METAL-METAL INTERACTION IN LIGAND-BRIDGED DIMOLYBDENUM(0,0) AND DIMOLYBDENUM(I,0) COMPLEXES WITH VERY SMALL FRONTIER ORBITAL GAPS - ELECTROCHEMISTRY AND SPECTROSCOPIC PROPERTIES OF 3 NEIGHBORING OXIDATION-STATES

Complexes (mu-L)[Mo(CO)(2)(P(n)Bu(3))(2)](2) with cis-carbonyl. ligands and symmetrically bridging bis(alpha-diimine) che)ate ligands L=2,2'-bipyrimidine (bpym) and 2,5-bis(2-pyridyl)pyrazine (2,5-bppz) were synthesized and studied in different oxidation states by cyclic voltammetry, H-1 NMR, EPR, IR and UV-Vis-NIR absorption spectroscopy. The combination of two very electron rich d(6) metal centers with one pi accepting bridging ligand results in rather small HOMO-LUMO energy gaps, as evident from differences of about 1 V between the potentials for reversible oxidation and reduction and from intense charge transfer absorptions at about 1 eV, i.e. in the near-infrared (NIR) region. Whereas the shifts of the carbonyl vibrational bands are about equal for the reduction (low-energy shift) and the oxidation (high-energy shift) of the more stable bpym complex, the EPR results reveal occupation of the alpha-diimine pi* orbitals in the anionic forms and an Mo(I)/Mo(0) mixed-valent state with a comproportionation constant K-c greater than 10(8) for the cation ((bpym)[Mo(CO)(2)(P(n)Bu(3))(2)](2)])(+). Electronic transitions observed by W-Vis-IR spectroelectrochemistry confirm these assignments of the redox orbitals; the maximum of the weak metal-to-metal charge transfer band of the mixed-valent complex ((bpym)[Mo(CO)(2)(P(n)Bu(3))(2)](2)}(+)) was found at 3700 nm (2700 cm(-1)).