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Kinetic studies show that azamacrocycles functionalised with pendent co-ordinating 2,2'-bipyridyl-yl-methyl arms co-ordinate to labile metal ions in two stages; the first stage is rapid and second-order, and involves formation of an intermediate with rate-determining co-ordination of the pendent bipyridyl arm, and the second stage is slower and first-order, involving chelation by the macrocyclic ring.Some studies have indicated that macrocycles functionalised with pendent co-ordinating arms may allow entry of a metal ion into the macrocyclic cavity at a more rapid rate than is possible with the unfunctionalised parent macrocycles. The proposed mechanism involves initial capture of the metal ion by the pendent arms, followed by transfer to the macrocyclic cavity.'--3 This conclusion, if generally true, would be of considerable value, since not only would the pendent arms enhance the stability of the final products, but they would also be beneficial in aiding the kinetics of the overall formation process. Entry of metal ions into the cavities of unfunctionalised macrocycles is often rather slow, partly because of the ligand conformational changes (e.g. chiral N-inversions) which often occur following initial metal ion binding.4-7Recently we reported the synthesis and characterisation of the azamacrocycles L1 and L2 which contain a pendent co-ordinating 2,2'-bipypyridyl-6-yl-methyl arm. L1 forms

to relieve electronic charge, namely the µ-CO arrangement.This permits charge delocalization by a -type of bonding. Thus, the 16-electron compound [Pd2^-CO)2Cl4]2" is more stable with respect to the other two possibilities: the 16electron species [Pd2(CO)2Cl4]2' (all groups terminal with a metal-metal bond, as experimentally found16 in the case of platinum) and the 18-electron complex [Pd2(CO)2(jit-C1)2C12]2". The latter is clearly unfavorable for electron-rich metals lacking an efficient mechanism of x-back-donation.If a -type of bonding is essentially operative for palladium in its binding to CO, decreasing the oxidation state does not necessarily correspond to the stabilization of Pd-COt bonds. Further support to this assumption can be obtained from the experimentally observed low stability24 of Pd(CO)4 as compared to that of Ni(CO)4 in spite of the lower enthalpy of sublimation of palladium (91 kcal/mol) with respect to that of nickel (103 kcal/mol).25Note should also be taken of the apparently peculiar nature of chemisorbed CO on palladium. On this metal, bridging carbonyl groups have been detected spectroscopically26 and suggested to contribute considerably to the coverage of the surface. This sets an example of how the information from molecular homogeneous systems can sometimes be transferred with some confidence to heterogeneous systems.These considerations, together with our earlier findings on gold(I) carbonyl complexes,27 suggest that for late and heavy transition elements, especially those of d10 configuration, the mechanism of stabilization of the M-CO, bond is substantially one of -donation to the metal. This makes the carbonyl carbon especially prone to nucleophilic attack, as evidenced by the high reactivity of these carbonyl complexes toward water and amines.28Once certain requirements are met (availability of empty valence orbitals with subsequent sufficient Lewis acidity of the metal center), -bonding of CO should not be too surprising, though still uncommon.

New penta-and hexa-azamacrocycles have been synthesised, based on tri-and tetra-azamacrocycles functionalised with a single pendent co-ordinating 2,2'-bipyridyl-6-ylmethyl-arm; for the quinquedentate ligand 1 -(2',2"-bipyridyl-6'-ylmethyl)-l,4,7-triazacyclononane, the formation of a six-co-ordinate complex of Ni2+, and a