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We have synthesized two mononuclear cobalt(II) complexes (1 and 2) of pyrazole-based bidentate (NN) and tridentate (NNN) tripodal ligands. X-ray crystal structure determination reveals that complex 1 has a tetrahedral geometry, while complex 2 has a trigonal–bipyramidal geometry. Both the complexes have been characterized by variable-temperature magnetic measurements between 2 and 300 K. A weak ferromagnetic exchange interaction (J = +1.5 cm−1) is observed for complex 2. Due to the presence of supramolecular CH···Cl and π···π interactions, a good magnetostructural correlation was found between the D parameter and angular distortion (δ) for complex 1 and related complexes reported in the literature.

Blue [{Cu(2,2'-bipy)(2)}(2){alpha-SiW12O40}] (bipy = bipyridyl) (1) and pale yellow [Mn(2,2'-bipy)(3)](2)[alpha-SiW12O40] (2) have been synthesized hydrothermally and characterized by IR spectroscopy and single crystal X-ray structure analysis. In 1, the [alpha-SiW12O40](4-) ion acts as a bridge between the two [{Cu(2,2'-bipy)(2)](2+) moieties via coordination through the terminal oxygen atoms, while in 2, the [Mn(2,2'-bipy)(3)](2+) ion balances the charge on the polyoxo anion without forming any covalent bond. To the best of our knowledge, this is the first example of transition metal-mediated transformation of [alpha-SiW9O34](10-) to [alpha-SiW12O40](4-).

A novel bis(glycinato)copper(II)paradodecatungstate Na8[{Cu(gly)2}2]-{H2(H2W12O42)}] · 24H2O (1) has been synthesized under hydrothermal conditions. The crystal structure of 1 reveals an infinite one-dimensional chain along the [100] direction and is built from paradodecatungstate (H2W12O42)10− clusters joined through [Cu(gly)2] moieties. Parallel chains are interlinked by NaO6 octahedra to generate a two-dimensional network.

Two cobalt complexes, [Co(LSe)(phen)] · CH2Cl2 (1) and [Co(LSe)(N,N-Me2en)(CH3COO−)] (2) have been synthesized and characterized by elemental analyses, magnetic measurements, i.r. studies etc. Single crystal X- ray studies reveal that in complex (1) cobalt atom is in +2 oxidation state with trigonal bipyramidal geometry, while in complex (2) it is in +3 oxidation state and surrounded octahedrally. The asymmetric unit of complex (2) contains two crystallographically independent discrete molecules. Complex (1) was found to be paramagnetic with μeff = 2.19 BM indicating a low spin cobalt(II) d7 system, whereas complex (2) is found to be diamagnetic with cobalt(III) in low spin d6 state. The kinetic studies on the reduction of (2) by ascorbic acid in 80% MeCN–20% H2O (v/v) at 25 °C reveal that the reaction proceeds through the rapid formation of inner-sphere adduct, probably by replacing the loosely coordinated AcO− group, followed by electron transfer in a slow step and is supported by a large Q (formation constant) value.

The e.p.r. studies of AgII complexes of three tetraazamacrocycles, 1,4,8,11-tetraazacyclo-tetradecane (cyclam)(I), 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-teradecane-4,11-diene (Me6CD)(II) and 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-teradecane (Me6Cy)(III) and the electron transfer between [AgII(cyclam)]2+ and thiosulfate ion are described. The e.p.r. studies reveal that the spectra are almost the same as those reported earlier, particularly, for polycrystalline material and are typical of a d 9 planar AgII complexes. Previous e.p.r. studies on these square planar polycrystalline complexes showed no ligand hyperfine splitting. Reinvestigation of the e.p.r. spectra of these complexes in both the solid state and in solution at room (T∼ 297 K) and at low (T= 120 K) temperature reveals resolved hyperfine structures in solution for [Ag(Me6CD)]2+ and [Ag(Me6Cy)]2+ complexes. Surprisingly, such a structure was not observed in solutions of [Ag(cyclam)]2+. Computer simulations of the hyperfine structure observed in solutions are in good agreement with structural formulae proposed. The oxidation of thiosulfate ion by [Ag(cyclam)]2+ follows the rate law:−d/dt[AgII(cyclam)2+]=(k 1 Q 1[H+]+k 2 Q 2 K a2)/([H+]+K a2)[AgII(cyclam)2+][S2O3 2−] and an inner-sphere mechanism is proposed, based on the spectral and kinetic evidence.

The oxidation of CoIIW by bromine(V) is a complex process involving an induction period. The reaction was found to be first-order in both [CoIIW] and [Brv], and exhibits a complex dependence on [H+]. These observations were successfully explained by considering HBrO2, one of the intermediates formed in the direct but slow reaction between CoIIW and bromine(V), as the reacting species. The first-order limiting dependence in [H+] was due to the involvement of a protic equilibrium of HBrO2. The induction period appears due to the scavenging effect of Br− inadvertently present in the medium. It appears to be the first report where HBrO2 was found to be the reacting intermediate in the oxidation of metal ions and complexes by BrO−3.

An N donor tetradentate manganese complex, [MnII(bispicen)Cl2] (A) [bispicen = N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine)] catalyses the oxidative cleavage of 4-tert-butylcatechol (1) in the presence of O2. The oxygenated products were isolated by t.l.c. and column chromatography and characterised by 1H-, 13C-n.m.r., DEPT, i.r. and u.v.–vis. spectroscopy. The oxygenated products as well as other spectral evidence suggest that the oxygenation occurs via a 4-tert-butylsemiquinone bound complex, [MnII(bispicen)(4-sq)]+ (4-sq = 4-tert-butylsemiquinone). 1H-n.m.r. spectroscopy suggests that the oxygenation follows multiple pathways. Isolation of the products suggests that the oxygenations proceed in an extradiol fashion and a probable mechanism is suggested. Some intradiol cleaved products have also been detected. E.s.r. spectroscopy suggests that manganese(II) is ultimately converted into the manganese(IV) species.

The kinetics of the interaction of trans-[Pd(PN)2Cl2] (PN = pyridoxine) with nitrogen heterocycles e.g., imidazole, benzimidazole and pyrazole, in dimethyl sulfoxide (DMSO) have been carried out at 30 °C using the stopped-flow technique and u.v.–vis. spectrophotometry. Trans-[Pd(PN)2(DMSO)2] was assumed to be the actual reactive species in solution. Four reaction steps can be proposed from an analysis of absorbance-time data and where [Pd(HB)4]Cl2 (HB – heterocyclic bases) is the final reaction product. Rate constants for each step have been evaluated and interpreted.

Electron transfer between [FeIII(L2)]+ and sulphur(IV) has been proposed to proceed via an inner-sphere mechanism involving formation of a transient hydrogen-bonded intermediate between the acidic proton of SO2 · xH2O/HSO3− and the oximato oxygen of the coordinated ligand, providing the ready availability of the proton for the reduced complex. In the case of SO32−, this is not possible and the reaction is believed to proceed via an outer-sphere scheme.