Mixed-valence complexes delocalization

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

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). 

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 ... 

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... 

Mixed-valence complexes terminated in other than ammine and cyano ligands include [([9]aneS3)ClRu(/x-243)RuCl([9]aneS3)] which exhibits a strong intermetallic interaction consistent with a Robin-Day class III (delocalized) species. The synthesis of the Ru Ru complex [ P(OMe)3 2(MeCN)Ru(u-S2)(/u-NH2NH2)Ru P(OMe)3 2(MeCN)] + has been reported. Oxidation of the (M = Ru or Os) complexes [ 4 -(4-MeC6H4 tpy)M(u-L)M 4 -(4-... 

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.

From the A 1/2 = 320 mV, K om is 1.6 x 10, indicating this complex is a Robin and Day class II mixed-valence complex. Electronic absorption spectroelectrochemistry in the near infrared (NIR) shows an osminm-based intervalence charge transfer (IVCT) at 2440 cm. The extent of electronic delocalization, a, can be calcnlated nsing eqnation (16). 

The ligand-bridged complexes [Rh2(CO)4 (PPh3) (/x-RNXNR)2] (n = 1 or 2, R = aryl, X = N or CMe) undergo reversible one-electron oxidation to isolable monocations (275), which appear to be fully delocalized mixed valence complexes (276). Oxidation also leads to enhanced susceptibility to carbonyl substitution ( = 1, X = CMe), and a drastic shortening in the metal-metal bond distance implies electron loss from an anti-bonding dimetal orbital (275) (Section III,F). 

Core vibrational frequencies obtained by Raman or resonance Raman spectrophotometric technique are summarized in Table V (53, 113, 116-118). The p(Pt—Pt) bands for the pop complexes [Pt2(pop)4] " (PtdDa), [Pt2(pop)4X] - (Ptdl,111)2), and [Pt2(pop)4X2] - (Pt(III)2) increase in the order expected from increasing Pt—Pt interaction (53). Resonance Raman spectra have been used frequently for the assignment of the electronic transitions of complexes as listed in the table. Also, observation of the vibrational structure in the low-temperature luminescence spectra coupled with the data in the table has been useful for considering the nature of the transition of these diplatinum complexes. The resonance Raman spectra at the intervalence band of the three mixed-valence complexes, which are stable only in the solid state, indicate that [Pt2(pop)4Cl] is a localized valence species and the bromo and iodo analogues are nearly delocalized valence species (53). 

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