Basis Of Electronic Spectral Data Research Articles

Synthesis, Characterization, Antimicrobial, Anti-Diabetic, Anti-Inflammatory and Anti-Cancer studies of Schiff Base metal(II) complexes derived from 4-hydroxy-3,5 dimethoxybenzaldehyde and 3-aminoquinoline

The new Schiff base ligand (E)-2,6-dimethoxy-4-((quinolin-3-ylimino)methyl )phenol (HL) and its Cu(II), Co(II), Ni(II) and Zn(II) metal complexes were synthesized and characterized by various spectroscopic (UV-vis, IR, NMR), SEM, Mass and magnetic susceptibility measurement. The ligand (HL) was synthesized by condensation of 4-hydroxy-3,5-dimethoxybenzaldehyde and 3-aminoquinoline. On the basis of electronic spectral data and magnetic susceptibility measurement the tetrahedral geometry has been proposed for all the complexes. The ligand and metal complexes have been screened for their antimicrobial activities against bacteria ( Staphylococcus aureus, Escherichia coli) and antifungal activity against the fungi ( Candida albicans) . Further the ligand and its Cu(II) complex was also screened for anticancer activity on human cancer cell lines such as breast (MCF7) by MTT assay method . Interestingly, Cu(II) complex shows better anticancer activity than the free schiffbase ligand . The in vitro anti-inflammatory and anti-diabetic activities of the ligand and Cu(II) complex were studied. The Cu(II) complex show higher inhibition activity than that of the free ligand.

SYNTHESIS, CHARACTERIZATION AND ANTIMICROBIAL ACTIVITY OF MIXED LIGAND COMPLEXES OF INNER TRANSITION METAL WITH L-ASCORBIC ACID, ITS DERIVATIVE AND ADENINE BASE

Complexes of La(III), Ce(III), Nd(III) and Gd(III) with mixed Ligand of 2-aminopurene and L-ascorbic acid derivative were prepared in aqueous ethanol. The two types of mixed ligand complexes [M(AA)(ADE)(H2O)2]+and [M(IPAA) (ADE)(H2O)2]+ have been synthesized. All complexes were characterized on the basis of elemental analysis, molar conductance, magnetic susceptibility measurements, IR, UV-VISSIBLE, NMR, ESR and Mass spectral studies. IR spectra of the complexes reveal that the complex formation occurred through both nitrogen and oxygen atoms. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for the complexes. The ESR spectral data of the M (III) complexes showed that the metal-ligand bonds have considerable covalent character. The electrochemical behaviour of mixed Ligand M(III) complexes was studied which showed that complexes of IPAA appear at more positive potential as compared to those for corresponding AA complexes. In addition, biological activity of the synthesised metal complexes against Salmonella typhi, Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae bacteria and Aspergillus niger, Aspergillus flavus, Fusarium oxisporium and Trichoderma viride fungi respectively were examined in-vitro. Some of the metal complex displayed pounced biological activity.

Synthesis, spectroscopic characterization, dyeing performance and corrosion inhibition study of transition metal complexes of a novel azo derivative formed from 2-aminothiophene

Abstract A novel bi-heterocyclic azo derivative is synthesized by the diazo-coupling reaction of 2-amino-3-carbethoxy-4,5-dimethylthiophene and 2-amino-4-phenylthiazole and its ligational behaviour towards Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) salts is investigated. The complexes are characterized on the basis of elemental analyses, molar conductance, magnetic susceptibility measurements, UV-Visible, IR and 1H NMR spectral data. Spectral data have indicated that the ligand is coordinated to the metal ion in a neutral bidentate manner. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry is proposed for each complex. Thermogravimetric studies of copper(II) complex are used to characterize the complex. The dyeing performance of the azo derivative and its metal complexes is examined on silk fabric. The corrosion inhibition efficiency of the ligand and its metal complex in 0.5 M H2SO4 for mild steel coupons is monitored by weight-loss and open circuit potential measurements .

Molecular docking, photocatalytic activity and biomedical investigations of some metal complexes

Metal(II)–(furfural-L-his) complexes with a potentially bidentate furfural-L-his have been synthesized. Furfural-L-his and its Co/Ni/Cu/Zn(II)–(furfural-L-his) complexes have been optimized by DFT. The structural features were determined from their elemental analyses, molar conductance, magnetic, UV–Vis, IR, mass, 1H NMR and EPR spectral studies. On the basis of electronic spectral data and magnetic measurements, suitable geometry has been proposed for each complex. The redox behaviour of Cu(II)–(furfural-L-his) complex has been studied by cyclic voltammetry. Thermal decomposition profiles are consistent with the proposed formulation of Co/Ni/Cu/Zn(II)–(furfural-L-his) complexes. PXRD studies reveal that furfural-L-his and Zn(II)–(furfural-L-his) complex are of nanomeric structure. SEM images of furfural-L-his exhibit flake-like morphology. Photodegradation of methylene blue dye indicates that they are photocatalyticaly efficient. NBO and NPA shown considerable reduction in the formal charge on metal ions. Docking analysis with EGFR and cyclooxygenase-2 receptor has been performed to find the best binding energy. Antimicrobial, antioxidant and anti-inflammatory activity against standard at variable concentrations revealed that the Co/Ni/Cu/Zn(II)–(furfural-L-his) complexes show enhanced antimicrobial, free radical scavenging and anti-inflammatory activities as compared to furfural-L-his. Furfural-L-his and Cu(II)–(furfural-L-his) complex have been tested against human ovarian cancer cells, which showed that Cu(II)–(furfural-L-his) complex exhibited promising anticancer activity. Communicated by Ramaswamy H. Sarma

Characterization and Antimicrobial Activity of New Cobalt-Nickel-Copper (Ii) Complexes with Benzoin and 3-Amino Benzoic Acid

Condensation of Benzoin and 3-Amino Benzoic acid yields a novel Schiff base ligand i.e. 3-(2-hydroxy-1,2-diphenylethylideneamino) benzoic acid of type HL. Also, we synthesized the [C 42 H 32 N 2 O 6 Co.2H 2 O], Ni(II) [C 42 H 32 N 2 O 6 Ni.2H 2 O] and Cu(II) [C 42 H 32 N 2 O 6 Cu.2H 2 O] metal complexes. The characterization study of the ligand and metal complexes were carried out by elemental analysis, UV-visible, IR, 1 H NMR spectroscopy, thermal studies and magnetic susceptibility measurement. On the basis of electronic spectral data and magnetic susceptibility measurement the octahedral geometry has been proposed for Co(II), Ni(II) and Cu(II) complexes. The X-ray diffraction study reveals that ligand and all the complexes are crystalline in nature and also possess orthorhombic structure. In vitro antibacterial and antifungal activities assay were performed for the ligand and metal(II) complexes. The micro-organisms Staphylococcus aureus and Pseudomonas aeruginosa and the fungi Candida albicans and Aspergillus Niger were used. Our findings are very promising in the field of clinical research as the highest antimicrobial and antifungal activity is observed for copper complexes.

Supramolecular structure, spectroscopic, thermal studies and antimicrobial activities of Schiff base complexes

Metal(II) complexes of 4-(((2-hydroxynaphthalen-1-yl)methylene)amino)-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one (HL) were prepared, and their compositions and physicochemical properties were characterized on the basis of elemental analysis, with1HNMR, UV–Vis, IR, mass spectroscopy and thermogravimetric analysis. All results confirm that the novel complexes have a 1:1 (M:HL) stoichiometric formulae [M(HL)Cl2] (M = Cu(II)(1), Cd(II)(5)), [Cu(L)(O2NO)(OH2)2](2), [Cu(HL)(OSO3)(OH2)3]2H2O(3), [Co(HL)Cl2(OH2)2]3H2O(4), and the ligand behaves as a neutral/monobasic bidentate/tridentate forming a five/six-membered chelating ring towards the metal ions, bonding through azomethine nitrogen, exocyclic carbonyl oxygen, and/or deprotonated phenolic oxygen atoms. The XRD studies show that both the ligand and Cu(II) complex (1) show polycrystalline with monoclinic crystal structure. The molar conductivities show that all the complexes are non-electrolytes. On the basis of electronic spectral data and magnetic susceptibility measurements, a suitable geometry has been proposed. The trend in g values (gll > g⊥ > 2.0023) suggest that the unpaired electron on copper has a \(d_{{x^{2} - y^{2} }}\) character, and the complex (1) has a square planar, while complexes (2) and (3) have a tetragonal distorted octahedral geometry. The molecular and electronic structures of the ligand (HL) and its complexes (1–5) have been discussed. Molecular docking was used to predict the binding between HL ligand and the receptors of the crystal structure of Escherichia coli (E. coli) (3t88) and the crystal structure of Staphylococcus aureus (S. aureus) (3q8u). The activation thermodynamic parameters, such as activation energy (Ea), enthalpy (ΔH), entropy (ΔS), and Gibbs free energy change of the decomposition (ΔG) are calculated using Coats–Redfern and Horowitz–Metzger methods. The ligand and its metal complexes (1–5) showed antimicrobial activity against bacterial species such as Gram positive bacteria (Bacillus cereus and S. aureus), Gram negative bacteria (E. coli and Klebsiella pneumoniae) and fungi (Aspergillus niger and Alternaria alternata); the complexes exhibited higher activity than the ligand.

Polymer complexes. LXVI, thermal, spectroscopic studies and supramolecular structure of N-[β-(ethylamino)] acrylamide polymer complexes

Novel polymeric complexes with a potentially bidentate ligand N-[β-(ethylamino)] acrylamide (EA), formed by amidation of 1,2-diaminoethane with acryloyl chloride were synthesized and characterized on the basis of elemental analyses, IR, 1H NMR, UV–Vis, magnetic susceptibility measurements, ESR, molar conductance and thermal analysis. Molecular docking was used to predict the binding between EA ligand (keto and enol forms) and the receptor of prostate cancer mutant 2q7k-hormone and the receptor of breast cancer mutant 3hb5-oxidoreductase. The molecular and electronic structures of the different forms of the ligand are optimized theoretically and the quantum chemical parameters are calculated. The molar conductance data reveal that all the polymer complexes are non-electrolytes. Ligand (EA) has been shown to behave as neutral bidentate via its nitrogen atom of NH2 group and nitrogen atom of NH group in polymer complexes. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry has been proposed for each polymeric complex. The ESR spectral data of the Cu(II) complex showed that the metal-ligand bonds have considerable covalent character. The activation energies of the degradation of the polymer complexes were calculated.

Geometrical structures, molecular docking, spectroscopic characterization of mixed ligand and Schiff base metal complexes

The mixed ligand mononuclear complexes were synthesized by reaction of MX2·nH2O (M=Cu(II) or Zn(II)) with 4-aminoantipyrine (Lp) and heterocyclic base 1,10-phenanthroline (L). Molecular docking was used to predict the binding between the Schiff base ligands (L1 and L2) and the receptors of prostate cancer mutant 2q7k-hormone and breast cancer mutant 3hb5-oxidoreductase. Also the corresponding Schiff base mixed ligand complexes were prepared by condensation of mixed ligand complex [Zn(Lp)(L)]Cl2 with 1,2-diaminoethane and/or benzaldehyde. All the compounds have been characterized by elemental analysis, conductance measurements, magnetic moments, spectral (UV–vis, IR, 1H &13C NMR and ESR) and thermal studies. The IR showed that the ligand (Lp) act as neutral bidentate through the amino nitrogen atom and carbonyl oxygen moiety. The presence of anion, viz., Cl−, NO2−, SO4−− or NCS− in the coordination sphere is also inferred by the IR spectral data and conductance measurements. The molar conductivities show that all the complexes are non-electrolytes except Zn(II) complexes. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry has been proposed for each complex. The molecular and electronic structures of the Schiff base ligands (L1 and L2) and complexes (1–4) are optimized theoretically and the quantum chemical parameters are calculated. It was found that the ligand (L1) is more stable than the other ligand (L2). The ESR spectral data of copper complexes provided information about their structure on the basis of Hamiltonian parameters and degree of covalency. ESR calculations support the characterization of the structures of the complex geometries.

Polymer complexes. LV. Spectroscopic, thermal studies, and coordination of metal ions N-[3-(5-amino-1,2,4-triazolo)] acrylamide polymer complexes

Novel polymeric complexes with a potentially bidentate ligand formed by amidation of 3,5-diamino-1,2,4-triazole with acryloyl chloride were synthesized and characterized on the basis of elemental analyses, IR, 1H-NMR, UV-Vis, magnetic susceptibility measurements, molar conductance, and thermal analyses. The molar conductance data reveal that all the polymer complexes are non-electrolytes. Spectral studies reveal that the free ligand coordinates bidentate to the metal ion through the oxygen of the carbonyl and azomethine of the heterocyclic ring. Elemental analyses of the polychelates indicate the metal to ligand ratio of 1 : 1/1 : 2. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry has been proposed for each polymeric complex. The electron spin resonance spectral data of the Cu(II) complex showed that the metal–ligand bonds have considerable covalent character. The thermal behavior of these chelates shows that the polymer complexes lose coordinated water in the first step immediately followed by decomposition of the anions and ligand molecules in a subsequent step.

Synthesis, characterization, thermal decomposition studies and dyeing properties of some novel transition metal complexes of an azo derivative formed from 2-aminothiophene

An azo derivative was synthesized by coupling diazotized 2-amino-3-carbethoxy-4,5-dimethylthiophene with 2-naphthol and this new ligand formed a series of metal complexes with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) salts. These complexes were characterized on the basis of elemental analyses, molar conductance, magnetic susceptibility measurements, UV-Visible, IR and 1H NMR spectral data. Analytical data revealed that all the complexes exhibited 1: 1 metal-ligand ratio. Spectral studies showed that the ligand existed in an internally hydrogen bonded azo-enol form rather than the keto-hydrazone form and coordinated to the metal ion in a tridentate fashion. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry was proposed for each complex. The ligand and its cobalt(II) complex were subjected to X-ray diffraction study. The thermal behaviour of the ligand and its copper(II) complex was examined by thermogravimetry. The ligand and its copper(II) and nickel(II) complexes were applied to silk fabric and their fastness properties were evaluated.

Synthesis, characterization and antimicrobial activity of new cobalt (II), nickel (II) and copper (II) complexes with 2-(2-hydroxy-1, 2-diphenylethylideneamino) benzoic acid

Abstract The new Schiff base ligand 2-(2-hydroxy-1, 2-diphenylethylideneamino) benzoic acid [HL] and its Co(II), Ni(II) and Cu(II) metal complexes were synthesized. The ligand and metal complexes were characterized by elemental analysis, UV-visible, IR, 1H NMR spectroscopy, thermal studies and magnetic susceptibility measurement. The ligand [HL] was synthesized by condensation of benzoin and 2-amino benzoic acid. On the basis of electronic spectral data and magnetic susceptibility measurement the octahedral geometry has been proposed for Cu(II), Ni(II) and Co(II) complexes. The ligand and metal (II) complexes were also screened for their antimicrobial activity against microorganisms Staphylococcus aureus and Pseudomonas aeruginosa and antifungal activity against the fungi Candida albicans and Aspergillus niger.

Polymer complexes. LIII. Supramolecular coordination modes and structural of novel sulphadrug complexes

Novel polymer complexes of copper(II), palladium(II), platinum(II) and cadmium(II) containing homopolymer [4-acrylamido benzene sulphonyl guanidine; (HL)] and various anions (SO 4 2−, CH 3COO −, NO 3 −, Br − or Cl −) have been designed and carried out. Their structures were investigated by elemental analyses, spectral (IR, UV–vis, 1H NMR and ESR) and magnetic moments. The modes of interactions between the ligand and the metals were discussed, where oxygen (of O S O group) and nitrogen atom [of imino nitrogen (NH/N) of the guanidine group] are involved in chelation. The homopolymer shows two types of coordination behaviour. In mononuclear polymer complexes 4 and 6– 10, it acts as a neutral bidentate ligand chelated through the NH and O atoms, whereas in the polymer complexes 1– 3, 5 and 11, monobasic bidentate ligand is coordinated through the –N and –O atoms. The poly-chelates are of 1:1/1:2 (metal–homopolymer) stoichiometry and exhibit four coordination. On the basis of electronic spectral data and magnetic susceptibility measurement square planar geometry has been proposed. The ESR spectral data provided information about their structure on the basis Hamiltonian parameters and degree of covalency. From the electron paramagnetic resonance and spectral data, the orbital reduction factors were calculated.

Synthesis, structural characterization, thermal and electrochemical studies of Mn(II), Co(II), Ni(II) and Cu(II) complexes containing thiazolylazo ligands

Some thiazolylazo derivatives and their metal complexes of the type [M(L)(H2O)Cl]; M=Mn(II), Co(II), Ni(II), Cu(II) and L=6-(2′-thiazolylazo)-2-mercapto-quinazolin-4-one (HL1), 6-(4′-phenyl-2′-thiazolylazo)-2-mercapto-quinazolin-4-one (HL2), 6-(2′-thiazolylazo)-2-mercapto-3-(m-tolyl)-quinazolin-4-one (HL3) and 6-(4′-phenyl-2′-thiazolylazo)-2-mercapto-3-(m-tolyl)-quinazolin-4-one (HL4) have been prepared. All the complexes were characterized on the basis of elemental analysis, molar conductance, magnetic moment, IR, UV–vis, ESR, TG-DTA and powder X-ray diffraction studies. IR spectra of these complexes reveal that the complex formation occurred through thiazole nitrogen, azo nitrogen, imino nitrogen and sulfur atom of the ligands. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for the Mn(II), Co(II) and Ni(II) complexes and distorted octahedral geometry for the Cu(II) complexes. Electrochemical behavior of Ni(II) complexes exhibit quasireversible oxidation corresponding to Ni(III)/Ni(II) couple along with ligand reduction. X-ray diffraction study is used to elucidate the crystal structure of the complexes.

Synthesis, spectroscopic characterization, electrochemical behaviour and thermal decomposition studies of some transition metal complexes with an azo derivative

Complexes of manganese(II), cobalt(II), nickel(II), copper(II) and zinc(II) with a novel heterocyclic azo derivative, formed by coupling diazotized 2-amino-3-carbethoxy-4,5-dimethylthiophene with acetylacetone were synthesized and characterized on the basis of elemental analyses, molar conductance, magnetic susceptibility measurements, UV-vis, IR, (1)H NMR and EPR spectral data. Spectral studies revealed that the ligand existed in an internally hydrogen bonded azo-enol form rather than the keto-hydrazone form and coordinated to the metal ion in a tridentate fashion. Analytical data revealed that all the complexes exhibited 1:1 metal-ligand ratio. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry was proposed for each complex. The nickel(II) complex has undergone facile transesterification reaction when refluxed in methanol for a long period. The ligand and the copper(II) complex were subjected to X-ray diffraction study. The electrochemical behaviour of copper(II) complex was investigated by cyclic voltammetry. The thermal behaviour of the same complex was also examined by thermogravimetry.

Synthesis, structural characterization, thermal and electrochemical studies of mixed ligand Cu(II) complexes containing 2-phenyl-3-(benzylamino)-1,2-dihydroquinazoline-4-(3H)-one and bidentate N-donor ligands

Some mixed ligand Cu(II) complexes of the type [Cu( L)( en)X 2] ( 1a–3a), [Cu( L)( en)](ClO 4) 2 ( 4a), [Cu( L)( phen)X 2] ( 1b–3b) and [Cu( L)( phen)](ClO 4) 2 ( 4b) [where L = 2-phenyl-3-(benzylamino)-1,2-dihydroquinazoline-4-(3H)-one; en = ethylenediamine; phen = 1,10-phenanthroline; X = Cl −, N 3 − and NCS −] have been prepared. The complexes were characterized on the basis of elemental analysis, molar conductance, magnetic moment, IR, UV–vis, mass, ESR and thermal studies. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for 1a–3a and 1b–3b and square-planer geometry for 4a and 4b. The ESR spectral data of complexes provided information about their structure on the basis of Hamiltonian parameters and degree of covalency. The electrochemical behaviour of mixed ligand Cu(II) complexes was studied which showed that complexes of phen appear at more positive potential as compared to those for corresponding en complexes.

Characterization and electrochemical studies of Mn(II), Co(II), Ni(II) and Cu(II) complexes with 2-mercapto-3-substituted-quinazolin-4-one and 1,10-phenanthroline or ethylenediamine as ligands

Some mixed ligand complexes of the type [M(L 1 or L 2)( phen or en)(H 2O)Cl], where M = Mn(II), Co(II), Ni(II) and Cu(II); HL 1 = 2-mercapto-quinazolin-4-one; HL 2 = 2-mercapto-3-phenyl-quinazolin-4-one; phen = 1,10-phenanthroline; en = ethylenediamine have been prepared. All complexes were characterized on the basis of elemental analysis, molar conductance, magnetic susceptibility measurements, IR, UV–vis, ESR and powder X-ray diffraction studies. IR spectra of these complexes reveal that the complex formation occurred through both nitrogen and sulphur atoms. On the basis of electronic spectral data and magnetic susceptibility measurement octahedral geometry has been proposed for the complexes. The ESR spectral data of the Cu(II) complexes showed that the metal–ligand bonds have considerable covalent character. X-ray diffraction studies of Cu(II) complexes are used to elucidate the crystal structure. The electrochemical behaviour of mixed ligand Ni(II) complexes was studied which showed that complexes of phen appear at more positive potential as compared to those for corresponding en complexes.

Synthesis, spectroscopic characterization, electrochemical behaviour, reactivity and antibacterial activity of some transition metal complexes with 2-( N-salicylideneamino)-3-carboxyethyl-4,5-dimethylthiophene

Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with a potentially tridentate Schiff base, formed by condensation of 2-amino-3-carboxyethyl-4,5-dimethylthiophene with salicylaldehyde were synthesized and characterized on the basis of elemental analyses, molar conductance values, magnetic susceptibility measurements, UV–vis, IR, EPR and NMR spectral data, wherever possible and applicable. Spectral studies reveal that the free ligand exists in a bifunctionally hydrogen bonded manner and coordinates to the metal ion in a tridentate fashion through the deprotonated phenolate oxygen, azomethine nitrogen and ester carbonyl group. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry has been proposed for each complex. The EPR spectral data of the Cu(II) complex showed that the metal–ligand bonds have considerable covalent character. The Ni(II) complex has undergone facile transesterification reaction when refluxed in methanol for a lengthy period. X-ray diffraction studies of Cu(II) complex showed that the complex has an orthorhombic crystal lattice. In view of the biological activity of thiophene derivatives, the ligand and the complexes were subjected to antibacterial screening. It has been observed that the antibacterial activity of the ligand increased on chelation with metal ion.

ChemInform Abstract: ELECTRONIC SPECTRA OF SOME RHODIUM(III) COMPLEXES OF SATURATED CYCLIC TETRAMINES

AbstractStereochemische Zuordnung der Komplexe (I), die nach bekannten Methoden dargestellt werden.

Electronic spectra of some rhodium(III) complexes of saturated cyclic tetramines

Rhodium(III) complexes of 12-, 13-, 14-, 15-, and 16-membered saturated cyclic tetramines have been synthesised with Cl– and PF6– as counter ions. Molecular formulae for the complexes have been arrived at on the basis of analytical and conductance studies. The stereochemistry of the complexes has been assigned on the basis of electronic spectral data. It is observed that the rhodium (III) complexes are cis with 12- and 13-, both cis and trans with 14-, and only trans with 15- and 16-membered ring macrocyclic amines.

Syntheses and solution equilibria of nickel(II) chelates. II. Dinuclear mixed ligand species

A new class of mixed ligand complexes of nickel(II) are reported. They have the stoichiometry LKNi where L is a potentially tridentate Schiff base anion and K is the corresponding bidentate carbonyl species. The complexes show clearly defined CO and CN stretches in the infrared. Where solubility permitted the determination of molecular weights, the compounds were found to be dimeric in freezing benzene and/or bromoform. The complexes are uniformly high-spin and have magnetic moments in the range 3.0–3.2 BM. On the basis of electronic spectral data it is concluded that the compounds have, in general, pseudooctahedral environment around each nickel(II) in the solid state. In one case the environment is believed to be square pyramidal. Structures which accommodate these geometrical requirements are suggested within the dimeric framework. Solution spectra are solvent and temperature dependent. The results are interpreted in terms of equilibria involving the two geometries. The possible occurrence of monomeric species at low concentration is examined