D-electron delocalization

In particular, poly(amidoamine) dendrimers were peripherally modified with diimide moieties (see the structure shown in Scheme 1.43). After rednction with dithionite, this dendrimer was cast into a film, the electronic properties of which were isotropic. (This means that on the molecular and macroscopic levels, there is a three-dimensional (3-D) electron delocalization.) The conductivity was humidity dependent. Water molecules integrate into the material s crystal structure and take part in long-distance electron transfer. Such an effect of water was also observed to enhance electric... 

Eg state, that is k6 ks. No simple correlation has been observed between k6 and vibrational energies of the complexes or with the extent of d electron delocalization onto the ligands as measured by the 2Eg - A2g splitting. It was pointed out by Forster that actual dissociation-recombination of the complex while in the aEg state might affect the magnitude of k6 in some cases. This would not, strictly speaking, be a radiationless decay process and would not be expected to correlate with electron delocalization, etc.17... 

The effective hamiltonian representation of equations [8-10] is readily extensible to systems with more than one TM. The Hdd< matrix for such cases contains terms that account for d-electron delocalization (via the second term in Equation 9) from one metal to another - indirectly via the bridging ligands or directly via the corresponding off-diagonal terms of Hdd [33]. This interaction produces magnetic-exchange coupling, which is reviewed elsewhere [34, 35],... 

For example, consider the Nephelauxetie Effeet. Analysis of the interelectron repulsion parameters derived from analyzing the d-d speetrum invariably leads to lower values than in the free ion. The interpretation is that, in the eomplex, the d eleetrons are, on average, further apart which is consistent with expanded rf-funetions in the eomplex and/or with d electron delocalization onto the ligands. Analysis of the electron density distribution from X-ray diffraction in trans-[Ni(NH3)4(N02)2] yields a rf-orbital radius larger than that for free Ni. However, the unpaired electron density derived from polarized neutron diffraction (PND) data yields a if-orbital radius less than for free Ni " prompting Figgis to propose an anti-Nephelauxetic effect. DFT calculations support LFT in that the d-orbitals expand upon complex formation but also provide an explanation of the diffraction data. 

The following d-orbital energy levels are available for trigonal bipyramid complexes " when 7 Cp/12 Dq = 2, corresponding to less d-electron delocalization by the surrounding Ugands. 

Reactions at the a-carbons have been of considerable kiterest because it is at these positions that enzymatic oxidation, which is beheved to initiate the events leading to carcinogenic metaboUtes, generally occurs (5,7,8,73). The a-hydrogens exchange readily as shown in the following where D represents H. This exchange apparentiy results from stabilization of an anionic intermediate by electron delocalization (74,75). 

As in the preceding transition-metal groups, the refractory behaviour and the relative stabilities of the different oxidation states can be explained by the role of the (n — l)d electrons. Compared to vanadium, chromium has a lower mp, bp and enthalpy of atomization which implies that the 3d electrons are now just beginning to enter the inert electron core of the atom, and so are less readily delocalized by the formation of metal bonds. This is reflected too in the fact that the most stable oxidation state has dropped to +3, while chromium(VI) is strongly oxidizing ... 

A cyclic conjugate molecule composed of n bonds, A, B,..., X, and Y interacts at A with a reactant Z (Scheme 12). When the molecule is an electron donor (Scheme 12a), electrons delocalize from a to z. The resulting electron hole in a is supplied with an electron by the neighboring b. Similar delocalization sequentially follows from c to b, from d to c and so on. This is also the case with the opposite side Y, X,. It follows that the cyclic orbital interaction of a, b,.x, andy is important in the conjugated molecule. The orbitals are all electron-donating orbitals. When each neighboring pair of orbitals is combined out of phase, the interaction of the cyclic... 

The cyclic conjugation is continuous if the donors are on one side of the cyclic chain and the acceptors are on the other side (Scheme 14a). Electrons delocalize from a donor Dj to Aj. The electron accepted by Aj can readily delocalize to the neighbor on the other side because it is an acceptor (A ). An electron can delocalize from D, to A. The delocalization can take place along the other path. donates an electron to A. The resulting electron hole in can be supplied with an electron by the neighbor Dj. This is equivalent to the delocalization from Dj to A. Electrons can delocalize in a cyclic manner. Thermodynamic stability of continuously conjugated molecules is under control of the orbital phase property or determined by the number of n electrons. 

Important changes in the electronic structure occur. Electron delocalization decreases, which is reflected in a narrowing, especially, of the d-valence electron... 

A pure transition metal is best described by the band theory of solids, as introduced in Chapter 10. In this model, the valence s and d electrons form extended bands of orbitals that are delocalized over the entire network of metal atoms. These valence electrons are easily removed, so most elements In the d block react readily to form compounds oxides such as Fc2 O3, sulfides such as ZnS, and mineral salts such as zircon, ZrSi O4. ... 

The term D of Eq. (3.14) is called the delocalization stabilization, which is usually positive. This term comes from the electron delocalization between the molecules A and B. The physical meaning of the denominator of each term in the right side of Eq. (3.14) can be discussed in relation to the Koopmans theorem 58)... 

Noodleman, L., C. Y. Peng, D. A. Case, and J. M. Muesca. 1995. Orbital Interactions, Electron Delocalization and Spin Coupling in Iron-Sulfur Clusters. Coordination Chemistry Reviews 144, 199. 

A positive charge in an empty rc-orbital can be stabilized by the + M effect exerted by the free electron pair on an adjacent atom X, or by a filled tr-orbital of a double bond C = D. In delocalizing a positive charge into a double bond C = D, the larger orbital coefficient is again on atom D. The higher the electronegativities of the orbitals on X or on D, the less they are available for donation of electron density into the... 

The next cyclic alkadiene, 1,3-cyclopentadiene, has been experimentally studied by MW, GED and XR methods. The carbon skeleton is planar (C2v symmetry), and the small C=C—C angles compared to those in 1,3-butadiene (124.3°) or d.s-1-butene (126.40)58 do not seem to influence noticeably the lengths of the CC bonds, although other effects, such as 7r-electron delocalization, might have an opposite effect. The apparently normal structure parameters observed for 1,3-cyclopentadiene might therefore be a result of different forces having opposite effects on the structure parameters. 

Figure A.6 Simplified density of states of metals showing the broad band of the delocalized s- and p-electrons, and the narrower band of the more localized d-electrons.

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