A large number of complexes of the first-row transition metals with non-innocent ligands has been characterized in the last few years. The localization of the oxidation site in such complexes can lead to discrepancies when electrons can be removed either from the metal center (leading to an M((n+1)+) closed-shell ligand) or from the ligand (leading to an M(n+) open-shell ligand). The influence of the ligand field on the oxidation site in square-planar nickel complexes of redox-active ligands is explored herein. The tetradentate ligands employed herein incorporate two di-tert-butylphenolate (pro-phenoxyl) moieties and one orthophenylenediamine spacer. The links between the spacer and both phenolates are either two imines ([Ni(L1)]), two amidates ([Ni(L3)]2-), or one amidate and one imine ([Ni(L2)]-). The structure of each nickel(II) complex is presented. In the noncoordinating solvent CH2Cl2, the one-electron-oxidized forms are ligand-radical species with a contribution from a singly occupied d orbital of the nickel. In the presence of an exogenous ligand, such as pyridine, a Ni(III) closed-shell ligand form is favored: axial ligation, which stabilizes the trivalent nickel in its octahedral geometry, induces an electron transfer from the metal(II) center to the radical ligand. The affinity of pyridine for the phenoxylnickel(II) species is correlated to the N-donor ability of the linkers.

The two tetradentate ligands H2L and H2LMe afford the slightly distorted square‐planar low‐spin NiII complexes 1 and 2, which comprise two coordinated phenolate groups. Complex 1 has been electrochemically oxidized into 1+, which contains a coordinated phenoxyl radical, with a contribution from the nickel orbital. In the presence of pyridine, 1+ is converted into 1Py+, an octahedral phenolate nickel(III) complex with two pyridines axially coordinated: An intramolecular electron transfer (valence tautomerism) is promoted by the geometrical changes, from square planar to octahedral, around the metal center. The tetradentate ligand H2LMe, in the presence of pyridine, and the hexadentate ligand H2LPy in CH2Cl2 afford, respectively, the octahedral high‐spin NiII complexes 2Py and 3, which involve two equatorial phenolates and two axially coordinated pyridines. At 100 K, the one‐electron‐oxidized product 2Py+ comprises a phenoxyl radical ferromagnetically coupled to the high‐spin NiII ion, with large zero‐field splitting parameters, while 3+ involves a phenoxyl radical antiferromagnetically coupled to the high‐spin NiII ion.

The MEP pathway for the biosynthesis of isoprene units is present in most pathogenic bacteria, in the parasite responsible for malaria, and in plant plastids. This pathway is absent in animals and is accordingly a target for the development of antimicrobial drugs. LytB, also called IspH, the last enzyme of this pathway catalyzes the conversion of (E)-4-hydroxy-3-methylbut-2-enyl diphosphate (HMBPP) into a mixture of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) using an oxygen sensitive iron sulfur cluster. The exact nature of this iron sulfur cluster is still a matter of debate. We have used (57)Fe Mössbauer spectroscopy to investigate the LytB cluster in whole E. coli cells and in the anaerobically purified enzyme: In LytB an unusual [4Fe-4S](2+) cluster is attached to the protein by three conserved cysteines and contains a hexacoordinated iron linked to three sulfurs of the cluster and three additional oxygen or nitrogen ligands.