Abnormal bond lengths

Unlike the iron acid the crystal structure of molybdenum analogue 7r-CjH5Mo(CO)3CH2COOH, shows no abnormal bond lengths and the C-O distances of the carboxyl group are unequal (1-36 and H9 A) as is... 

The lack of reactivity of (45) and CEP-pyrrole towards alcohols is attributed, at least partly, to the double bond character of the phosphorus-nitrogen bond as evidenced by the crystallographically determined abnormally short P-N bond lengths. 

The values for the covalent radii of N and 0 given in the table dp not differ significantly from the Pauling values, but the value for fluorine is a little smaller. They were obtained by extrapolation of the values for the other period 2 elements (Robinson et al., 1997). In any case the covalent radii of oxygen and fluorine are of little use because, as we shall see later, essentially all bonds formed by these elements, except the O—O, O—F, and F—F bonds, which are abnormally weak and long, have too great an ionic character to justify the use of covalent radii to calculate bond lengths. 

It has been known for a long time that amines which have a hydrogen on a carbon attached to the nitrogen so that the C—H bond is antiperiplanar to the lone pair, show abnormally low stretching frequencies for those C—H bonds. In order to reproduce this (Bohlmann) effect MM3 corrects the natural bond lengths and force constants of such C—H bonds by31 ... 

Rieker s report (77) on the isolation and X-ray structure characterization of the hexaphenylethane derivative 5 aroused a lot of interest, since the observed central bond length was abnormally short (1.47 0.02 A), in sharp contradiction to the MM prediction (1.635 A) for the parent hydrocarbon (78). However, the X-ray data need to be further refined before a final conclusion can be reached, and MM calculations of 5 itself confirm the long C-C bond (79). 

To be consistent, the same conclusion should be drawn for the dimethyl-oxime complexes. The difficulty here is to answer the question as to why other flat, uncharged, molecules do not stack in a similar fashion. It is believed that in fact they would do so were it not for factors such as mutual repulsion of u-electrons, as in condensed hydrocarbons (214). An examination of the bond lengths shown in Fig. 10 indicates that in the dimethyl-glyoxime complexes the ir-bonding is not nearly so extensive as commonly imagined. Similarly, it is known that aromatic donor-acceptor complexes, e.g, quinhydrone, stack in a fashion very similar to the dimethylglyoximc complexes (166a), and also show abnormal dichroism (182). 

B—B. not gi /en in Chapter 3) are about as expected (Sec. 3-5). The bond lengths of these bonds are also larger than would be expected in comparison with those of their congeners (Chap. 7). These abnormalities are to be attributed to the strong repulsion of the unshared electron pairs of the bonded atoms (and of the electrons involved in the other bonds in the case of N—N and 0 0). 

Some molecules with abnormally long C-C bonds (lengths in pm) (a) oxalic... 

Asa result of resonance a shortening of bond lengths nearly always occurs. The abnormally large N—Cl distance (1.95 A calc. 1.69 A) in nitrosyl chloride, NOC1, however forms a... 

The cyclopropyl group is often equivalent to the vinyl group (-CH=CH2) thus the exocyclic bond length in 1 is abnormally short and not far from the C-R bond length found in CH2=CH-R (R = vinyl, carbonyl, acid and ester groups). 

Interaction between the Oxygen p-type Lone Pair and Adjacent q-bonds The interaction between the oxygen p-type lone pair and the adjacent antibonding orbital of a carbon-halogen bond was first considered in 1959 by Lucken as a possible explanation of the abnormally low NQR frequencies of Ot-haloethers (33). Later on, Altona showed that the same phenomenon could explain the peculiarities of some bond lengths in similar compounds (2). The spectroscopic properties of pyranose sugars which we have described in the first two sections of this article seem to imply that the existence of such an interaction lies beyond all possible doubt in these molecules. It does not follow that they can explain the Anomeric Effect (34) (35). For this, we need a quantitative estimation of the stabilization introduced in a molecule by such interactions. 

Pauling commented many years ago on the abnormally long M-X bonds in and Te X ions, comparing them with the sums of the tetrahedral covalent radii, which for Se and Te correspond to M(ii). A more direct comparison could be made of the M—X bond lengths in the following pairs ... 

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