Mixed-valence complexes spectroscopy

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

Electric fleld gradient, 22 214-218 Electroabsorption spectroscopy, 41 279 class II mixed-valence complexes, 41 289, 291, 294-297 [j(jl-pyz)]=+, 41 294, 296 Electrocatalytic reduction, nickel(n) macro-cyclic complexes, 44 119-121 Electrochemical interconversions, heteronuclear gold cluster compounds, 39 338-339 Electrochemical oxidation, of iron triazenide complexes, 30 21 Electrochemical properties fullerene adducts, 44 19-21, 33-34 nickeljll) macrocyclic complexes, 44 112-113... 

For diiron complexes Mossbauer spectroscopy allows to asses (1) oxidation and spin states of the iron atoms, (2) diamagnetism and ferromagnetism of the groimd state for diferric and mixed-valent oxidation levels and (3) valence (de)localisation in the solid state for mixed-valence complexes [2,3]. Isomer shifts (IS) in the range 0.35-0.60 mm/s are characteristic of 5- or 6-coordinate high-spin diferric p-hydroxo complexes [2,3], Tetrahedral high-spin ferric iron has lower isomeric shifts in the range of 0.22 mm/s [2,3]. For isolated ferric iron with S =... 

Infrared spectroscopy of (x 4-cyclopentadiene)Fe(CO)3 moiety-attached ferrocene and its oligomers, 2, gave information on the electronic structure of the mixed-valence complexes 19 the v(CO) bands of the Fe(CO)3 moiety20,21 could sense the oxidation state of the nearest terminal ferrocenyl group. 

The heteronuclear mixed-valence complexes 16b,c were obtained from H2L 15, triethylamine, and iron(III) chloride in the presence of an excess of cobalt(II) chloride or copper(II) chloride. In neutral 16b,c iron is present only in the oxidation state 3 , as unambiguously confirmed by Mossbauer spectroscopy, which reveals only one quadruple doublet. Cyclic voltammetric investigation of the redox-active metal centers of neutral 16b shows a reversible three-potential one-electron transfer process. The half-wave potentials of -660 and -1310 mV correspond to the redox processes [(Fe )2Co"OLf ] [Fe Fe Co OLf ]" [(Fe")2Co"OLf ] , whereas... 

As first shown by Boxer et al. electroabsorption (or Stark effect) spectroscopy provides an experimental method to determine the extent of delocalization in mixed-valence complexes. Unfortunately, the technique is experimentally very difficult and this has limited its application. Nevertheless, Stark effect measurements of parallel and orthogonal intervalence transitions in... 

It would be of some theoretical and practical importance to know the magnitude of resonance exchange energy required to achieve a delocalized state in a mixed-valence complex. In this regard, infrared spectroscopy has been recognized as a powerful means of examining electron transfer in mixed-valence complexes, as its time scale (10 s) gives an almost instantaneous view of the state of a fiuxional molecule. 

Oxidation of dinuclear Pd(II) complexes by one electron is predicted to afford dinuclear Pd(II)/Pd(III) mixed valence complexes with a Pd-Pd bond order of 0.5 (Fig. 20) [92]. Dinuclear Pd(II)/Pd(III) mixed valence complexes have been detected by EPR spectroscopy, although, similar to mononuclear systems, the unpaired electron is not always metal-centered [87,93], Two examples of dinuclear Pd(ll)/Pd(lll) complexes bearing a metal-centered unpaired electron have been reported [94, 95]. In 1988, Bear reported the EPR spectrum of tetrahridged dinuclear complex 38, prepared by electrochemical oxidation of 37 (Fig. 22a) [95]. In 2007, Cotton reported the only crystaUographically characterized mixed valence Pd (ll)/Pd(ni) complex (40 Fig. 22b) [94]. Both 38 and 40 are paramagnetic and have EPR spectra consistent with a metal-based oxidatimi. The metal— metal distance in 40 is 0.052 A shorter than the corresponding distance in Pd(ll) complex 39, consistent with a Pd-Pd bond order of 0.5. 

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