0-0 bond lengthening

The double-bond length in 1,3-butadiene is 0.134 nm, and the ingle-bond, 0.148 nm. Since normal carbon—carbon single bonds are 0.154 nm, this indicates the extent of double-bond character in the middle single-bond. Upon complexing with metal carbonyl moieties like Fe(CO)2, the two terminal bonds lengthen to 0.141 nm, and the middle bond shortens even more to 0.145 nm (18). 

Extent of bond lengthening increases with electron-accepting capacity of OR. 

Comparison with data (mainly obtained from EXAFS measurements) on osmium diarsine complexes (Table 1.14) shows that as the oxidation state increases, osmium—halogen bonds shorten whereas Os-P and Os—As bonds lengthen. Bond shortening is predicted for bonds with ionic character,... 

An accurate MO study of the inversion barrier in dimethyl sulphoxide9 showed that the height of the calculated barrier is much more sensitive to the overall quality of the basis set, and to geometry optimization, than to the presence of 3d functions. This study predicts an S—O bond lengthening to 1.55 A, and the best estimate of the barrier is 39.9 kcal mol 1. This was the difference in energy between optim um planar and pyramidal... 

We pointed out that these results can be attributable to the a-n interaction. At the transition state, the o orbital at C5 on the anti side of the dienophile is parallel with the n orbital, the o bond electrons are able to delocalize much more effectively than that on the syn side. Since the electron donating o bond on the anti side stabilizes the transition state, the a-n interaction can contribute to rr-facial selectivity. These results suggested that the bond lengthening cannot necessarily be convincing evidence for the Cieplak effect, but can be explained in terms of the a-n interaction without assuming the incipient a bonds at the transition state (Scheme 30). 

The crystal structures of a number of diphosphine disulphides (121) and (122) show a remarkable constancy in the bond lengths. Two types of molecule are observed in the crystal of the tetramethyl compound (121, X = Y = Me). The crystal structure of triphenylphosphine oxide (P—C 176 pm, P—O 164 pm) varies little from that observed in the uranium oxide complexes, and does not confirm P—O bond lengthening in complexes, as indicated by vp=.o (see Section 3C). 

The above relationships between the thiiranes (20) and their dioxides (17) are reminiscent of those between cyclopropane and cyclopropanone. The entire phenomena of the C—C bond lengthening and the concomitant C—S bond shortening in the three-membered ring sulfones and sulfoxides can be accounted for in terms of the sulfur 3d-orbital participation and the variation in the donor-acceptor capacities of the S, SO and S02 . The variations of the calculated valence-state orbital energies, together with the corresponding variations of the C—C overlap populations, can be used to understand the discontinuous variations of the C—C and the C—S bond lengths in the series thiiranes -... 

CHD Re (CO) j. are very close to the gas phase positions and the shift (Mn to Re) is almost identical in gas phase and matrix. Thus, V(C-H). in the matrix will provide information on the C-H bond length. On generation of the unstable 1 2 complexes, there is a downward shift in V(C-H). of 11-14 cm implying a bond lengthening of. 0011-.00 X. This is clearly a very sensitive probe of the effect of slight electronic perturbation of the metal centre and is likely to be useful for other systems. 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

Figure 3.69 C—F bond-order-bond-length correlation for rotamers of tri-ami nomethy 1 fluoride 29 (cf. Fig. 3.68), showing stereoelectronic bond lengthening and bond-order reduction with each new anti nN—ocf interaction. (Points are connected by straight lines to aid visualization.)...

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