The rhenium(I) compounds [(NwedgeN)Re(CO)(3)(MQ)](PF(6))(2) (NwedgeN = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 3,3'-bipyridazine (bpdz), or 1,4,7,10-tetraazaphenanthrene (tap) and MQ(+) = N-methyl-4,4'-bipyridinium) undergo four one-electron reduction steps which could be analyzed using cyclic voltammetry, EPR, IR, and UV/vis spectroelectrochemistry. Due to the rather low-lying pi orbital of tap, the corresponding compound shows electron uptake by NwedgeN each time before MQ(+) is reduced. The opposite is observed for the complexes of the other chelate ligands NwedgeN, however, and the pi(NwedgeN) orbital aproaches the pi(MQ(+)) level in the order bpy < bpym < bpdz. Remarkably, the reduction processes of MQ(+) and bpdz in [(bpdz)Re(CO)(3)(MQ)](PF(6))(2) are separated by only 74 mV as deduced from IR spectroelectrochemical analysis. On reduction of the related compound [(bpy)Re(CO)(3)(mpz)](PF(6))(2) (mpz(+) = N-methylpyrazinium), the first two electrons are added to the axial ligand which has a lower-lying pi orbital than MQ(+) and cannot undergo intramolecular twisting.

The molecular rectangle [(bp)2(bpym)2(OC)12Re4]4+ (bp = 4,4‘-bipyridine and bpym = 2,2‘-bipyrimidine) undergoes several reversible reduction processes in DMF solution. UV/vis and IR spectroelectrochemistry shows that the two bpym ligands are reduced twice by one electron each before the bp components pick up additional electrons in one-electron steps at more negative potentials.

Complexes between the chlorometal(III) cations [(C5Me5)ClM]+, M = Rh or Ir, and the 1,10-phenanthroline-derived alpha-diimine (N--N) ligands dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4,7,10-tetraazaphenanthrene (tap), or 1,10-phenanthroline-5,6-dione (pdo) were investigated by cyclic voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry with respect to either ligand-based or metal-centered (and then chloride-dissociative) reduction. Two low-lying unoccupied molecular orbitals (MOs) are present in each of these three N wedge N ligands; however, their different energies and interface properties are responsible for different results. Metal-centered chloride-releasing reduction was observed for complexes of the DNA-intercalation ligands dppz and tap to yield compounds [(N--N)(C5Me5)M] in a two-electron step. The separation of alpha-diimine centered optical orbitals and phenazine-based redox orbitals is apparent from the EPR and UV-vis-NIR spectroelectrochemistry of [(dppz)(C5Me5)M](0/*-/2-). In contrast, the pdo complexes undergo a reversible one-electron reduction to yield o-semiquinone radical complexes [(pdo)(C5Me5)ClM]* before releasing the chloride after the second electron uptake. The fact that the dppz complexes undergo a Cl(-)-dissociative two-electron reduction despite the presence of a lowest lying pi* MO (b1(phz)) with very little overlap to the metal suggests that an unoccupied metal/chloride-based orbital is lower in energy. This assertion is confirmed both by the half-wave reduction potentials of the ligands (tap, -1.95 V; dppz, -1.60 V; pdo, -0.85 V) and by the typical reduction peak potentials of the complexes [(L)(C5Me5)ClM](PF6) (tap, -1.1 V; dppz, -1.3 V; pdo, -0.6 V; all values against Fc(+/0)).