Divalent electronic structures

The divalent carbon(O) atom in L C L has two lone electron pairs which makes CL2 a particular class of ligands which may bind as a bidentate Lewis base to one and to two monodentate Lewis acids. The nature of the ligand L determines whether a divalent carbon atom behaves as a carbone or as a carbene. The newly gained insight into the electronic structure of carbones opens a large field for theoretical and experimental research. 

In magnetite Fe304, the divalent ions are iron and the interaction between ions on adjacent octahedral sites is particularly strong. One way of looking at the electronic structure of this oxide is to consider it as an array of ions and Fe ions. 

The periodic system developed from Bohr s atomic theory is of the greatest importance in chemical science because it demonstrates that the properties of the elements depend on their positions in the system. It is immediately apparent that chemical valency depends on the number of loosely-bound electrons in the atom. Thus, the alkali metals have one such electron while the divalent alkaline-earth metals have two, etc. Valency is therefore closely connected with electronic structure and provides the foundation for the modern theory of the chemical bond, the basis of which is to be found in the coupling or transfer of the valency electrons. 

We consider in this section a third reactive intermediate, the carbene.141 A carbene is a molecule containing a divalent carbon that bears an unshared pair nf electrons- (Structure 56) Carbenes may be considered as the conjugate bases of... 

Divalent Synthetic Equivalents. These species display a fascinating ambiguity by showing a formal appearance of divalent complexes, but having in reality electronic structures of higher valent species. Yet, the chemical reactivity is what one could predict for genuine divalent species. 

Simple considerations of the electronic structure of carbenes indicate that, of the six valence electrons associated with the divalent carbon, four are taken up by the two covalent bonds and two are non-bonding. The divalent carbon atom has two orbitals available to accommodate... 

By the MEM charge density studies, the different features of encapsulated metal atoms in C82 were revealed for La C82 [2] and Sc C82 [3]. To compare the three-dimensional shape of the metal-atom charge distribution, a section of the equi-density surface of a La C82 molecule is presented in Fig. 9 together with the result for Sc C82 (Isomer I). The equi-density level is 1.8 e/A3. The number of electrons belonging to the Sc atom of Sc C82 was 18.8(2) e, which is close to the 19.0 e of a divalent state of the scandium atom, Sc2+. This indicates that the Sc C82 (I) is an endohedral metallofullerene whose formal electronic structure is Sc2+C82. This result has brought the long discussion as to whether the Sc atom is in a divalent or trivalent state inside the carbon cage [26-29] to an end experimentally. 

Further evaluations [164,165] have demonstrated the applicability of the fitted system of parameters for calculations of the electronic structure and spectra of numerous complexes of divalent cations using merely the CNDO parametrization for the /-system. In [140,169] the EHCF method is also extended for calculations of the ligands by the INDO, MINDO/3, and SINDO/1 parametrizations. In all calculations the experimental multiplicity (spin) and spatial symmetry of the corresponding ground stales were reproduced correctly. The summit of this approach has been reached in [170] by calculations on the i.s-[Fe(NCS btbipyjo] complex. Its molecular geometry is known both for the HS and LS isomers of the said compound. The calculation... 

Effect of Water and Divalent Metal Ions on the Electronic Structure of DNA. As a first step in investigating the effect of impurities on the electronic structure of DNA, the band structure of poly(G-C) was computed in the PPP CO approximation assuming that one or two water molecules are bound by hydrogen bonds to the NH2 group of each C molecule or/and to the 0=0 group of each G molecule.94 According to the results obtained for these systems the additional 7r-orbital in the H20 molecules produced an extra 71-band between the lowest filled bands, while the other bands remained practically unchanged. 

It is significant that the electronic state of Sc is variable, and sometimes controversial. Both experimental and theoretical results recommend the divalent state for Sc in mono-metallofullerenes and di-metallofullerenes [ 119], but it was determined that the three Sc atoms in Sc3N Cso take the - -3 valency. Variable electronic states of Tm in different EMFs were also found. For example, Tm was found to be -1-2 in Tm Cs2, but -1-3 in Tm3N Cso [120]. It is worth noting that a purely ionic picture is not valid to describe the electronic structure of the encaged metals because the charges are not observable quantities [2]. 

Isocyanides have attracted much attention in synthetic organic chemistry, and have led to the development of a variety of useful synthetic transformations [1]. In particular, carbon-carbon bond forming reactions using isocyanides as key reagents have been extensively studied during the past 50 years, as they allow the realization of useful multi-component assembly reactions, such as the Ugi and Passerini reactions [2]. These have once again been highlighted in recent years with the development of combinatorial chemistry. The characteristic reactions of isocyanides are mostly ascribed to the unique reactivity of the isocyano carbon atom, which can be represented by a divalent, carbene-like electronic structure (Scheme 1). In this respect,... 

Inoue et al. [303] found that the correlation between log (hydrous Nb20s) at pH 3 for four trivalent and three divalent metal cations and logarithm of stability constant of l-I acetate complex (r=0.96) was better than with the first hydrolysis constant (r = 0.91). An equally good correlation between log Ap (hydrous Zr02) at pH 4 for three trivalent and four divalent metal cations and logarithm of the stability constant of 1-1 acetate complex (r = 0.98) and of the first hydrolysis constant (r = 0.99) was found [407]. In another publication Inoue et al, [304] arranged the cations into three groups on basis of the valence and electronic structure, and the... 

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