One mononuclear cobalt(II) compound of the type [Co(bnzd)₂(NCS)₂(OH₂)₂] (1) (bnzd = benzidine) has been isolated through a single-pot reaction of the molecular building components in MeOH-H₂O solvent mixture at room temperature and characterized by physico-chemical and spectroscopic methods. The crystal structure of complex 1 has an octahedral geometry with trans, trans, trans orientations in the order (N$^a$,N$^a$), (N$^t$,N$^t$), (O$^w$,O$^w$) [N$^a$ = N(amine), N$^t$ = N(thiocyanate) and O$^w$ = O(water)]. In crystalline state, individual units of 1 are associated by weak cooperative O-H…N, O-H…S and N-H…S hydrogen bonds resulting in a 3D network structure. The compound is redox active and shows luminescence in MeOH solution. Thermal decomposition pattern of 1 reveals the presence of two coordinated water molecules. Variabletemperature magnetic susceptibility measurement shows significant orbital contribution and numerical matrix diagonalization method gives the best fit parameters: 𝛼 = A^∗k = 1.43; 𝜆 = −130 cm^-1; 𝛥 = −864 cm^-1; TIP = 0.000975; R = $1.19 \times 10^{−4}$.

Synthesis and characterization of a mononuclear Fe(II) compound [Fe(L)](ClO4)2 (1) [L = N-(1-pyridin-2-yl-phenylidene)-N'-[2-({2-[(1-pyridin-2-ylphenylidene)amino]ethyl}amino)ethyl] ethane-1,2-diamine] (1) is reported. 1 crystallizes in P-1 space group with a = 11.9241(3) Å, b = 12.1994(3) Å and c = 13.0622(4) Å. The binding property of the complex with DNA has been investigated using absorption and emission studies, thermal melting, viscosity experiments and circular dichroism studies. The binding constant ($K_b$) and the linear Stern-Volmer quenching constant ($K_{\text{sv}}$) of the complex have been determined as $3.5 \times 10^3$M^-1 and $2.73 \times 10^4$M^-1, respectively. Spectroscopic and hydrodynamic investigations revealed intercalative mode of binding of 1 with DNA. 1 is also found to induce oxidative cleavage of the supercoiled pUC 18 DNA to its nicked circular form in a concentration dependent manner.

Synthesis and X-ray structural characterization of three complexes of type cis-[M(dafone)₂(NCS)₂] (M = Co(II), 1; Ni(II), 2; Zn(II), 3; dafone = 4,5-diazafluoren-9-one) and a polymer cis-[Cd(dafone)(NCS)₂]$_n$ (4) have been reported. Each of the four complexes is crystallized in orthorhombic crystal system. Structural study reveals that each metal(II) centre in the four complexes adopts distorted octahedral geometry with MN₆ chromophore in 1-3 and MN₄S₂ chromophore in 4. The room temperature steady-state fluorescent intensity of dafone in dimethyl formamide at 402 nm is found to be quenched in these reported dafone complexes (1-4).

A new nickel(II) complex [Ni(L)] (1) [H₂L = 1,1′-(1E,1′E)-(propane-1,3-diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene)dinaphthalen-2-ol] was synthesized and X-ray crystallographically characterized. 1 crystallizes in orthorhombic crystal system with Cmc21 space group. The unit cell dimensions of the crystal are a = 30.6345(4)Å, b = 8.45340(10)Å, c = 7.75180(10)Å. Structural analysis reveals a tetradentate chelation behaviour of the dianionic ligand H₂L having a distorted square planar geometry around Ni(II) with NiN₂O₂ chromophore in 1. The title complex 1 behaves as an effective catalyst towards oxidation of 3,5-ditertiarybutyl catechol (3,5-DTBC) in acetonitrile to its corresponding quinone derivative in air. The reaction follows first-order reaction kinetics with rate constant 4.28 × 10⁻⁵ min^-1. The reaction follows Michaelis-Menten enzymatic kinetics with a turnover number of ($K_{cat}$) 140.72 h^-1 in acetonitrile.

Two mononuclear compounds of the types pentacoordinated [Zn(L1)(OAc)]PF₆·H₂O (1) and hexacoordinated [Zn(L2)(OAc)]PF₆ (2) [L1 = N,N′-(bis(pyridin-2-yl)formylidene)-1,3-propanediamine; L2 = N,N′-(bis(pyridin-2-yl)benzylidene)-1,4-butanediamine] have been synthesized using one-pot reactions of a 1:1:1 molar ratio of Zn(OAc)₂.2H₂O, L1/L2, ammonium hexafluorophosphate in MeOH at room temperature. Compounds 1 and 2 are characterized on the basis of microanalytical, spectroscopic, thermal and other physicochemical results. Single crystal X-ray structural study reveals that the zinc(II) centre in 1 is coordinated by four N atoms of the Schiff base L1 and one O atom of terminal acetate with a ZnN₄O chromophore, whereas in 2, the zinc(II) centre is bound by four N atoms of the tailored Schiff base L2 and two O atoms of the chelated acetate. In the crystalline state, mononuclear units in 1 are engaged in weak cooperative intermolecular O-H⋯O and C-H⋯F hydrogen bonds affording a 1D chain. The individual units of 2 are packed by 𝜋 ⋯ 𝜋 and anion⋯ 𝜋 interactions to form a 2D sheet structure. The complexes show reasonable thermal stabilities and display intraligand 𝜋 $\to$ 𝜋^∗ fluorescence in solid state at room temperature.

Synthesis and characterization of a mononuclear Ni(II) compound [Ni(L)(H₂O)₂](NO₃)₂ [L = N,N'-bis((pyridine-2-yl)phenylidene)-1,3-diaminopropan-2-ol] (1) is reported. 1 crystallizes in triclinic P-1 space group with a = 8.1911(2) Å, b = 11.6624(3) Å, c = 16.5356(4) Å and𝛼 = 108.8120(10)° , 𝛽 = 91.2010(10)° , 𝛾 = 91.1500(10)° . The binding property of the complex with DNA has been investigated using absorption and emission studies, and viscosity experiment. The binding constant (K_b) and the linear Stern-Volmer quenching constant (K_sv) of the complex have been determined as 9.23 × 10 4 M⁻¹ and 2.0 × 10 4 M⁻¹, respectively. Spectroscopic and hydrodynamic investigations revealed groove or electrostatic nature of binding of 1 with DNA. 1 is also found to induce oxidative cleavage of the supercoiled pUC 18 DNA to its nicked circular form in a concentration dependent manner.

One nickel(II)-dithiolene complex bis(1,2-diphenylethylene-1,2-dithiolate)nickel(II) has been synthesized and crystallographically characterized. The complex is square planar at its metal centre. The compound in its solid-state is found to conduct electricity at room temperature. Variation of conduction is also found on raising the temperature. Instead of a metal…metal orbital overlap, the electron density is expected here to pass through some other covalent and electrostatic bonds of the compound in solid-state resulting in electricity conduction.