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 VIB azobipyridine bipyrimidine bischelant carbonyl;Group carbonyl;MO carbonyl;electronic carbonyl;energy electrochem electrochem;bipyrimidine electrochem;electrochem electrochem;pyridyltetrazine level pyridylpyrazine pyridyltetrazine;pyridylpyrazine transition]