Charge-delocalized ligands

As seen, in confirmation of the charge delocalization of the electron transfers, most distances inside the dmit ligand remain essentially unaltered. Nevertheless, the progressive shortening of the Ni-S bond suggests a non negligible contribution of the metal d orbitals to the frontier orbitals. 

Fig. 1. Simplified pictorial representation of the transmission paths for cis and tram effects in metalloporphyrins according to Phillips and Williams (2). A 3 d orbital is shown for the metal, two p-orbitals of opposite N-atoms representative of the empty, antibonding porphyrin (rr ) orbitals, and a p-orbital for each of the axial ligands. Lobes containing arrows are occupied by electrons the arrows indicate the direction of charge delocalization. The types A-F are explained in Sect. 5...

The purpurate anion (53) has a mononegative charge delocalized over more than one donor atom.30 The kinetics of its complexation with alkali metal cations have been reviewed.290 The structures of several purpurates have been solved and the ligand consistently acts as a tridentate ligand. However, a variety of bonding modes are exhibited with the cation and polymeric structures resulting.30... 

Some electrophilic reagents have been found to react with unsaturated macrocyclic ligands, particularly those containing acetyl groups attached to charge-delocalized chelate rings.2856,2857 The acetyl groups are displaced with a reaction mechanism which resembles the substitution... 

The oxidative substitution and radical coupling reactions of [Fc(CO)3(PPh3)2]+ have led to a number of unusual iron(O) and iron(ll) complexes, e.g. equation (25), which cannot be obtained234 from its diamagnetic precursor [Fe(CO)3(PPh3)2], The iron(l) species is relatively unstable, probably due to poor charge delocalization by the phosphine ligands. The presence of an imidazolidinylidene... 

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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