Isomers, electronic

Ylidic four Tr-electron four-member X -phosphorus heterocycles as electronic isomers of heterocyclobutadienes 98AG(E)270. 

Spin-state transitions have been studied by the application of numerous physical techniques such as the measurement of magnetic susceptibility, optical and vibrational spectroscopy, the Fe-Mbssbauer effect, EPR, NMR, and EXAFS spectroscopy, the measurement of heat capacity, and others. Most of these studies have been adequately reviewed. The somewhat older surveys [3, 19] cover the complete field of spin-state transitions. Several more recent review articles [20, 21, 22, 23, 24, 25] have been devoted exclusively to spin-state transitions in compounds of iron(II). Two reviews [26, 27] have considered inter alia the available theoretical models of spin-state transitions. Of particular interest is the determination of the X-ray crystal structures of spin transition compounds at two or more temperatures thus approaching the structures of the pure HS and LS electronic isomers. A recent survey [6] concentrates particularly on these studies. 

In a solution where a nonzero volume change between the electronic isomers, HS and LS, is encountered, the position of the spin equilibrium will depend on pressure. The volume change, usually denoted here AF°, may be obtained from the study of the pressure dependence of equilibrium properties such as the magnetic susceptibility or the electronic spectrum. In favorable cases, A F° values may be derived from the amplitude of sound absorption observed in ultrasonic relaxation measurements of a spin equilibrium as will be shown in the... 

Spin-state equilibria between low-spin (LS Si) and high-spin (HS S2) electronic isomers in solution may be characterized by ... 

The description of states participating in a spin-state transition as electronic isomers with discrete nuclear configurations, in particular different metal-ligand distances, requires that separate electronic and vibrational spectra of the two spin states exist. Indeed, a superposition of the individual vibrational spectra of the two states is in general observed, the relative contribution of the states being a function of temperature [41, 139, 140, 141, 142]. This observation sets a lower limit for the spin-state lifetime longer than the nuclear vibrational period, i.e.,... 

The monocationic form of the dimer 192 gives peaks at wavenumbers between those for the neutral and dicationic forms, and the peak width is broadened, which can be attributed to the rate of electron exchange between two electronic isomers, Red-Ox-Fe(CO)3 and Ox-Red Fe(CO)3, where Fe(CO)3 refers to an [(r 4-cyclopentadiene)Fe(CO)3]... 

In summary, some aspects of the behavior of Rh(bpy)3 + suggest its formulation as [Rh111(bpy)2(bpy )]2+ while others are more in accord with its description as a rhodium(II) complex. It may be that two electronic isomers in chemical equilibrium i.e.,... 

The mechanism can be described by using a R —> M = M — Ox scheme. The fully reduced complex R is oxidized by a one-electron process to the thermodynamically unstable electronic isomer, M, which is formed by attack on the Ru11 center, in a kinetically controlled reaction. Then, M7 rapidly isomerizes via intramolecular electron transfer to M, the thermodynamically stable isomer. As the latter species has again a very reactive Ru11 site, the complete oxidation to Ox is consumed. A similar mechanism has been described for the related oxidations of the [(edta)RuIIpzRuII(NH3)5] complex (129). 

In this alternative route, the first one-electron oxidized product in Eq. (40) is the kinetically favored as well as the thermodynamically stable electronic isomer, M. The evolution of the oxidation process, is now facilitated by the rapid isomerization to the unstable electronic isomer M7 in Eq. (41), which has again an available Ru11 site for the reaction proceeding to Ox as in Eq. (42). This route has been also found for R= [(NC)5FeIIbpaRuII(NH3)5], containing the non-communicating ligand bpa= Li-l,2-bis(4-pyridyl)ethane (131), and for... 

Solvent Effects on the Stabilization of Electronic Isomers of Mixed-Valence Ions... 

From these considerations it is clear that complexes in spin equilibrium do not exist at the crossover point between high-spin and low-spin configurations represented on a Tanabe-Sugano diagram. The two states are electronic isomers with geometric and electronic structures well separated on either side of the crossover point. The energy required to reach the crossover point represents at least part of the activation energy for the spin state interconversion. 

The description of spin states as electronic isomers with different metal-ligand distances requires that their vibrational spectra be a superposition of the spectra of the two separate spin states. The relative contribution of the two states to the observed spectrum will change with temperature as the population of the spin states changes. This has been observed (76, 77, 122, 144, 145). Difficulties occur with the assignment of the metal-ligand vibrational frequencies of particular interest for the analysis of the dynamics of spin state transitions. Some success has been achieved with the use of metal isotopic labeling (82, 151), but there are few reliable assignments. 

Most reported phthalocyanine derivatives (sulfo-, nitro-, amino-, triphenylmethyl-, polymeric, etc.) are copper complexes, although at present the synthetic chemistry of other d- and /-metal Pc derivatives is being rapidly developed (Examples 30-36) [5,6,116-118]. Some of them (in particular, copper phthalocyanine sulfonic acids) are of industrial interest because of their usefulness as dyes. Phthalocyanine sulfonic acids themselves are prepared both by urea synthesis from sulfonated phthalic anhydride and by the sulfonation of the phthalocyanine [6], Some substituted metal phthalocyanines can be obtained by chemical or electrochemical reduction [118e]. Among a number of reported peculiarities of substituted phthalocyanines, the existence of three electronic isomers for magnesium derivative PcMn was recently confirmed [118f]. 

There are many examples of redox-induced structural rearrangements in organometallic complexes the reader is referred to a review by Connelly. The simplest rearrangement is cis/trans or fac/mer isomerization, usually induced by oxidation. Thus, cw-[Mn(CO)2(dppe)2] slowly converts to the trans isomer with a rate constant of 10 s at room temperature. Upon oxidation, the cis — trans isomerization increases in rate by 7 powers of 10. The process is not catalytic, so that stoichiometric oxidation followed by reduction is required to synthetically utilize the increased reactivity of the radicals in the conversion of 18-electron cis-[Mn(CO)2(dppe)2l to frani-[Mn(CO)2(dppe)2]. An example of oxidatively induced fac mer isomerization is given in Scheme 10. The fac mer reaction for the neutral 18-electron isomer is slow, with 2 = 2 X 10 s , K2 = 4. The reaction fac mer is much faster and... 

One of the available ways for investigating intramolecular electron transfer is that of inducing a charge shift by bimolecular PET (Figure Ic). The feasibility of this relies on the fact that the bimolecular event producing the thermodynamically unfavored electronic isomer of the dyad is faster than the intramolecular electron transfer process to be observed. When this requirement is met, since the back recombination reaction is a bimolecular process, relatively slow intramolecular electron transfer processes can be studied. This type of process has been extensively used for studying electron transfer in proteins [206], but has not been frequently applied to dyads. In an early study [207], the Rh(III)-diquat dyad (28) was... 

Scheme 11 summarizes the various unimolecular transformations of the [ct, ct]-, [ct, 7t]-, and [%, 7i]-type dioxyl diradicals, which have been disclosed by our qualitative analysis of the orbital orientations in these electronic isomers. The answer to the original question as to the reasons for the persistence of dioxiranes rests on the appreciable electronic barrier towards rearrangement into the ester for the thermally produced [ct, ct] diradical instead, the [ct, ct] dioxyl species recloses to the dioxirane Consequently, these most highly strained cyclic peroxides can be prepared, isolated, handled, and utilized for synthetic purposes even under ambient conditions. On photochemical activation (n, CT excitation), however, the [ct, it] diradical results, which prefers P-scission into a carboxy-alkyl radical pair rather than rearranging into the ester product. For the latter process,... 

III. If we consider the interaction between two valence-localised electronic isomers, in which the metal centres are of oxidation state +2 or -f3 (e.g. and there exist particular equilibrium ge-... 

XYfi-type molecule is XtF. In their earlier work, Claassen et al suggested the distortion of XeF from Oi, symmetry since they observed two stretching bands in infrared and three stretching bands in Raman spectra. It was not possible, however, to determine the precise structure of XeFe until they carried out a detailed infrared, Raman and electronic spectral study of XeFe vapor as a function of temperature. They were then able to show that XeF consists of the three electronic isomers shown in Fig. 11-17, and to explain subtle differences in spectra at different temperatures as a shift of equilibrium among these three isomers. As expected, the isotope shift of the... 

To give semantic weight to this conception, otherwise identical molecules that differed in their electronic structure came to be known as electronic isomers, and molecules with the same electronic structure but different multiphcity as spin isomers [3]. An example of electronic isomers are molecules of acetone, the electronic structure of which may be formalized as ... 

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