Macrocyclic ligands electron delocalization

On coordination, the porphyrin macrocycle loses two protons (to yield a neutral complex when the central metal ion is divalent). The extensive electron delocalization throughout the ligand will normally extend to the central metal when the latter is covalently bound to the porphyrin. As expected, such complexes are extremely stable this is undoubtedly important to the biological role of these complexes. 

Optical Properties. The double-decker complexes of porphyrazines have characteristic electronic absorption spectra (Table V). The intense Soret bands of the double-decker complexes are blue shifted with respect to the single pz ligand as a consequence of the strong n-n interactions. Another characteristic of sandwich compounds is the additional appearance of absorption bands shifted to the red (termed Q ) and to the blue (termed Q") of the normal g-band region. These new transitions are thought to result from orbitals delocalized over the two macrocyclic ligands (33, 82). 

In chelate and macrocyclic complexes, electronic states may exist which are of a delocalized nature they pertain to the system of metal and ligands. Such states are not simply derived from metal d—d states or from free ligand states and transitions involving delocalized states are often quite intense. 

A review of expanded porphyrin ligands can be found. The texaphyrins (8) can be considered as 22- 7r-electron benzannulene systems with an IS-tt-electron delocalization path, based on crystal structure data as well as NMR. The cadmium complex of the macrocycle is found to be planar with pentadentate coordination of the macrocycle to cadmium, which becomes seven-coordinate as a result of axial coordination to two pyridine molecules. The cavity is neariy circular with a center-to-nitrogen distance of 2.39 A. Because of the larger size of this macrocycle, metal ion coordination is generally seen with the larger transition metals and lanthanides. 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

The binuclear nickel-thiolate macrocyclic complex (101) displays noteworthy redox behavior, in which one-electron oxidation yields a Ni. Ni product with significant delocalization of the unpaired electron density onto the bridging thiolate ligands and not onto the second nickel ion. The charge delocalization consequently lies between the two redox extremes of nickel(III)-thiolate and nickel(II)-thiyl radical, thus mimicking the Ni-C state in [NiFeJhydrogenase (see 8ection 8). 

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