Valence Complexes

Since the publication of Creutz s review, many new complexes have been synthesized and important developments have occurred. This chapter surveys the literature from 1982 to August of 1993, and it is meant to be an extension of the Creutz review and a guide to the experimentalist in the design of novel dinuclear ruthenium complexes for the study of superexchange phenomena. 

The reader should be familiar with the above reviews, and it would be useful to have a basic understanding of excited-state (8) and thermal (9) electron transfer. A quantum mechanical approach to these subjects can be found in a recent review by Newton (10). 

The Theory of Mixed-Valence Systems 1. The Hush Model 

The conceptual framework that is used to understand thermal and photoinduced electron transfer is illustrated by a classical potential energy-configuration diagram. The simpliest case for electron transfer in which the electron is coupled between donor and acceptor by a single oscillator having the same frequency in both the initial and the final states is illustrated in Figs, la and lb. 

For the weak coupling case (class II complexes), in which the distortion seen in Fig. 2 is small, Hush derived the following relationships of intervalence band properties For the symmetric system (Fig. la), 

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Walker G 0, Barbara P F, Doom S K, Dong Y and Hupp J T 1991 Ultrafast measurements on direct photoinduced electron transfer in a mixed-valence complex J. Rhys. Chem. 95 5712-15... 

Mixed valence complexes of cP-

metal centres. C. Creutz, Prog. Inorg. Chem., 1983, 30, 1-73 (149). 

A similar delocalization has been proposed for the mixed-valence complex... 

Azo-bridged ferrocene oligomers also show a marked dependence on the redox potentials and IT-band characteristics of the solvent, as is usual for class II mixed valence complexes 21,22). As for the conjugated ferrocene dimers, 2 and 241 the effects of solvents on the electron-exchange rates were analyzed on the basis of the Marcus-Hush theory, in which the t/max of the IT band depends on (l/Dop — 1 /Ds), where Dop and Ds are the solvent s optical and static dielectric constants, respectively (155-157). However, a detailed analysis of the solvent effect on z/max of the IT band of the azo-bridged ferrocene oligomers, 252,64+, and 642+, indicates that the i/max shift is dependent not only on the parameters in the Marcus-Hush theory but also on the nature of the solvent as donor or acceptor (92). 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

Another type of complex for which it is obvious thatQlarge structural changes occur on excitation (probably 0.2-0.3 A) along particular coordinates are the infinite-chain mixed-valence complexes of platinum and palladium (class II, or localized mixed-valence complexes) (11).. These Pti [/PtIV, PdIVPtIV or Pdi [/PdIV... 

These results suggest that the critical factor in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. The interaction of adsorbates with each other through the mediation of the substrate is of fundamental importance in surface science. The theoretical treatment, however, involves complicated many-body potentials which are presently not well-understood (2.). It is instructive to view the present case of Pt-substrate-mediated DHT-DHT interactions in terms of mixed-valence metal complexes (2A) For example, in the binuclear mixed-valence complex, (NH3)5RU(11)-bpy-Ru(111) (NH 3)5 (where bpy is 4,4 -bipyridine), the two metal centers are still able to interact with each other via the delocalized electrons within the bpy ligand. The interaction between the Ru(II) and Ru(III) ions in this mixed-valence complex is therefore ligand-mediated. The Ru(II)-Ru(III) coupling can be written schematically as ... 

Creutz, Carol, Mixed Valence Complexes of d5-d6 Metal Centers. 30 1... 

Notice that the excited states are similar to those formed from the localized ground states. Again the first transition is of higher energy than the second. Hush has made a theoretical study of these transitions123. He concluded that, in the case of symmetrical delocalized mixed valence complexes, the two XPS peaks will occur at energies... 

One final point should be noted. Theoretical discussions of electron transfer processes have focused almost entirely on outer-sphere processes. When we have an inner-sphere mechanism, or sufficient electronic interaction in a dynamically trapped mixed-valence complex to produce a large separation between upper and lower potential surfaces, the usual weak-interaction approach has to be abandoned. Thus a detailed knowledge of a potential surface which is not describable as an intersection surface of perturbed harmonic surfaces, for example, is required. For this purpose, detailed calculations will be required. The theory of these processes will be linked more... 

The Electronic Interaction Term. A question raised earlier was is the magnitude of J sufficiently small to justify the weak-interaction assumption This is a question which can be answered, at least for mixed-valence complexes, by interpretation of the oscillator strength of the intervalence band. In Table III, some data (24, 25) for the intervalence band of the... 

There are at least two ways in which detailed information about electron-vibrational coupling strengths can be obtained for mixed-valence complexes. Both are based on the fact that such coupling will be reflected in modifications of the vibrational spectrum. Thus, for example, coupling to antisymmetric modes in a symmetric ion will modify intensities and frequencies of the modes involved. 

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