Abstract
Complexes (m-L)[Mo(CO)2(PBu3)2]2 with cis-carbonyl ligands and sym. bridging bis(a-diimine) chelate ligands L = 2,2'-bipyrimidine (bpym) and 2,5-bis(2-pyridyl)pyrazine (2,5-bppz) were synthesized and studied in different oxidn. states by cyclic voltammetry, 1H NMR, EPR, IR and UV-visible-NIR absorption spectroscopy. The combination of two very electron rich d6 metal centers with one p accepting bridging ligand results in rather small HOMO-LUMO energy gaps, as evident from differences of $\sim$1 V between the potentials for reversible oxidn. and redn. and from intense charge transfer absorptions at $\sim$1 eV, i.e. in the near-IR (NIR) region. Whereas the shifts of the carbonyl vibrational bands are about equal for the redn. (low-energy shift) and the oxidn. (high-energy shift) of the more stable bpym complex, the EPR results reveal occupation of the a-diimine p* orbitals in the anionic forms and an Mo(I)/Mo(0) mixed-valent state with a comproportionation const. Kc \textgreater108 for (bpym)[Mo(CO)2(PBu3)2]2+. Electronic transitions obsd. by UV-visible-IR spectroelectrochem. confirm these assignments of the redox orbitals; the max. of the weak metal-to-metal charge transfer band of the mixed-valent complex (bpym)[Mo(CO)2(PBu3)2]2+ was found at 3700 nm (2700 cm-1). [on SciFinder(R)]
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