Four bridging bis chelate ligands with very low lying p* orbitals. MO perturbation calculations, electrochemistry, and spectroscopy of mononuclear and binuclear group 6 metal tetracarbonyl complexes. Inorganic Chemistry, (26)1:68--77, 1987. [PUMA: Group MO electrochem;bipyrimidine level azobipyridine electrochem;electrochem electrochem;pyridyltetrazine electrochem carbonyl;MO bipyrimidine carbonyl;Group VIB transition pyridylpyrazine bischelant carbonyl;energy carbonyl;electronic pyridyltetrazine;pyridylpyrazine]