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Valence (chemistry)

About: Valence (chemistry) is a research topic. Over the lifetime, 24937 publications have been published within this topic receiving 645252 citations. The topic is also known as: valency.


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TL;DR: Wang et al. as discussed by the authors reported experimental and theoretical evidence of d aromaticity in the [Ta3O3] cluster through combined photoelectron spectroscopy and ab initio studies.
Abstract: The concept of aromaticity was introduced into organic chemistry to describe delocalized p bonding in planar, cyclic, and conjugate molecules possessing (4n+2) p electrons. In recent years, this concept has been advanced into main-group molecules including organometallic compounds with cyclic cores of metal atoms and, in particular, all-metal clusters. It has been shown that main-group clusters may exhibit multiple aromaticity (s and p), multiple antiaromaticity (s and p), and conflicting aromaticity (s aromaticity and p antiaromaticity or s antiaromaticity and p aromaticity). Here, we report experimental and theoretical evidence of d aromaticity, which is only possible in transition-metal systems. It is discovered in the [Ta3O3] cluster through combined photoelectron spectroscopy and ab initio studies. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of [Ta3O3] and are compared with ab initio calculations, which show that the [Ta3O3] cluster has a planar D3h triangular structure. Chemical-bonding analyses reveal that among the five valence molecular orbitals involved in the multicenter metal–metal bonding, there is a completely bonding d and p orbital formed from the 5d atomic orbitals of Ta. The totally delocalized multicenter d bond renders d aromaticity for [Ta3O3] and represents a new mode of chemical bonding. [Ta3O3] is the first d-aromatic molecule confirmed experimentally and theoretically, which suggests that d aromaticity may exist in many multinuclear, lowoxidation-state transition-metal compounds. In 1964, Cotton and co-workers published a milestone work on K2[Re2Cl8]·2H2O, [7] in which they showed the presence of a new type of chemical bond—a d bond between the two Re atoms. Since then, a branch of inorganic chemistry has been developed that involves multiple metal–metal bonding with bond orders higher than three, the maximum allowed for main-group systems. Power and co-workers recently reported the synthesis of a Cr2 compound with a quintuple bond (spd) between the two Cr atoms. This work, along with recent quantum chemical studies of multiple bonds in U2 and [Re2Cl8] 2 , has generated renewed interest in multiple metal–metal bonding. The presence of d bonds between two transition-metal atoms suggests that multicenter transition-metal species with a completely delocalized cyclic d bond may exist, thus raising the possibility of d aromaticity analogous to p or s aromaticity in main-group systems. We have been interested in understanding the electronic structure and chemical bonding of early transition-metal oxide clusters as a function of size and composition, and in using them as potential molecular models for oxide catalysts. During our investigation of tantalum oxide clusters, we found the presence of d aromaticity in the [Ta3O3] cluster, in which each Ta atom is in a low oxidation state of Ta and still possesses three electrons for Ta–Ta bonding. The experiment was conducted by using a magneticbottle-type photoelectron spectroscopy apparatus equipped with a laser vaporization cluster source. [TamOn] clusters with various compositions were produced by laser vaporization of a pure tantalum disk target in the presence of a helium carrier gas seeded with O2, and were size-separated by time-of-flight mass spectrometry. The [Ta3O3] species was mass-selected and decelerated before photodetachment by a pulsed laser beam. Photoelectron spectra were obtained at two relatively high photon energies, 193 nm (6.424 eV) and 157 nm (7.866 eV), to guarantee access to all valence electronic transitions (Figure 1). Three well-resolved bands (X, A, and B) were observed at the lower-binding-energy side. The X band is much more intense and shows a discernible splitting at 193 nm (Figure 1a). Surprisingly, no well-defined electronic transitions were observed beyond 3.7 eV, where continuous signals were present, probably as a result of multielectron transitions. The vertical detachment energies (VDEs) of the observed transitions at the low-bindingenergy side are given in Table 1, where they are compared with theoretical calculations by two different methods. [*] Dr. H. J. Zhai, Prof. Dr. L. S. Wang Department of Physics Washington State University 2710 University Drive, Richland, WA 99354 (USA) and Chemical & Materials Sciences Division Pacific Northwest National Laboratory MS K8–88, P.O. Box 999, Richland, WA 99352 (USA) Fax: (+1)509-376-6066 E-mail: ls.wang@pnl.gov

114 citations

Journal ArticleDOI
TL;DR: The Mossbauer spectra of both reduced and oxidized high-potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian and a model of the four-iron four-sulphide active centre is interpreted which is consistent with its electronic and magnetic properties in both redox states.
Abstract: The Mossbauer spectra of both reduced and oxidized high-potential iron protein (Hipip) from Chromatium have been analysed using computer fits to theoretical spectra derived from a spin Hamiltonian. Fits to spectra obtained over a range of temperatures between 4.2 and 195 K and in applied magnetic fields up to 10.0 T lead to a consistent set of hyperfine parameters. These results are interpreted in terms of a model of the four-iron four-sulphide active centre which is consistent with its electronic and magnetic properties in both redox states. In the model for the reduced centre all four iron atoms have essentially the same valence, intermediate between ferric and ferrous, with the spins being coupled antiferromagnetically to give the centre zero net spin. The oxidized centre has one less electron which at low temperatures appears to have come predominantly from one pair of iron atoms which thus become ferric with the other pair remaining substantially unchanged. It is clear from the Mossbauer hyperfine parameters obtained from the computer fits to the low-temperature spectra that a larger magnetic moment is associated with the ferric/ferrous pair of iron atoms than with the ferric pair of iron atoms. This also explains the g values with an average of greater than 2 which are observed in electron paramagnetic resonance (EPR) measurements. At higher temperatures the differences between the electron charge density at the different iron atoms in the oxidized centre appear to become smeared out.

114 citations

Journal ArticleDOI
TL;DR: In this article, the authors describe how the Hubbard model in the atomic limit implemented by taking into account molecular vibrations can give a description of mixed valence binuclear systems with both metallic ions simultaneously magnetic.
Abstract: This paper describes how the Hubbard model in the atomic limit implemented by taking into account molecular vibrations can give a description of mixed valence binuclear systems with both metallic ions simultaneously magnetic. A FeIII (high spin) FeII (high spin) binuclear complex would constitute an example. In such systems electron transfer and electron exchange are expected. If the compound belongs to class II (Robin and Day classification) we find that the activation energy of the thermal electron transfer and the intensity of the intervalence band are spin dependent but that the energies of spin states are given by the Heisenberg Hamiltonian with a new expression for the exchange parameter. For class III binuclear complexes a quite different behavior is found. The energy of the intervalence band is spin dependent but energies of spin states are no longer given by the Heisenberg Hamiltonian.

114 citations

Journal ArticleDOI
TL;DR: Etude du transfert d'electrons dans les heteropolysels contenant du tungstene a partir des variations des profils de raies en fonction de la temperature, de ces spectres RPE of ces composes as mentioned in this paper.
Abstract: Etude du transfert d'electrons dans les heteropolysels contenant du tungstene a partir des variations des profils de raies en fonction de la temperature, dans les spectres RPE de ces composes

114 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,521
20222,997
2021616
2020611
2019584
2018577