Formal Valence Bond Structures

The vast majority of presently known phosphorus compounds correspond to those arrangements shown within the closed rectangles, with the AV and varieties being the most common. The remaining bond arrangements represent either unknown, comparatively rare, or only contributing states to a molecule. 

Some 60 years ago, almost the whole of known phosphorus chemistry was divided between trivalent pyramidal pentavalent tetrahedral (AV) and a few pentavalent trigonal bipyrami-dal (AV) compounds (3.1c) through (3.1e). Only since the 1960s have significant numbers of A% compounds and some of the other varieties listed in Table 3.8 been synthesised. The synthesis of many of these latter compounds has clearly invalidated the double bond rule which had come to be fairly widely accepted by 1950. This now obsolete rule stated that the formation of double bonds between P and first row elements was virtually impossible. 

The still uncommon A% and A o arrangements are represented by compounds (3.12a) and (3.12b) respectively, while examples of A% compounds are provided by (3.13a,b). The AV and type compounds are represented by (3.13d,e) and (3.13f), respectively moreover, A o compounds have also been reported (3.13g). 

Formal Valence Bond Structures of Phosphorus Compounds 

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The first spectroscopic observation of the radical SiN, a silicon compound that can be formulated as triply bonded in one of its formal valence-bond structures ( Si=Nf) dates back to 19 1 3323 -325 Despite this early appearance it was not until recently that enhanced effort was applied to the synthesis of triply bonded silicon species. In this connection a group of authors photolysed and pyrolysed the geminal triazide, 719, in the attempt to isolate a silicon analogue of a nitrile or an isocyanide.326... 

The bonding in I5+ (and in Br5+) can be described in terms of valence bond structures 325a and 325b showing a formal bond order of 1 for the terminal I—I bonds.788 The bond order of the central bonds is 0.5, and these bonds may be considered as three-center four-electron bonds. 

The 1,2- and 1,4-quinone methides are formally neutral molecules. However, the zwitterionic aromatic valence bond resonance structures (Scheme 1) make an important contribution to their structure. This combination of neutral and zwitterionic valence bond structures confers a distinctive chemical reactivity to quinone methides, which has attracted the interest of many chemists and biochemists. 

The bond valence of Re4(/u-3 -H)4(CO) 12 is 8, and the tetrahedral Re4 skeleton can be described in two ways (I) resonance between valence-bond structures, leading to a formal bond order of lj, and (II) four 3c-2e ReReRe bonds. Since there are already four 1x3 -11 capping the faces, description (II) is not as good as (I). 

Although the bis(dithiocarbamate) complexes of Fe(II) are relatively unstable to air oxidation, early studies (12, 15) produced stable adducts of NO and CO. Both 5-coordinate [Fe(NO)(I dtc)2] and 6-coordinate [Fe(NO)2(R2dtc)2] complexes are known. There has been considerable interest in the mode of attachment of the NO molecule, as there are six possibilities (see Scheme 4). (A) and (B) represent valence-bond structures of the linear Fe-NO bond. Structure (C) involves a symmetric Fe-NO TT-bond. Structure D illustrates the bent mode of attachment, in which nitrosyl is coordinated to the metal through the nitrogen atom, but the Fe-NO bond-angle differs greatly from 180°. Structures (E) and (F) are valence-bond formalisms of an unsymmetrical, metal-NO 7r-bond. The structure of [Fe(NO)(R2dtc)2] (R = Me or Et) has been shown (230,231) to be square pyramidal, with four sulfur atoms in... 

Individual formal valence structures of conjugated hydrocarbons are excellent substrates for research in chemical graph theory, whereby many of the concepts of discrete mathematics and combinatorics may be applied to chemical problems. The lecture note published by Cyvin and Gutman (Cy-vin, Gutman 1988)) outlines the main features of this type of research mostly from enumeration viewpoint. In addition to their combinatorial properties, chemists were also interested in relative importance of Kekule valence-bond structures of benzenoid hydrocarbons. In fact, as early as 1973, Graovac et al. (1973) published their Kekule index, which seems to be one of the earliest results on the ordering of Kekule structures These authors used ideas from molecular orbital theory to calculate their indices... 

Using graph-theoretical technique, Hearndon and Ellzey have identified, within the Hiickel MO formalism, a new class of n-stnictures containing even numbers of 7t-orbitals of which 2-cyclopropenylallyl is the smallest member. This has a closed shell of electrons according to HMO theory, but is required to have a biradical valence bond structure, and dicationic species should be obtainable. ... 

Two strong electron-pair 7i-bonds can also be formed by overlapping the doubly-occupied 2p i (or 2p7i) orbitals of the 0 with the vacant Cg orbitals, to give valence-bond structures of type (10). In stmcture (10), the unpaired electrons are localized in the d, y and dy orbitals, and the formal charge on the Fe is zero. 

Using CIO2 as the example, the Addendum 2014 provides an alternative, non d-orbital approach that can be used to reduce the magnitudes of the atomic formal charges of valence-bond structures (30), (31), (41) and (42) for CIO2, CIO3 and SOi. 

As indicated already in Chapter 2, unless stated otherwise (see for example Chapter 23), the equivalent Lewis structure resonance theory assumes that electron-pair bond wavefunctions are of the Heitler-London atomic orbital type - for example y(l)a(2) + a(l)y(2) a.nAy l)b(2) + b )a 2) for structures (6) and (7). Atomic formal charges are not indicated in the generalized valence bond structures that involve the Y, A, B, C and D atoms. 

In Fig. 23.1, we show the canonical stractures and formal charges that correspond to Pi to Piv for Hs. The formal charges are also those for the corresponding valence-bond structures for NO, HCOj and CjH". The corresponding canonical stmctures for O3 are displayed in Table 2-1. 

Both of these structures satisfy the formal valence rules for carbon, but each has a serious fault. Each structure shows three of the carbon-carbon bonds as double bonds, and three are shown as single bonds. There is a wealth of experimental evidence to indicate that this is not true. Any one of the six carbon-carbon bonds in benzene is. the same as any other. Apparently the fourth bond of each carbon atom is shared equally with each adjacent carbon. This makes it difficult to represent the bonding in benzene by our usual line drawings. Benzene seems to be best represented as the superposition or average of the two structures. For simplicity, chemists use either one of the structures shown in (30) usually expressed in a shorthand form (SI) omitting the hydrogen atoms ... 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

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