Carbon bond order

Since a similarity between the rates of decomposition of thiirene dioxide complexes and those of thiirane dioxides was found, it was suggested103 that upon coordination the carbon-carbon bond order of thiirene dioxides decreases and the ligand becomes thiirane dioxide-like. The role of the metal is thus to saturate the carbon-carbon double bond so that the reactivity of the coordinated thiirene dioxide approaches that of the thermally less stable thiirane dioxide. 

Harris s study, which also concluded that nitrogen-a-carbon bond order in these transition states is between 0.2 and 0.3. 

Unfortunately it is difficult to resolve the X-ray diffraction of the carbon atoms in these compounds from that of the heavy elements present, and in no case are the metal-carbon or carbon-carbon distances known with sufficient accuracy to enable pronouncements about the carbon-carbon bond order to be made. In this respect the infrared spectra ought to be helpful, but there is considerable controversy over the assignment of absorption... 

The tungsten-carbon single, double, and triple bond lengths in this compound are 225.8, 194.2. and 178.5 pm, respectively, and the accompanying W—C—C bond angles are 124°. 150°. and 175 , all of which is quite consistent with tungsten-carbon bond orders of 1,2. and 3. 

An experiment for obtaining He c values. The names are characteristic of the carbon-carbon bond order. For example, Jc c = 35-45 for a C-C single bond while He c = 65 for a double bond. The main decoupled signals are removed and the satellite peaks appear as positive and negative signals. The experiment requires C-enriched or very concentrated samples. 

The origins of the chemical shifts are probably not sufficiently well understood (as yet), to allow a quantitative discussion of aromatic character in the annulenes. If such a concept is considered meaningful it would probably best be defined in terms of the degree of bond alternation therein, which is of pivotal importance to the jr-electron properties (see Sections B and C). Apart from theoretical calculations, a number of physical methods have demonstrated their ability to estimate the extent of bond alternation in annulenes (crystallographic analysis, electronic/vibronic spectral analysis, diamagnetic anisotropy/susceptibility exaltation measurements and of course n.m.r.), see ref. > for a full discussion. (Furthermore the known correlation between n.m.r. vicinal coupling constants and carbon-carbon bond orders is of potential utility in any determination of bond alternation 65>). 

The carbon-to-carbon bond order in benzene is 9 electron pairs/6 bonded-atom pairs, or I5 also. For the carbonate ion, C03 , three resonance structures can be drawn. Each has 4 electron pairs shared among 3 bonded-atom pairs, so the bond order is 4/3, or I5. One of the three resonance structures for C03 is... 

The central bond has some double-bond character, and the end bonds have some single-bond characte (In VB theory, this is explained by contributions from such resonance structures as CH2—CH=CH—CH2.) The sum of the bond orders is 5.235, exceeding 5. [This is because the 7r-electron energy of butadiene exceeds that of two isolated double bonds see Eq. (16.64).] For benzene each carbon-carbon bond order is found to be 5/3 = 1.667. 

Thus, the carbon-carbon bond order of the olefin in such a complex deaeases upon coordination, and tire C-C bond length is longer than that of the free olefin. In addition, the hybridization at the olefinic carbons increases from sp toward sp, and the substituents on the olefim are bent away from tiie metal. Such olefin ligands imdergo electrophilic attack, as is discussed in Chapter 12. 

In general, the metal-nitrogen bond orders of metal-nitrosyl complexes are larger (i.e., the M-N bonds are stronger) than the metal-carbon bond orders for the corresponding... 

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