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
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Inorganic Chemistry 26 (1): 68--77 (1987)

Coordination characteristics of the 4 bridging, p-delocalized bis chelate ligands 2,2'-bipyrimidine (bpym), 2,5-bis(2-pyridyl)pyrazine (bppz), 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz), and azo-2,2'-bipyridine (abpy) were evaluated with use of p MO perturbation calcns. and were studied exptl. by example of mononuclear and binuclear complexes with the fragments M(CO)4 (M = Cr, Mo, W). The substantial stabilization of the p* (LUMO) levels in the order bpym, bppz, bptz, abpy strongly facilitates redn. of the complexes and causes shifts of the intense metal-to-ligand charge-transfer (LMCT) absorption bands out into the near-IR region. Despite a notable activation barrier for the formation of binuclear abpy complexes with 6-coordinate metals, (abpy)Mo(CO)42 was prepd. and it exhibits most remarkable spectral properties such as an intense yet rather narrow and solvent-insensitive MLCT absorption band at 11,000 cm-1, another detectable absorption max. \textgreater30,000 cm-1, a redn. potential close to that of the ref. electrode, SCE, and closely spaced yet well-resolved CO stretching frequencies in the IR spectrum. All these spectral results can be correlated to p MO calcn. data, which strongly suggest the use of bptz and abpy for studies concerned with ligand-mediated electronic interactions between metal centers. A particular asset of binuclear abpy complexes is the unusually short metal-metal distance caused by their coordination-induced S-frame conformation. on SciFinder(R)
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