Mixed valence dimers

Cl,MeSi)2Pt2(PPh3)4 Mixed valence dimer with bridging (SiMeCL) groups 110a... 

In a related example, the mixed-valence V2O3 core forms when vanadyl sulfate is oxidized in a pH = 7.5 HCOs /COs buffer. This complex has a reversible oxidation whose potential is pH dependent since the mixed-valence dimer forms by the following equilibrium (Fq. 6) ... 

This classification distinguishes two degrees of mixing of two different configurations of a mixed-valence dimer Ma - and - Mb", where A and B... 

A few examples in which aromatic cation radicals have been isolated as crystalline salts, actually consist of mixed valence units. For example, crystal structure analysis showed that the naphthalene radical cation (NAP) + forms a mixed valence dimer (NAPy4 in which the two components are arranged face to face in n-stacks with an interplanar separation significantly closer than van der Waals contacts. Such an intermolecular organization arises from the sponta-... 

Dimers formed by exchange-coupled nickel(II) ions will be reviewed first, starting with dimers with monoatomic bridges. Mixed valence dimers and heterobimetallic dimers containing one nickel(II) ion will follow. 

Nonetheless, in Table 4 are summarized the results of a series of rate constant comparisons between calculated values using optical absorption data and experimental values.83 The experimental data are derived from self-exchange rate constants for couples which are structurally related to the mixed-valence dimers shown in the table. The experimental values cited are calculated values for electron transfer within the association complex between reactants as estimated from k kQ >JKA. values were calculated using equation (33) and it was assumed in the calculations that vet = 5 x 1012 s-1. Calculations of this kind have been extended to unsymmetrical dimers like (NHJ)5RuIII(pz)RuIICl(bipy)24+ 83 and even to outer-sphere ion-pairs like (4).88... 

For the Ru111/11 or Fe111 11 mixed-valence dimers of the previous section, the metal ion sites are sufficiently separated spatially for direct overlap between pure metal-based d-orbitals to be mini-... 

The question of the applicability of the semiclassic adiabatic approach to the vibronic problems in the JT systems is rather complicated in general and the thorough answer can be done with regard to a particular problem. In our brief discussion of this question we will refer to two areas - thermodynamic properties and resonance (optical) problems. In Ref. [9] the magnetic properties of mixed-valence dimeric and trimeric clusters are considered in the framework of... 

Indeed, these reactions are strongly retarded by Co(II). In the stoichiometric oxidation of ethylbenzene by cobalt(III) acetate in acetic acid, no reaction was observed253 when [Co(III)] = [Co(ll)]. This result was attributed to the formation of mixed Co(III)-Co(II) dimers that are inactive.251-253 Similarly, cobalt-(III) acetate oxidation of toluene is strongly retarded by other metal acetates, such as Mn(II), Mn(III), Ce(IV), and Cu(II), due to the formation of inactive polynuclear complexes (mixed valence dimers).251... 

A number of these have been prepared using the synthetic procedures shown in Scheme 2. Of particular interest is [Os2(pyz)(NH3)10]5+ which has a radically different electronic absorption spectrum from either [Os 2(pyz)(NH3)10]4+ or [Osin2(pyz)(NH3)10]6+, and is believed to be a class III delocalized mixed-valence dimer, as is the dinitrogen analogue [Os2(N2)(NH3)10]s+ (see p, 556). A similar situation probably obtains for [Os2 (pyz)Cl(NH3), ]4+.1 l4a... 

Reaction of the monomer with 2,2 -bipyridine results in the formation of the mixed-valence dimer, Mn2(III,II)(biphen)2(biphenH)(bipy)2... 

X-ray crystal structures of both dimers have been determined. The Ni—Ni distance of the mixed-valence dimer (2.514(5) A) is shorter than that of the Ni(II) dimer (2.564(1) A), suggesting stronger direct interaction between the two nickel ions. The crystal structure of [Ni2(CH3CS2)4l] consists of infinite chains of Ni2(CH3CS2)4 I Ni2(CH3CS2)4 — I —, which is similar to that of the Pt analogue. All the Ni atoms are equivalent. Vibrational spectra have been discussed (36). The reaction of nickel salt with dithiophenylacetic acid instead... 

Structures of both [Ni2(form)4] and [Ni2(form)4]BF4 have been determined. The crystal structure of [Ni2(form)4]-2H20 is isostructural with that of [Pd2(form)4]-2H20. Again the mixed-valence dimer has a shorter Ni—Ni distance (2.418(4) A) than that of the Ni(II) dimer (2.485(2) A). Interestingly, the torsion angle of the mixed-valence dimer (27.4 ) is greater than that of Ni(II) dimer (16.8°), in spite of the smaller difference between the bite distance and the metal—metal distance. EPR measurement indicated that the metal centers are oxidized in this case. 

The electronic factor k i is a function of the electronic interaction between the reduced (A) and oxidized (B) metal center, expressed as the electronic coupling matrix element Hab- Hab can be calculated from first principles (195-197) or can be estimated from the intensity of the intervalence band of chemically closely related mixed-valence dimers (198). When Hab is large, Kei is equal to 1 and the reaction is said to be adiabatic. 

Yield of isolated (and often recrystallized solid) reactions often essentially quantitative prior to workup. Includes Zn reduction and subsequent air oxidation steps to mixed valence dimer. 

Fig. 8 Effect of the double exchange on the energy pattern of mixed valence dimer...

Vibronic Effects in Mixed-Valence Dimers Containing Orbitally Degenerate Ions... 

Halogen oxidation of [Re2Xg] " (X = Cl, Br) produces Re dimers, [Re2X9] , isolated as [n-Bu4N] salts The [ReiXg]" ions add one electron to form mixed-valence dimers, [ReaXg] " (Scheme 1) ... 

Zinc reduction of [TcClg] in HCl (aq) produces the mixed-valence dimer [TcaClg] ". The analogue of Ref[TciClg] , is isolated by workup of products from reduction of pertechnetate with hypophosphorus acid. The [n-Bu4N]2 [TC2CI8] is isolated and converted to [Tc2Br8] salts . 

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