Intervalence absorption spectrum

The highly unstable, active titanocene 8, reported by van Tame-len et al., also forms a blue complex with dinitrogen, [(CjH Ti] (25) (Section III,C). Interestingly, the visible absorption spectrum of this titanocene 8 is identical with that of -(rj1 T75-CjsH4)(i -CsHs)3Ti2 (10) (bands at 486 and 640 nm) (3). Unfortunately however for comparative purposes, the reported spectrum of 8 does not cover the near infrared region at —1120 nm where 10 shows a characteristic intervalence absorption band. 

Figure 2 Electronic absorption spectrum of the Creutz-Taube ion in the intervalence region, in nitro-...

The absorption spectrum of a supramolecular system can differ substantially from the sum of the spectra of the molecular components. Aside from those small shifts that can be dealt with in terms of perturbation of the spectra of the single components upon bridging, some totally new bands can be present in the spectrum of the supermolecule. These bands correspond to optical electron transfer transitions, commonly denominated charge-tranter and intervalence transfer transitions, in the organic [26] and inorganic [23-25] literature, respectively (process 7 in Fig. 4, eq 1). The factors that determine the spectroscopic characteristics of such bands... 

FIGURE 35.6 Spectra obtained during oxidation with bix(pyridyl)-phenyliodonium top open-ring isomer 17a bottom closed-ring isomer 17b exhibiting an intervalence absorption in the 1000-1500 range 1 initial spectrum 2 at half-oxidation 3 spectrum of the fully oxidized solution. 

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... 

Similar oxidative coupling [Cu(OAc)2/pyridine/dbu, 40 °C] of Fe(C=CC=CH)(dppe)Cp gives Fe(dppe)Cp 2 r-(C=C)4 (159) as an air- and thermally-stable burnt-orange compound.307 Oxidation with [FcH]+ gave forest-green [ Fe(dppe)Cp 2( r-C8)]+ which has two v(CC) absorptions at lower frequencies (1784, 1879 cm-1), intermediate between the diyndiyl and cumulenic forms. The UV/vis spectra contain MLCT bands at 389, 481 nm (neutral) and 403, 423, 556, 624, 707 nm (cation), while the near IR spectrum contains a narrow intense intervalence CT band... 

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. 

The main difference in the spectra concerns the presence of an intense absorption band at around 700 nm, which can be attributed to Intervalence Charge-Transfer between Mo5+ and Mq6+ in the Keggin unit [22,23], thus indicating the presence of a partially reduced POM for the catalyst which has been unloaded after reaction under isobutane-rich conditions (spectrum... 

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. 

Nitrogen-to-iron weight ratios of 0.71 and 0.88 in 6 and 8, respectively, established the 71% and 88% oxidation ratios. The near-IR spectra of 12 and 13 had a broad absorption at 1000-2000 nm with maximum intensity at 1400-1700 nm. This band in BFD" (TCNQ)2 had previously been assigned to a photon-assisted intramolecular intervalence exchange (8, 9) and it confirmed that monooxidation of BFD units to [Fe(II)Fe(III)] occurred (BFD was dioxidized to its [Fe(III)Fe(III)] salts by Mueller-Westerhoff and Eilbracht (19) these salts had no absorption at 1400-1700 nm). Absorption at 600 nm was also pronounced. The Mossbauer spectrum of 12 was dominated by a single symmetrical absorption with a quadrupole splitting of 1.73 mm/sec which further confirms the BFD structure. Neutral BFD s doublet (2.40 mm/sec) was also observed. 

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 case isovalence-electronic with BaBiOj is K2SbClg, a salt referred to as Setterberg s salt, which was discussed in Chapter 10. The average oxidation state for Sb in the salt is -1-4, but this is a missing valence state. Absorption, giving the strong color, is ascribed to Sb +Sb + Sb" +Sb" + by Day et al. The understanding of the intervalence spectrum is based on the local model. 

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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