Substituent effects bond length

Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

They display the considerable trans-influence of hydride and aryl groups (the Pt-Cl bond lengths should be compared with the value of c. 2.31 A in PtCl4 ). The Pt-P bond lengths are more insensitive to the phosphine, but by synthesis of ds-Pt(PR3)(PR3)Cl2 [150] complexes, it has been possible to study the effect of electron-withdrawing substituents on the Pt-P bond, as well as on the trans-influence of the phosphine (Figure 3.94). 

The effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

Fig. 25. The effect of metal ion size on porphyrin ruffling. Very small metal ions [P(V) with an ideal P-N bond length of 1.84 A and low-spin Ni(II) with an ideal Ni-N length of 1.90 A in (a) and (b)) cause extensive S4 ruffling. Metal ions close to the right size (M-N = 2.035 A) give planar structures [Zn(II) in (c)]. Metal ions that are too large [Pb(II) at (d) with ideal Pb-N of 2.39 A] are extruded from the plane of the porphyrin and cause it to dome. For clarity, substituents on the porphyrins such as phenyl or ethyl groups have been omitted. Modified after Ref. (77).

There has been a HO study of the stereoelectronic effects in methy1phosphines (90). Steric effects were found to concentrate in the HOHO and accounted for half the substituent effects on the pK values, whilst electronic effects on the HOHO was minimal.242 The relative basicities of polymethoxytriarylphosphines have been measured. Tris(2,4,6-trimethoxyphenyl)phosphine was considerably more basic than piperidine.243 The basicities of P-N compounds have been reviewed and their correlation with P-N bond lengths invest igated.2 4 4... 

An interesting conclusion that Apeloig and Kami have made, based on additional calculations, is that the effects of substituents on the Si=C bond length are approximately additive. Thus, they are able to predict the... 

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