Intervalence absorption spectrum mixed-valence complex

The difficulties of interpretation presented by some of these complexes are well illustrated by the sequence of ferrocene derivatives (VIII to X). The spectrum of VIII shows a near-infrared absorption as expected for a mixed valence complex, but the band has a shoulder, and at low temperatures is resolved into two bands. Complex IX has a well-defined band almost certainly ascribable to intervalence transfer, but complex X with a similar molecular structure has no such... 

An intervalence electron-transfer band, not present in the Fe analogue, was observed in the room temperature electronic spectrum at 13 800cm . Mossbauer spectra indicated distinct Fe" and Fe " sites at 17K while at 300 K a single absorption was observed. The thermal barrier to electron transfer in the trimer was estimated as about 470 cm. Triiron clusters of this type, in the presence of zinc powder, acetic acid, aqueous pyridine and oxygen, are reported " to effect the oxidation of saturated hydrocarbons. The exchange interactions in the series of complexes [Fe2 M 0(02 CMe)spyj] py (M = Mg, Mn, Co, Ni or Zn) which, M = Ni excepted, are isomorphous with the mixed valence complex referred to above have been measured. 

One of the most powerful methods of evaluating the properties of mixed-valence complexes is spectroelectrochemistry. A key experiment is to reversibly cycle a dinuclear complex through fully oxidized, mixed-valence and fully reduced states. A near IR absorption band that appears only in the electronic spectrum of the mixed-valence state is strong evidence for an intervalence transition. Determination of the nature of the mixed-valence state can also be achieved by spectroelectrochemical methods. 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

When the charge is 4+ or 6-I-, the complexes are Ru(II)/ Ru(II) or Ru(III)/Ru(III) species respectively. For n = 5, a mixed-valence Ru(II)/Ru(III) species might be formulated but spectroscopic and structural data show the Ru centres are equivalent with charge delocalization across the pyrazine bridge. Such electron transfer (see Section 26.5) is not observed in all related species. For example, [(bpy)2ClRu(p-pz )RuCl(bpy)2] exhibits an intervalence charge transfer absorption in its electronic spectrum indicating an Ru(II)/Ru(III) formulation. The complex [(H3N)5Ru (p-pz )Ru Cl(bpy)2]" is similar. 

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