Jt delocalization

Acyclic Tricoordinate Radicals As one can easily note, the above examples of the cyclic radicals of Si and Ge atoms have rather particular structures featuring the cyclic delocalization of the odd electrons. The simple tricoordinate acyclic radicals of the type RsE (E = Si, Ge, Sn, Pb), lacking the stabilization effects of the cyclic Jt-delocalization, constitute another, more general and even more challenging, class of stable organometallic radicals. " Consequently, the search for such highly symmetrical species appeared to be of primary importance for synthetic chemists. 

Muller et al.332 have characterized a number of a-aryl-, (3,(3 -disilyl-substituted vinyl cations (130). Both 13C NMR data and calculations show the effect of both Tt-conjugation with the aromatic moiety and O-delocalization of the (3-Si—C bonds. Jt-Delocalization is manifested by the marked low-field shift of the ortho and para carbon atoms compared to those of the precursor alkynes, whereas o-delocalization is shown by the deshielding of the 29Si NMR resonance relative to the precursor silylalkyne (A829Si 29.5-41.4). Computed structures [B3LYP/6-31G(d) level] are all very similar, that is, calculation is not able to show the subtle interplay between O- and 7t-delocalization, which is evident from the NMR spectroscopic data. 

The He(l) photoelectron spectrum of trithiadiazepin 5 was assigned by Koopmans correlation with PM3 eigenvalues based on the structural data, and by the 7i-perfluoro effect observed. The 7i-system can be rationalized by heteroatom first-order perturbation, which reduces the cyclic Jt-delocalization. Replacement of the four F substituents by FI affected neither the long-wavelength absorption band in the UV/Vis (Vis = visible) spectrum nor the S1SN shift in the 15N NMR <1997CBR247>. 

Like porphyrins, heteroporphyrins are aromatic and confirm the [4n+2] it-elecfron Huckel rule (00MI3). Replacing N with O, S, Se, and Te perturbs jt-delocalization and electronic properties. The alteration in jt-delocalization is evidenced from downfield shifts in H NMR spectra. The extent of shift in monoheteroatom is more than diheteroatom-substituted porphyrins (1979JA7055, 19840MR561). The large heteroatoms... 

Cyclic voltammetry revealed that the N-atoms of 86a, 88a and 91a are oxidized at lower potentials than the trivalent P atoms. Comparison of these data with those observed with model compounds shows a very weak electronic delocalization via the P centers for copolymers 86a and 91a. In contrast, the low first oxidation potential observed for 88a (Table 4.5) is assumed to result from an electronic communication between the N moieties through the connecting P centers [59b]. The equivalence of the oxidation potentials for the oxidized polymers (Table 4.5) suggests the presence of electronically isolated triarylamine fragments in these derivatives. Note that the involvement of the P lone pair in jt-delocalization... 

Cremer and co-workers [48] assessed the degree of electron delocalization in 61a by comparing aryl-substituted silylium cations with regard to calculated rotational energies, the calculated electron population of the 3pw(Si) orbital, the 13C NMR chemical shift of the p-positioned C atom, and the 29Si NMR chemical shift values (Table 9). They came to the conclusion that about 70% jt-delocalization of... 

Electronic, geometric, energetic, and magnetic conventional criteria of aromaticity when applied to cobalta- and iridacyclopentadiene showed that these heterocycles are firt-electronic and aromatic <2004JOM(689)1050>. However, there is controversy in the literature concerning the existence and extent of Jt-delocalization in metalla-cyclopentadienes, -trienes, and metallacyclopentynes, and more systematic theoretical computations are needed. 

The extensive jt-delocalized system of metal-dithiolene complexes is also responsible for the nonlinear optical properties (NLO) which have been recently reviewed . The interaction of radiation with the matter induces an instantaneous displacement (polarization Pq = /X = aE, where a is the linear polarizability) of the electronic density away from the nucleus at small field (linear optics). At high fields (laser light) the polarizability of the molecule can be driven beyond the linear regime and a nonlinear polarization is induced (NLO) = aE + fiE" + y E + and for the bulk material... 

The quaternization of one phosphorus atom in the cation of 91 causes a further shortening of the P-P bond to 2.095(1) A accompanied by an equalizing of both PC contacts to 1.737(4) and 1.747(4) A. Again a short CN bond [1.322(4) A] points to an extensive Jt-delocalization as expressed by the canonical formulas 91-91". Thus, the cations also show the structural features of a phosphinylidene [Pg.729]

Figure 1.11. Three-center bonding possibilities for some cationic and neutral hydrocarbon systems, (a) Some o delocalized systems (b) some jt delocalized systems (c) some related electron-precise hydrocarbons.

The degree of jt-delocalization between atoms A and B was quantified in the framework of the —> AIM theory, by the delocalization index DIab, which is defined as ]Poater, Fradera et al., 2003a, 2003b Poater, Garcia-Cruz et al., 2004 Krygowski, Ejsmont et al., 2004]... 

Protonation from above the plane at Cl and at C2 gives tertiary carbenium ions, which are both relatively stable, but the phenyl group allows extensive Jt-delocalization of the positive charge and, consequently, the ion with its charge adjacent to Ph is more stable and formed preferentially (Scheme 2.33). 

The first examples of d-based double (o- and n-) aromaticity were reported by Chi and Liu in bare Xj (X = Sc, Y, La) clusters [116]. According to their calculations, 2a/- and la2"-MOs (both formed by d-AOs of transition-metal atoms) are responsible for delocalized bonding in equilateral triangular global minimum structures of Xj". The 2a -MO is regarded as o-delocalized MO and the la2"-MO is regarded as Jt-delocalized MO, which are responsible for their o- and ji-aromaticity, respectively. 

Diphenyl porphyrins and their Zn° complexes substituted in the meso position by a Jt-delocalized substituent carrying an electron-donor or an electron-withdrawing group (49) were also investigated by the EFISH technique [169]. These second-order NLO chromophores have confirmed the ambivalent role of the polarizable porphyrin ring, which, in the ground state, already acts as a donor or acceptor. 

EPR spectroscopy suggests that part of this stabilization arises from Jt-delocal-ization. g values in [Co(9S3)2] (Table 5) deviate little from g, a reflection of appreciable delocalization of metal electron density onto the ligands [22, 99]. 

L. Latos-Grazytiski, J. Lisowski, L. Szterenberg, M. M. Ohnstead and A. L. Balch, Crystal and molecular structure of 21-thia-5,20-diphenyl-10,15-bis(p-nitrophenyl)porphyrin and 21,23-dithiatetraphenylporphyrin. The influence of sulfur on the Jt-delocalization pattern, J. Org. Chem., 56, 4043-4045 (1991). 

The synthesis of anionic compounds derived from squaric acid possessing one or more jt-delocalized substituents, such as dicyanomethylene groups. 

Squarate compounds with transition metal ions are rarely described in the chemical literature. For example, the crystal structure of cis-bis(dicyanomethylene) squarate 27 in a complex with Cu(I) has been reported [48b]. Galibert and collaborators synthesized a complex of Cu(II) with fraws-bis(dicyanomethylene) squarate 26 [86]. The pseudo-oxocarbon ring in this complex was found to be planar. However, a significant deviation of the cyano groups from the best plane formed by the ring contributes to the diminished planarity of the complex, a phenomenon that is also reflected by the loss of Jt-delocalization in the ring. 

S.S. Shaik, A. Shurki, D. Danovich, P.C. Hiberty, A different story of Jt-delocalization—The distortivity of Jt-electrons and its chemical manifestations, Chem. Rev. 101,1501, 2001. 

Driven by these results, other ligands have been successively developed, bearing a Jt-delocalized aromatic linker between the two carbenes (17—19) and whose main interesting feature relies on the possibility to enable electronic communication between the two carbene moieties, and as a consequence between the two coordinated metal centers. 

The type of resonance that we see in the allyl and benzyl cations is called jt-delocalization. Cations can also be stabilized by sharing a lone pair on an adjacent atom—called w-delocalization (h is for nonbonding electrons). Examples are provided by 4.67 and 4.68, and the resonance structures with the charge on the heteroatom are very significant, since in these, all the atoms have octets of electrons. 

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