After a short account of the model approach to the coordination chemistry of Vitamin B 12 • previous evidence concerning the influence of organic groups as axial ligands on ground state properties and reactivity of several series of cobalt chelates with tetradentate equatorial macrocycles is summarized. The recent results dealing with the influence of the nature of the equatorial chelating agent on ground state properties of the molecule and on coordination chemistry and reactivity at the axial positions are compared. All properties dependent on the macrocyclic ligand show a consistent trend which can be interpreted in terms of the electron affinity of the particle mainly influencing the equilibria involving the cobalt-axial Iigand bond, as well as the reaction mechanism at the cobalt-carbon bond.The interest in the chemistry of cobalt chelates, from which stable crcobalt-carbon bonds can be produced, was at tirst mainly due to striking analogies with Vitamin B 12 group complexes where organic groups are directly bonded to the meta] atom 1 . Compounds containing conjugated systems which act as bisbidentate or tetradentate chelating agents lying in the equatorial plane of square pyramidal or octahedral structures were thus proposed as models for the naturally occurring complexes of the corrin ring 1The relevant properties induced by the conjugate chelating systemarenot limited to the stability of the cobalt-carbon bond in a formally Com-d 6 low spin complex but also include, for example, the reversible formation of oxygen adducts from Con-d 7 paramagnetic complexes4-8 and the relative stability of the Co 1 -d 8 oxidation state. A variety of surprising new reactions which are still being currently reported stimulated studies directed to the understanding of the mechanistic concepts underlying this versatile and unusual chemical behaviour. This aim was pursued through the study of the coordination chemistry of the transition metal atom in these chelates. The effect of the axial Iigand has already been discussed in a series of derivatives with a single equatorial Iigand including the Vitamin B 12 group complexes 9 and bisdimethylglyoximates 1 • 10 -14 . We prepared a number of complexes with ditTerent tetradentate ligands, all of which induce the same type of 335 P .A.C.-30/3-B
