Three Donor-carbon Atoms

Cyclohexyl rings have been omitted for clarity (Reproduced by permission from Angew. Chem. Internal. Edn., 1971,11, 137) 

C-C distances are unequal, as are the Ni-C(terminal) distances, that to the carbon atom involved in the long C-C bond being greater On the other hand, in the accurately determined [cr(C-Q a 0.006 A] centrosymmetric dinuclear complex (42), the C-C(allyl) distances of 1.408 and 1.411 A are 

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Similarly, all diamine ligands with three aliphatic carbon atoms between the nitrogen donors will... 

Alternatively, since the platinum atom in trimethyl(nonane-2,4-dione)platinum(IV) is directly above the carbon atom, one could conceive of this as a pseudo-n-allylic complex over the three ring carbon atoms. A final description of bonding in these bridge-bonded complexes can be given in terms of valence-bond theory which suggests that there is a predominence of a third resonance contributing form (II) which enhances the donor ability of the unique ring carbon atom 27). 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Other organic ligands can be bound in more than one way also. For example, the allyl group can be bound by a cr-bond between the metal and one carbon atom (rf) or as a 7r donor encompassing all three carbon atoms (ry3) ... 

The method chosen to achieve this stabilization is to have the olefinic group form part of a molecule containing a class b donor atom, usually P or As but occasionally 0 or S. It is found, as expected, that the strongest chelate effect occurs when the chelate ring has 5.5 or 6.5 members, that, is, when there are two or three carbon atoms between heavy atom and vinyl group. Typical ligands, with the abbreviations used here, are shown in Tables 1 and 2. This review excludes compounds such as the one in Fig. 1, where no true chelation, considered as the... 

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