Delocalized electrons structure

It is also worthy to note that the pentacarbonyl-tungsten complex of the phosphirenylium cation (5) has been recently reported, but no X-ray structure has been obtained. B3LYP/6-311G(d) optimization of the presumed -Cr(CO)5 complex of 5 resulted in an 7 -form (Figure 1) when starting from the bound chromium complex. Such a structure is in complete accord with a delocalized electronic structure of the cation. 

In this orthorhombic structure, superconductivity is not observed, and two inequivalent Bi sites are observed. A cubic, but non-superconducting specimen has been reported (10) for x =. 13. In this structure, all Bi sites are equivalent, and a metallic or delocalized electronic structure would be expected to exist. This structure apparently persists into the superconducting compositions, where it has been observed for x =. 374 (4) which displays a Tc of 30.5 K. A cubic phase has also been reported for x =. 6, with Tc = 34K. The limit of this solid solution which may be at or beyond this x =. 6 composition, may be dependent upon synthesis temperature and technique. Indications of phase disproportionation at these high x values have been published by Jones et al. (11). 

Note that, because electron correlation often stabilizes delocalized electronic structures over localized ones, HF theory tends to be inaccurate for such systems in the opposite direction from DFT, and thus, again, hybrid ACM functionals tend to show improved performance by an offsetting of errors. 

XVI), most of the aforementioned change does not appear to be caused by an electron density increase at C-5 but by the change to a dihydropyrimidine structure. This is conceivable if the adduct is not a free anion but is strongly associated with lithium. Further information about the structure of adduct 96 comes from a comparison between the anionic adduct 30 with the neutral adduct 33, formed from pyrimidine and 1-methylpyrimidinium cation, respectively, by reaction with NH2". Here the C-5 position of the anionic adduct is found to be 10.2 ppm upheld with respect to the neutral adduct. If it is assumed that in ammonia the anionic adduct is not associated with the positive counterion, the aforementioned phenyllithium adduct is more likely to possess a slightly delocalized electronic structure (104) resembling that of a dihydropyrimidine. This fact is not surprising, also in view of the low polarity of the solvent. 

The novel four-center two-electron delocalized o-bishomoaromatic species 593, 594,599,601a, and 603 are representatives of a new class of 2jt-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations985 (Figure 3.25, 604). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 605 having a conventional p-type delocalized electron structure (Figure 3.25). 

Compound 12, incorporating two heterocyclic nuclei, is very polarizable and shows a large solvatochromic behavior.9 A polar solvent shifts the equilibrium toward the opened form as shown in Table 2.7. Nuclear magnetic resonance (NMR) experiments (400 MHz 1H) showed that the open forms of merocyanines are transoid toward the azomethine bridge. The delocalized electronic structure tends to become more quinoidal with decreasing polarity of the medium.9... 

Biferrocene 1 (M = Fe) oxidizes in two separate one-electron steps (6, 7) (Ei/2 = 0.31 and 0.64 V), of which only the first is chemically reversible (i.e., the monocation is stable but the dication is not) mild oxidation yields the monocation [l]" (M = Fe) as a mixed-valence Fe(II)Fe(III) species (8-10). Bis(fulvalene)diiron 3 (M = Fe) similarly undergoes two successive one-electron oxidations (the dication is also stable), but in this case the Mdssbauer spectrum of the monocation shows the iron atoms to be equivalent, implying a delocalized electronic structure and an oxidation state of 2.5 for each metal (9,11). The separation of... 

Consistent with the dithiolene structure proposed for the oxidized derivative of molybdopterin (3), no nonexchangeable H resonances are observed in the region expected for the CH protons of a dithiolate structure (5b). The mass spectrum of 3 is also consistent with the proposed dithiolene structure (19, 34). Resonance Raman spectra of DMSO reductase show bands at 1575 cm (oxidized form) and 1568 cm (reduced form) that are assigned to the C=C stretch of the dithiolene unit of 2 (40). However, the delocalized electronic structure of dithiolene ligands makes it difficult to assign the C=C stretch with certainty. As Rajagopalan notes (19), ultimate proof of the structure [of molybdopterin] will have to await either X-ray studies on a molyb-doenzyme or unequivocal chemical synthesis of the molecule. ... 

V(NCMe)6)[VCl2(odt)]2, whose structures have been determined. The tris-dithiolene complex [V(SCHCHS)3] has Dah symmetry and the compound has a delocalized electronic structure involving a spread of charge over the dithiolene ligands. " ... 

The d oxidative addition may seem unfamiliar because there are many more examples 47) of d d and d d processes. However, ruthenocene, which is sl (P ruthenium (II) complex with a delocalized electronic structure, undergoes two-electron oxidative addition by I2 and Br2 to give the Ru(IV) complexes Ru(cp)2r and Ru(cp)2Br (48). X-ray studies of Ru(cp)2r show that it is eflFectively a seven-co-ordinate complex (48). 

Although the geometrical parameters, in particular the Cu-Cu and S-S distances, are quite different between the 1V54B and 1V540 models (Fig. 30.3a), the shapes and symmetries of the Ou and Ku RAMOs are equivalent to each other. This orbital similarity results in the similarity in the electronic structures between two models. It indicates that the Cua site can transfer electrons despite the distortion of the diamond core, implying that the Cua site can be regarded as a flexible electron mediator. This flexibility is useful for the incorporation of the Cua site to protein for electron transfer inside the protein. In the previous study [23], we exhibited that heme a in CcO can also keep the delocalized electronic structure in spite of the deformation of the porphyrin ring. These results indicate that metal cofactors, which are involved in the electron transfer in proteins, have such robustness of the delocalized state. 

Resonance is a way that chemists represent delocalized electronic structure. Let s take that statement apart to understand what it means. Delocalized means that an electron, or a pair of electrons, is not located entirely around a single atom or bond. Take a look at the two structures of nitrogen dioxide (NO2) on the following page. The n ative charge is located... 

The earliest period of work on pterin models for Moco followed the discovery of the pterin unit within Moco, and occurred prior to the confirmation of the dithiolene chelate. These early studies explored the coordination chemistry between molybdenum and pterins or other structurally related molecules such as pteridines (Figure 2.1, top). The resulting themes of this body of work include the favorable coordination by molybdenum in several oxidation states to the 04, N5 chelate site in pterin (see Figure 2.1 for numbering), a variety of reactivities exhibited by Mo -tetrahydropterin systems and the highly delocalized electronic structures in molybdenum-pterin complexes that defy formal oxidation state assignments to Mo and pterin. 

In the case of molybdenum complexes of oxidized pterin and pteridine ligands, a similar argument leads to the formulation of these complexes as Mo(v) bound to a protonated, one-electron reduced pteridine radical, or Mo -(Hpterin ) (Scheme 2.18). This interpretation is consonant with earlier work on Ru"-flavin complexes. Radical character on flavin arising from intramolecular electron transfer was consistent with a structural distortion of flavin observed crystallographically, interpreted as partial Ru" -flavinsemiquinone character. Similarly, the Mo complexes of oxidized pterins and pteridines display short M=N5 bonds and bent flavin or pterin planes, consistent with a similar delocalized electronic structure. 

A] are consistent with a delocalized electronic structure to which the canonical form (84b) probably makes a significant contribution. Similar... 

---