Carbon-oxygen double bond lengths

Actual measurement shows that all the carbon-oxygen bonds in CaC03 have the same length, 1.31 A, as compared with a usual length of about 1.36 A for a carbon-oxygen single bond and about 1,23 A for a carbon-oxygen double bond. What is a better representation of the CO3" ion ... 

The structures of some known carbonyl compounds show that this picture is a reasonable one. Bond angles are close to the 120° required by pure sp hybridization, and of course we would not expect a molecule of less than perfect trigonal symmetry to have exactly sp hybridization. Bond lengths are similar to those in simple alkenes, although the carbon-oxygen double bond is a bit shorter and stronger than its carbon-carbon coimterpart (Fig. 16.3). 

X-Ray studies have shown that carbon-oxygen double bonds are shorter than single bonds. The same kind of study of the carbonate ion shows, however, that all of its carbon-oxygen bonds are of equal length. One is not shorter than the others as would be expected from representations 1,2, and 3. Clearly none of the three structures agrees with this evidence. In each structure, 1-3, one carbon-oxygen bond is a double bond and the other two are single bonds. None of the structures, therefore, is correct. How, then, should we represent the carbonate ion ... 

All the carbon-oxygen bonds are found to be of equal length and intermediate between carbon-oxygen single and double bond length. 

The carbon-oxygen distance in this molecule is known6 to be 1.159 A. If one structure 0=C=0 alone represented the molecule the distance should be 1.18 A, the double-bond length with the adjacent-bent-bond... 

While many stable carbenes tend to be unreactive towards oxygen in the absence of a catalyst, Denk and co-workers found that a stable carbene could be oxidised to the urea 40 in the presence of a catalyst or by reaction with NO (Scheme 21).49 The solid state structure exhibits an oxygen-NHC bond length of 1.237(3) A, and the C2 carbon resonates at 152.7 ppm in the 13C NMR spectrum, 60.2 ppm upheld from the uncoordinated carbene. The bond length is elongated compared to a typical C=0 double bond... 

Carbon dioxide has a linear structure. The simple double-bonded formula, however, does not fully explain the structure since the measured carbon-oxygen bond lengths are equal but intermediate between those expected for a double and a triple bond. A more accurate representation is, therefore, obtained by considering carbon dioxide as a resonance hybrid of the three structures given below ... 

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

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