Synthetic ferroverdins, NaFe¹¹(RQ)₃ afford trinuclear M[Fe(RQ)₃]₂ upon reaction with bivalent ions (M²⁺). A probable bonding mode is considered. Ferric ions oxidise ferroverdin to ferriverdin, Fe^III(RQ)₃. Bothfac andmer isomers can be observed for ferro- and ferriverdins but at equilibrium the preferred isomers arefac for ferro- andmer for ferriverdin. Mismatch in geometry with oxidation state gets spontaneously corrected by isomerisation. Equilibrium constants, isomerisation rates, redox potentials and EPR spectra are examined. A study on the pattern of isomer population as a function of oxidation for other 3d analogues of ferro- and ferriverdin is initiated.

The coordination chemistry of copper(II) with tridentate aroylhydrazones is briefly discussed in this article. Two types of aroylhydrazones derived from aroylhydrazines and ortho-hydroxy aldehydes or 2-pyridine-carboxaldehyde have been used. The characterization of the complexes has been performed with the help of various physico-chemical techniques. Solid state structural patterns have been established by X-ray crystallography. In the solid state, structural varieties of these complexes are seen to range from monomeric, dimeric, polymeric and one-dimensional self-assembly via hydrogen bonds and π-π interactions. EPR spectroscopy and variable temperature magnetic susceptibility measurements have been used to reveal the nature of the coordination geometry and magnetic characteristics of these complexes.

A facile method for the synthesis of a series of cyclometallated ruthenium(II) carbonyl complexes with 1-pyrenaldehyde 4-R-3-thiosemicarbazones (H₂Lⁿ where the two H’s represent the dissociable thioamide and pyrenyl protons; R = H, Me and Ph) has been described. The characterization of the complexes having the general molecular formula trans-[Ru(Lⁿ)(CO)(EPh₃3)₂] (where E = P or As) were accomplished by elemental (CHN) analysis, magnetic susceptibility and spectroscopic (ESI-MS, IR, UV-Vis, emission and ¹H-NMR) measurements. Electronic spectra of the complexes display multiple strong absorptions in the range 440–224 nm due to intraligand transitions. All the complexes exhibit emission bands that are characteristic of ligand centred emissive states. X-ray diffraction studies with representative complexes reveal a pincer-like 5,5-membered fused chelate rings forming CNS coordination mode of the thiosemicarbazonate ligand (Lⁿ)²⁻ via regioselective activation of 1-pyrenyl ortho C–H and formation of a distorted octahedral C₂NSE₂ coordination sphere around the ruthenium(II) centre.

Reactions of CeCl₃·6H₂O and Gd(NO₃)₃·5H₂O with biacetyl bis(benzoylhydrazone) (H₂babh) and KOH in 1:2:2 mole ratio in methanol afford the complexes [Ce(babh)₂] (1) and [Gd(babh)(Hbabh)]·H₂O (2·H₂O), respectively in good yields. Characterization of the complexes has been performed with the help of elemental analysis, magnetic susceptibility, spectroscopic (IR, UV-Vis, EPR and NMR) and X-ray crystallo-graphic measurements. 1 is diamagnetic and NMR active, while 2·H₂O is paramagnetic (𝜇_eff = 8.03 𝜇_B at 300 K) and EPR active. The complexes crystallize as 1·CH₂Cl₂ and 2·H₂O. X-ray structures show that the metal centre in each of 1 and 2 is in a distorted dodecahedral N₄O₄ coordination sphere assembled by two meridionally spanning ONNO-donor ligands. Self-assembly of 1·CH₂Cl₂ via intermolecular C−H···N and C−H···Cl hydrogen bonds and 𝜋-𝜋 interactions provides one-dimensional ‘ladder’ type structure. On the other hand, 2·H₂O assembles into a two-dimensional ‘sheet’ like network through intermolecular N−H···O and O−H···N hydrogen bonds.

The Schiff bases N-(acyl)-N’-(ferrocenylidene)hydrazines (HFcah (1) and HFcbh (2), where acyl = acetyl in 1 and benzoyl in 2 and H represents the dissociable amide proton) were synthesized in high yields (74 and 81%) by condensation reactions of equimolar amounts of ferrocene-carboxaldehyde and the corresponding acylhydrazines in presence of acetic acid in refluxing methanol.Reactions of Ni(OAc) ₂·4H ₂O, the Schiff bases (1 and 2) and NaOAc ·3H ₂O in 1:2:4 mole ratio in refluxing methanol afforded the diamagnetic iron(II)-nickel(II)- iron(II) species [Ni(Fcah) ₂] (3) and [Ni(Fcbh) ₂] (4) in 60 and 68% yields, respectively. Both Schiff bases and the two trinuclear complexes were characterized by elemental (CHN) analysis, mass spectrometric, various spectroscopic (IR, UV-Vis and ¹H NMR) and cyclic voltammetric measurements. Molecular structures of 2, 3and 4 were determined by single crystal X-ray diffraction studies. The cyclopentadienide rings in the ferrocene moieties are essentially in eclipsed conformation in all three structures. Deprotonated Schiff base ligands in each complex (Fcah⁻ in 3 and Fcbh⁻ in 4) act as five-membered chelate ring forming azomethine-N and amidate-O donors and assemble a square-planar trans-N ₂O ₂ coordination environment around the nickel centre. The redox active 1–4 exhibit an iron centred redox couple in the potential range 0.23–0.50 V (vs. Ag/AgCl).