Semipolar single bonds

Although the S—O bond lengths in sulphoxides and sulphones seem to indicate that these are covalent double bonds, the dipole moments of these compounds obtained by calculation or by experimental measurements support the semipolar single-bond character in these compounds, and they should be represented as S - O, with about 66% ionic character14,15. 

On the basis of these values one can conclude that, with increasing bond orders, the force constants rise, suggesting that the S—O bond of sulphoxides should have more semipolar character than that of sulphones. Furthermore, molecular diffraction measurements20 and Parachors21 for sulphoxides also suggest that the S—O bond in sulphoxides should have a semipolar single-bond representation while the S—O bond in sulphones is described by double bonds or better as the resonance hybride shown in Scheme 1. 

In all these cases the oxygen atom has a negative charge, while the nitrogen or sulphur atom carries a corresponding positive charge. In the old formulae double bonds were used, later also semipolar double bonds there is, however, here no difference in principle with the normal single electron pair bond. 

On the other hand, the highly polar nature of the S—O bond (2.8-4.5 D 1 D = 3.336 X 10" C m), as well as the small force constants for this linkage in sulfoxides (6-8 X ItF N m ), which are similar to those of N—O linkages in pyridine N-oxides, have been taken to suggest a semipolar single S-+0 bond [4,11],... 

From this, it is sometimes called a semipolar double bond - a combination of a single ionic bond and a single covalent bond. 

In this process an electron pair, shared by a N and an O atom, is moved completely to the O atom. This is equivalent to the transfer of one electron from the N to the O atom, and by this process the N atom acquires a positive charge of one unit, whereas the oxygen atom obtains a negative charge the double covalent bond is transformed into one single covalent bond plus an ionic one. This type of bond is called a semifiolar bond. By assuming a sufficient number of semipolar bonds, all the other oxides with an even number of... 

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