P-Jt-bonding

Transition Metal Bonding Involving P—P jt Bonds and Pnictogen Delocalized Systems 1044... 

Spectroscopic evidence that ethylene preferably hydrogenates directly via the jt-bonded adsorbate and not via the more stable di-<7 bonded state has been provided by Somorjai and coworkers by in situ sum-frequency generation measurements [P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai,y. Am. Chem. Soc. 118 (1996) 2942]. 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

Aryl- and alkylsulfonyl radicals have been generated from the corresponding iodides and added to, e.g., propadiene (la), enantiomerically enriched (P)-(+)-propa-2,3-diene [(P)-(lc)] and (P)-(-)-cyclonona-l,2-diene [(P)-(lk)] [47]. Diaddition of sulfo-nyl radicals may compete considerably with the monoaddition [48,49]. Also, products of diiodination have been purified from likewise obtained reaction mixtures, which points to a more complex reactivity pattern of these substrates towards cumulated Jt-bonds. An analysis of regioselectivities of arylsulfonyl radical addition to allenes is in agreement with the familiar trend that a-addition occurs in propadiene (la), whereas alkyl-substitution at the cumulated Jt-bond is associated with a marked increase in formation of /3-addition products (Scheme 11.7). 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

The bonding in ethylene is based initially on one C-C CT bond together with four C-H a bonds, much as we have seen in ethane. We are then left with a p orbital for each carbon, each carrying one electron, and these interact by side-to-side overlap to produce a IX bond (Figure 2.15). This makes the ethylene molecule planar, with bond angles of 120°, and the TX bond has its electron density above and below this plane. The combination of the C-C ct bond and the C-C Jt bond is what we refer to as a double bond note that we cannot have Jt bond formation... 

The bonding in acetylene has one C-C a bond together with two C-H a bonds the p orbitals on each carbon, each carrying one electron, interact by side-to-side overlap to produce two Jt bonds (Figure 2.19). Note again that the p orbitals can only overlap if their axes are parallel. 

When we consider double bonds to oxygen, as in carbonyl groups (C=0) or to nitrogen, as in imine functions (C=N), we find that experimental data are best accommodated by the premise that these atoms are sp hybridized (Figure 2.22). This effectively follows the pattern for carbon-carbon double bonds (see Section 2.6.2). The double bond is again a combination of a ct bond plus a jt bond resulting from overlap of p atomic orbitals. The carbonyl... 

To achieve this stabiiization, the p orbitai on nitrogen needs to be iined np with the carbonyi jt bond. The immediate conseqnences of this are that five bonds in the peptide iinkage must be copianar. There is no free rotation about the N-C bond, because it is invoived with a partiai doubie-bond system. Of... 

The conjugated C-atoms of a fullerene respond to the deviation from planarity by rehybridization of the sp o and the p Jt orbitals, since pure p character of Jt orbitals is only possible in strictly planar situations ]56]. The electronic struchue of non-planar organic molecules has been analyzed by Haddon using the Jt orbital axis vector (POAV) analysis. For Cjq an average o bond hybridization of sp and a fractional s character of 0.085 (POAVl) or 0.081 (POAV2) was foimd ]59-63]. Consequently, the Jt orbitals extend further beyond the outer surface than into the interior of Cgg. This consideration implies, moreover, that fullerenes and, in particular, Cgg are fairly electronegative molecules [64, 65] since, due to the rehybridization, low-lying Jt orbitals also exhibit considerable s character. 

Fully conjugated 1,2-thiazines have been prepared with both S(ll) and S(vi) oxidation states. While the zwitterionic compound 11 has been known for some time <1984CHECI(3)995, 1996CHEC-II(6)349>, thiazinylium salts 12 have only recently been prepared. Both fully unsaturated 1,2-thiazine derivatives are considered to be nonaromatic due to poor p-d Jt-bonding. Eurthermore, the six-membered ring of l-alkyl-l,2-thiazine 1-oxide 11 is not planar, but instead exists in a puckered, half-boat conformation thereby precluding aromaticity <1978CC197>. 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene n bond strengths vary with the level of fluonnation (Table 18) Both CHF=CF2 and CF2=CF2 have significantly weaker n bonds than CH2= CH2, CH2=CHF, and CH2=CF2, consistent with other data indicatmg that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low jt bond energy in CF2=CF2 underlies its propensity to undergo thermal biradical [2+2] cycloadditions [703] (see p 767 )... 

For MCAT proposes, two conditions must exist for resonance to occur 1) a species must contain an atom either with a p orbital or an unshared pair of electrons 2) that atom must be single bonded to an atom that possesses a double or triple bond. Such species are called conjugated u use hi rated systems. The adjacent p orbital in a conjugated system may contain zero, one, or two electrons (as in another Jt-bond - The p orbital allows the adjacent ji-bund to extend and encompass more than two nuclei. 

Situation (i) will apply for ligands like F, OH- etc. Situation (ii) holds for ligands where the capacity for jt bonding stems from the postulated availability of, e.g., the 3d orbitals of the P atom in PX3, or the Jt antibonding MOs of CO and other such ligands. 

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