SYNTHESIS AND CHARACTERIZATION OF CYANIDE BRIDGED HETEROBINUCLEAR COMPLEXES BASED ON GROUP-VI OCTACYANOMETALATE(IV) AND PENTAAMMINEOSMIUM(III) IONS - A COMPARISON WITH GROUP-VIII HEXACYANO ANALOGS

Two new mixed-valence complexes of the form, [(NC)7M'(IV)(mu-CN)Os(III )(NH3)5]Li.2H2O (M'=Mo, W) have been synthesised and characterised by spectroscopic and electrochemical techniques. The complexes exhibit mo derately intense intervalence charge transfer (IVCT) transitions for M '=Mo at 638 nm (epsilon(max) almost-equal-to 1000 M-1 cm-1) and for M' =W at 717 nm (epsilon(max) almost-equal-to 1500 M-1 cm-1). The Hush pa rameters reveal Robin and Day Class II behaviour (alpha2<0.6%; H(rP) a lmost-equal-to 1000 cm-1). The IVCT bandwidths are unusually narrow (D ELTAnu1/2 almost-equal-to 3400 M-1 cm-1) relative to the well-known Gr oup VIII hexacyano analogues, [(NC)5M'(II)(mu-CN)Os(III)(NH3)5]- (aq) (M'=Ru, Os), and are attributed to a combination of reduced Franck-Con don barriers to electron transfer and the exclusion of spin-orbit broa dening. Electrochemical measurements reveal an octacyanometalate(IVN) based irreversible couple at +0.72 V (M'=Mo) and a near-reversible cou ple at +0.445 V (M'=W) in 0.2 M KCI(aq) (versus SCE) for the novel com plexes.