O)Cl] and [M(Hpabh)(H₂O)₂ (SO₄)] where, M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II); Hpabh = p-amino acetophenone benzoyl hydrazone and Hpcbh = p-chloro acetophenone benzoyl hydrazone have been synthesized and characterized with the help of elemental analyses, electrical conductance, magnetic susceptibility measurements, electronic, ESR and IR spectra, thermal (TGA & DTA) and X-ray diffraction studies. Co(II), Ni(II) and Cu(II) chloride complexes are square planar, whereas their sulfate complexes have spin-free octahedral geometry. ESR spectra of Cu(II) complexes with Hpabh are axial and suggest as the ground state. The ligand is bidentate bonding through >C = N ? and deprotonated enolate group in all the chloro complexes, whereas, >C = N and >C = O groups in all the sulfato complexes. Thermal studies (TGA & DTA) on [Cu(Hpabh)(H₂O)₂(SO₄)] indicate a multistep decomposition pattern, which are both exothermic and endothermic in nature. X-ray powder diffraction parameters for [Co(pabh)(H₂O)Cl] and [Ni(Hpabh)(H₂O)₂(SO₄)] correspond to tetragonal and orthorhombic crystal lattices, respectively. The ligands as well as their complexes show a significant antifungal and antibacterial activity. The metal complexes are more active than the ligand.     

Biomimetic zinc(II) and cobalt(II) complexes with tri-tert-butoxysilanethiolate and imidazole ligands - Structural and spectroscopic studies

New, heteroleptic zinc and cobalt complexes with tri-tert-butoxysilanethiolate and imidazole co-ligands are characterized by crystal structure studies. The ligands exhibit different coordination modes to Co(II) ions: NOS₂ (with methanol as O-donor ligand) in 2′, NO₂S₂ in 2′′, N₂S₂ in 1, and to Zn(II) ions: N₂S₂ in 3 and N₃S in 4. Complex 2′ is a structural analog of cobalt-substituted active site of alcohol dehydrogenase. All four-coordinate Co(II) and Zn(II) complexes have tetrahedral geometry. Solution and solid state electronic spectra of cobalt(II) complexes are discussed and compared to literature data available for the cobalt-substituted liver alcohol dehydrogenase and sorbitol dehydrogenase. The EPR spectra of all cobalt complexes exhibit at 77 K a characteristic broad signal with g ∼3.6 and 5.6, strongly indicating a high-spin state, S = 3/2, of Co(II) complexes.     

Copper (II) and cobalt (II) complexes of chiral tetrathiafulvalene-oxazoline (TTF-OX) and tetrathiafulvalene-thiomethyl-oxazoline (TTF-SMe-OX) derivatives

Synthesis and single crystal X-ray structures of the first paramagnetic transition metal complexes containing chiral ethylenedithio-tetrathiafulvalene-oxazoline (EDT-TTF-OX) 1a-c and ethylenedithio-tetrathiafulvalene-thiomethyloxazoline 2 (EDT-TTF-(SMe)OX) ligands based on copper (II) and cobalt (II) are described. The racemic [EDT-TTF-OX][Cu(hfac)₂] complex 3a crystallizes in the triclinic centrosymmetric space group , whereas the enantiopure counterparts 3b-c crystallize in the triclinic non-centrosymmetric space group P1. Cu(II) adopts a distorted square pyramidal coordination geometry, a much weaker Cu?S_TTF interaction also being identified. The same coordination pattern around Cu(II) is observed in the complex [(rac)-EDT-TTF-(SMe)OX][Cu(hfac)₂] (4) in spite of the bidentate nature of the redox active ligand. DFT theoretical calculations afforded two equilibrium configurations for a corresponding model complex, in which the metal centre establishes secondary coordination either with one S_TTF or with the SMe group. The same ligand coordinates the cobalt (II) to afford the octahedral complex [(rac)-EDT-TTF-(SMe)OX][Co(hfac)₂] (5). In all these novel complexes, the paramagnetic centres are structurally and magnetically isolated. Cyclic voltammetry measurements show the stability of the radical cation species.     

New metal complexes of N2S2 tetradentate ligands: Synthesis and spectral studies

New tetradentate ligands 2-(2-mercaptoethylthio)-N-(pyridin-2-ylmethyl)acetamide H₂L¹ and 2-chloro-2-(2-mercaptoethylthio)-N-(pyridin-2-ylmethyl)acetamide H₂L² were synthesised from the reaction of 2-aminomethanepyridine with 1,4-dithian-2-one and 3-chloro-1,4-dithian-2-one, respectively. Monomeric complexes of these ligands, of general formulae K[Cr^III(Lⁿ)Cl₂], K₂[Mn^II(Lⁿ)Cl₂] and [M(Lⁿ)] (M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) or Hg(II); n = 1, 2) are reported. The mode of bonding and overall geometry of the complexes were determined through IR, UV-Vis, NMR and mass spectral studies, magnetic moment measurements, elemental analysis, metal content and conductance. These studies revealed octahedral geometries for the Cr(III), Mn(II) complexes, square planar for Ni(II) and Cu(II) complexes and tetrahedral for the Fe(II), Co(II), Zn(II), Cd(II) and Hg(II) complexes. The study of complex formation via molar ratio in DMF solution has been investigated and results were consistent to those found in the solid complexes with a ratio of (M:L) as (1:1).     

Synthesis and chemical characterization of Cu, Ni and Zn complexes of 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole ligands

The synthesis and structural characterization of Ni^II, Cu^II and Zn^II complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are [M(bipyOXA)₂(H₂O)₂](ClO₄)₂ and [M(pyOXA)₂(ClO₄)₂], respectively (M = Ni, Cu, Zn). X-ray crystallography, ¹H and ¹³C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5 of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains the observed paramagnetism of the d⁸ and d⁹ electron configuration of the nickel(II) and copper(II) complexes, respectively, as determined by the broadening of their NMR spectra.     

Synthesis and characterization of [H2NC(S)NP(S)(OiPr)2] complexes of Co(II), Ni(II), Zn(II) and Cd(II)

Reaction of the potassium salt of the N-thiophosphorylthiourea H₂NC(S)NHP(S)(OiPr)₂ (HL) with Co(II), Ni(II), Zn(II) and Cd(II) cations in aqueous EtOH leads to the chelate complexes [ML₂] all showing a 1,5-S,S′-coordination formed by the CS and PS sulfur atoms of two deprotonated ligands L⁻. The structures of the resulting compounds were studied by IR, UV-Vis, ¹H, ³¹P{¹H} NMR spectroscopy and microanalysis. The metal center is found in a tetrahedral environment in [CoL₂], [ZnL₂] and [CdL₂]. According to NMR and UV-Vis spectroscopy the metal cation of [NiL₂] exhibits square planar coordination geometry in CH₂Cl₂, CHCl₃ and C₆H₆, while tetrahedral geometry is observed in acetone, DMSO and DMF. Regardless of the solvent used for the crystallization of [NiL₂], the molecular structure in the solid is always square planar as was confirmed by XRD of single crystals and magnetic measurements of the polycrystalline material. The magnetic and photoluminescent properties of all complexes are also reported.     

Structural and equilibrium studies on mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with N-(2-benzimidazolyl)methyliminodiacetic acid and typical N, N donor ligands

Mixed ligand complexes: [Co(L)(bipy)] · 3H₂O (1), [Ni(L)(phen)] · H₂O (2), [Cu(L)(phen)] · 3H₂O (3) and [Zn(L)(bipy)] · 3H₂O (4), where L²⁻ = two -COOH deprotonated dianion of N-(2-benzimidazolyl)methyliminodiacetic acid (H₂bzimida, hereafter, H₂L), bipy = 2,2′ bipyridine and phen = 1,10-phenanthroline have been isolated and characterized by elemental analysis, spectral and magnetic measurements and thermal studies. Single crystal X-ray diffraction studies show octahedral geometry for 1, 2 and 4 and square pyramidal geometry for 3. Equilibrium studies in aqueous solution (ionic strength I = 10⁻¹ mol dm⁻³ (NaNO₃), at 25 ± 1 °C) using different molar proportions of M(II):H₂L:B, where M = Co, Ni, Cu and Zn and B = phen, bipy and en (ethylene diamine), however, provides evidence of formation of mononuclear and binuclear binary and mixed ligand complexes: M(L), M(H₋₁L)⁻, M(B)²⁺, M(L)(B), M(H₋₁L)(B)⁻, M₂(H₋₁L)(OH), (B)M(H₋₁L)M(B)⁺, where H₋₁L³⁻ represents two -COOH and the benzimidazole N₁-H deprotonated quadridentate (O⁻, N, O⁻, N), or, quinquedentate (O⁻, N, O⁻, N, N⁻) function of the coordinated ligand H₂L. Binuclear mixed ligand complex formation equilibria: M(L)(B) + M(B)²⁺ ? (B)M(H₋₁L)M(B)⁺ + H⁺ is favoured with higher π-acidity of the B ligands. For Co(II), Ni(II) and Cu(II), these equilibria are accompanied by blue shift of the electronic absorption maxima of M(II) ions, as a negatively charged bridging benzimidazolate moiety provides stronger ligand field than a neutral one. Solution stability of the mixed ligand complexes are in the expected order: Co(II) < Ni(II) < Cu(II) > Zn(II). The Δ log K_M values are less negetive than their statistical values, indicating favoured formation of the mixed ligand complexes over the binary ones.     

Suberoylanilide hydroxamic acid, a potent histone deacetylase inhibitor; its X-ray crystal structure and solid state and solution studies of its Zn(II), Ni(II), Cu(II) and Fe(III) complexes

Reaction of the potent hydroxamate-based histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), with hydrated metal salts of Fe(III), Cu(II), Ni(II) and Zn(II) yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) both in the solid state and in solution. Reaction of the secondary hydroxamic acid, N-Me-SAHA, also yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) in solution. These metal complexes have the hydroxamato moiety coordinated in an O,O’-bidentate fashion. Stability constants of the metal complexes formed with SAHA and N-Me-SAHA in a DMSO/H₂O 70/30%(v/v) mixture are described. A novel crystal structure of SAHA together with a novel synthesis for N-Me-SAHA are also reported.     

Metal complexes of cyclic hydroxamates. Synthesis and crystal structures of 3-hydroxy-2-methyl-3H-quinazolin-4-one (ChaH) and of its Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) complexes

The attempted acetylation of anthranilic hydroxamic acid (2-aminobenzohydroxamic acid) as a possible dual inhibitor of the catalytic sites in prostaglandin-H-synthase (PGHS) gave the cyclic hydroxamic acid 3-hydroxy-2-methyl-3H-quinazolin-4-one (ChaH) which was characterised by spectroscopy and X-ray crystallography. The length of the hydroxamic acid C-N bond, 1.3998(17) Å, in ChaH is longer than normal (∼1.33 Å) indicative of reduced delocalisation of the nitrogen lone pair of electrons into the hydroxamic acid π system. This is confirmed by the appearance of the ν(CO) band at a considerably higher wavenumber in the IR spectrum than normal. The complexes Fe(Cha)₂(Cl)(H₂O)·⁷/₂H₂O, Co(Cha)₂(EtOH)₂, Ni(Cha)₂(EtOH)₂, Cu(Cha)(H₂O)(Cl) and Zn(Cha)₂(H₂O), have been synthesised and their structures determined by X-ray crystallography. The X-ray data confirmed coordination by Cha^- through the carbonyl and deprotonated hydroxamate oxygen in all cases. The M-O (hydroxamate) bonds are shorter than the M-O (carbonyl) bonds by between 0.0930 Å for the Co(II) complex and 0.0448 Å for the Ni(II) complex. The geometries of all complexes conform to the coordination requirements of the particular metal ion involved. Speciation studies for ChaH and its complexes with Ni(II) and Zn(II) were carried out using pH-metric methods. The results show that ChaH is much more acidic than related acyclic hydroxamic acids and that its metal complexes are correspondingly less stable.