Creutz and Taube ion

Figure 2. Experimental intervalence band (10) and calculated contour for the Creutz and Taube ion. The parameters used in the calculation are c, —6.0 Ac, 1.1 Ag, 5.51 vc, 500 cm"1 and vB, 100 cm"1 at 300 K. The sticks show the relative calculated intensities, and the contour is obtained by replacing each stick by a Gaussian of FWHM = 1.4 vc. (Some of the vertical bars—in Figure 2 only— have been omitted for clarity.) The experimental peak height is normalized to the calculated value, which is arbitrary. The calculated relative intensities among Figures 2-4 are meaningful.

Variations in electronic coupling can lead to profound changes in properties, as illustrated by the series (bipy)2ClRu(pz)RuCl(bipy)23+, (NH3)5Ru(pz)Ru(NH3)55+ and (bipy)2ClRuORuCl-(bipy)23+. In the first case there is clear evidence for localization of the excess electron on the vibrational timescale, in the last case the excess electron appears to be delocalized over both metal sites, and in the intermediate case, the Creutz and Taube ion, the system appears to be poised between the localized and delocalized limits.94... 

Nonbridging ligands can aflFect the energy and apparently also the intensity of IT bands. In the dimers (NH3)5Ru (pyz)Ru L(NH3)4 (34) and (NH3)5Ru (pyz)Ru X(bipy)o 29, 30), the energy of the IT band increases as variations in L or X increase the energy asymmetry between the two ends. For the mixed-valence dimer (bipy)2ClRu(pyz)-RuCl(bipy)2 ESC A studies demonstrated that there are discrete Ru(II) and Ru(III) sites (36). In recent work, Callahan and Meyer (37) found an IT band for the ion (Amax 1300 nm, c = 450 in acetonitrile) which has the approximate band width and solvent dependence that were predicted by Hush. IR data are consistent with localized valences. The properties of this ion differ markedly from those of the Creutz and Taube ion (NH3)5Ru(pyz)Ru(NH3)s (33), and the differences are consistent with a much stronger metal-metal interaction in the pentaammine system (37). 

Perhaps the most interesting cases are complexes which appear to lie in the transition region between localized and delocalized systems. Much evidence points to trapped valences in the Creutz and Taube ion... 

There is by now an extensive chemistry of mixed-valence compounds where the distinguishing feature is the existence, at least in a formal sense, of different oxidation states within the same compound.The first deliberate synthesis of a discrete metal-based mixed-valence ion was the Creutz and Taube ion, (NH3)5Ru(pz)Ru(NH3)5 + (pz is pyrazine). A number of examples of this type are known based on metal complexes and organometallics, e.g. [(C5H5)Fe(C5H4CsH4)Fe-(CsHs)]" or chemically linked organics such as (3), especially for d -d cases involving Ru " and Ru . - ... 

Continuing the studies on the Creutz and Taube ion (5), an -ray crystal structure determination has been reported. Within the accuracy of the measurements, the... 

If there is symmetry, statistical effects appear. Tom, Creutz, and Taube noted that in the equilibrium in Reaction 3 (where 4,4 -bipy = 4,4 -bipyridine), the mixed-valence ion is favored by a statistical factor of 4 even in the absence of other effects (31), When one compares the... 

DR. SCHATZ Depending on the values of epsilon and lambda, they can have essentially any frequency or a vast range of intensities. So you can t make any simple general statement. For the Creutz-Taube ion, they are on the order of 50-100 wave numbers. 

Creutz-Taube ion [bis(pentaammine-ruthenium)pyrazine]D (30) provides an example of this. There is good reason to suppose (in spite of many earlier arguments to the contrary) that this is a fully delocalized mixed-valence system (27). In symmetry, the one-electron levels separated by energy gap 2J are calculated to have b u (bonding) and b (antibonding) symmetry,... 

Figure 5. One-electron representation of pir/dw orbitals involved in coupling in Creutz-Taube ion. Key a, 2b u (doubly filled) and 2bSg (half-filled) and b, localized orbitals 2"%(2b u . 2bsg) (21).

Mention should be made here of recent attempts by Piepho, Schatz and Krausz (46) to give a general interpretation of intervalence bandshapes in terms of a Hamiltonian equivalent to that of eq 6. They use vibronic eigenfunctions (following the method of solution of Merrifield (47)) rather than adiabatic Born-Oppenheimer (ABO) functions. Thus, the aim is to interpret an observed spectrum in terms of one vibrational coupling mode, which is antisymmetric. Their analysis of the spectrum of the Creutz-Taube ion yields a value of 0 of 1.215, i.e., a rather weakly localized ground state. Using their assumed unperturbed... 

The eigenvalues calculated either by the Merrifield or ABO method are nearly identical. As can be seen, this analysis leads to the prediction of a number of low-lying infrared absorption bands for this complex. In a discussion of the structure of the Creutz-Taube ion, I pointed out some time ago (27) that the evidence definitely favors a well-delocalized ground state (6 < 1), and predicted that these infrared bands would, thus, not be observed. In a recent publication, Krausz, et al., report that a search for the bands indeed gave negative results, and withdraw their proposed interpretation of the spectrum (48). 

Figure 7. Potentials for lower and upper surfaces in Creutz-Taube ion calculated by method of Piepho, Krausz, and Schatz, as function of dimensionless coordinate, y, with J = 0.4 eV,

Figure 8. Intervalence spectrum of Creutz-Taube ion (------) and of the corresponding ion with 1-methoxypyrazine (-----) instead of pyrazine (DtO solution,...

DR. PAUL SCHATZ (University of Virginia) If one tries to get additional width, for example, in the Creutz-Taube ion intervalence band by putting in a symmetrical mode, and one works through the arithmetic, one finds that there is no intensity unless the force constants are different in the two oxidation states. And, furthermore, unless they are very different, the amount of intensity obtained will be very, very small. So I m personally very skeptical of your calculation using the symmetrical modes to obtain additional width. I don t think it will work. 

I have predicted that the very unusual low-frequency IR behavior for the Creutz-Taube ion calculated by Piepho, Schatz and Krausz [Piepho, S. B. Krausz, E. R. Schatz, P. N. J. Am. Chem. Soc. 1978, 100, 2996] on the assumption of only antisymmetric mode involvement in electron-vibrational interaction would not be found, and that it was an artifact of the method. The failure of experiments designed to locate such IR bands has subsequently been reported by Krausz, et al. 

Mixed-valence di- and po nuclear complexes have been of wide interest ever since the discovery of the Creutz-Taube ion and progress in the field has been reviewed. In this section, we consider representative mixed-valence complexes which are organized according to the type of bridging ligand. 

The PKS model has been criticized (27-29) for the assumption of a single frequency and its unsatisfactory description of the magnetic circular dichroism (MCD) found for the intervalence band of the Creutz-Taube ion (30). In a series of papers (31), Ondrechen and coworkers developed a more realistic three-site model for delocalized (class III) bridged mixed-valence complexes. This model incorporates many features of the PKS model but differs in that it explicitly includes the bridge, it uses a molecular orbital basis of one-electron functions, and the choice of important vibrational modes is not the same. The near-IR band line shape of the Creutz-Taube ion has been reasonably... 

Table I compiles the electrochemical and metal-metal absorption band spectral data of novel mixed-valence complexes (35 46) that are good candidates for valence delocalized systems. The decision as to where to draw the line between class II and III mixed-valence complexes is a difficult one to make. The Creutz-Taube ion has undergone an extremely rigorous physical characterization by just about every method known (see below), and it is only recently that the preponderance of evidence strongly favors a class III description. The same degree of examination should be applied to the complexes of Table I to fully justify their class III assignment.

Modification of metal-metal coupling is not just restricted to the inner coordination sphere. Hupp and coworkers (56) have shown that addition of the crown ethers, dibenzo-36-crown-12 (DB-36-C-12) or di-benzo-30-crown-10 (DB-30-C-10), to a nitromethane solution of the Creutz-Taube ion or crown ether oxygens for ruthenium-ammine hydrogens. Figure 5 shows the changes observed for the MMCT of effects occur with the 1 1 stoichiometry of crown ether... 

Figures 6 and 7 show absorption and electroabsorption spectra of [ (NH3)5Ru 2(/A-pyz)]5+ and [ (NH3)5Ru 2(M,4 -bpy)]5+, respectively. The change in AA as a function of x is uniform for the bands, which indicates that the molecular properties that give rise to AA are identically oriented with respect to the transition dipole moment. The electroabsorption spectra in the near-IR region (MMCT bands) give the greatest differences between complexes when analyzed with Eq. (31) and these are shown in Fig. 8. For the Creutz-Taube ion (Fig. 8A), the spectrum does not satisfactorily reduce to a sum of derivatives but nevertheless shows that AA(p) line shape to be modeled primarily by a negative zeroth derivative (Ax) term, especially at energies below 6500 cm-1. The fit in this case yields a value for Ap. = 0.7 0.1 D, which when compared with the maximum permanent electric dipole moment ( A/u max = 32.7 D, assuming a metal-to-metal distance) is strong evidence for a delocalized ground state. Contrast this result with the analysis of the electroabsorption spectrum of [ (NH3)5Ru 2(ja-4,4 -bpy)]5+ shown in Fig. 8B.

The consequences of a smaller separation between metal ions has already been discussed in Section II.A.l, where the application of an ellipsoid model for outersphere reorganization energy was introduced. Hush theory calculations of ifad and the mixing coefficient a [in the Creutz review (7) and Table VI of this chapter], which use R = metal-metal distance, are largely underestimated. For example, Sutton, Sutton, and Taube (105) have reported H i and a values for [ (NH3)5Ru 2(jU-4,4 -bpy)]5+ of 460 cm-1 and 0.057 [R = 11.3 A in Eq. (15)]. Hupp, Dong, Blackbourn, and Lu (106) have estimated for the... 

The prototype, intentionally synthesized, ion (18-F-I) has been the most intensively studied and also has been named after the discoverers, Carol Creutz and Henry Taube. 

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