TABLE OF CONTENTS Supramolecular cyclic self-assembly in a number of organometallic compounds will be illustrated with selected examples. By no means this presentation tends to be exhaustive. Only monocyclic species will be presented, although various cage-like polycyclic oligomers perhaps could also be regarded as cyclic oligomers. For a more comprehensive presentation see ref. [191].
Among the chemical bonds acting in inorganic chemistry, very important are the dative bonds. Unlike normal covalent bonds, which are formed by pairing of electrons with a one-electron contribution from each atom, dative bonds are two-electron bonds formed by "donation" of an electron pair from one atom to another. There is a general tendency to assume that the two-electron bonds between a certain pair of atoms ( e.g. boron and nitrogen in H 2B-NH 2 and H 3B<-NH 3) are identical, regardless of the origin of electrons (i.e. covalent and dative), but it has been pointed out that a distinction between the two types should be made [192] , [193].
The normal covalent and dative bonds differ at least in three major aspects:
the nature of the fragments formed when the bonds are broken;
the nature of the bond rupture process;
the magnitude of the bond cleavage enthalpy.
Thus: a) free radicals are formed by breaking covalent bonds and neutral molecules result by breaking dative bonds; b) the covalent bond rupture is a homolytic process, whereas the...
