Ligand-centered reactivity

Strategy III. Cooperative ligand-centered reactivity based on redox active ligands... 

The ligand-centered reactivity of organometallic radicals has been previously reviewed, and will not be discussed extensively. Only several examples will be shown to illustrate the typical reactivity of these systems will be commented. 

Furthermore, steric bulk increase around the metal center has been proven useful to promote ligand-centered radical reactivity. Several examples of ligand-centered reactivity in metalloporphyrin systems have been previously discussed. The (TMP)RhCO (TMP = tetramesitylporphyrin) couples through the CO ligand to form the diketone dimer.90... 

SCHEME 10.14 Metal and ligand-centered reactivity exhibited by the [(Me3tpa) Irn(ethene)]2 + (Me3tpa — /V,/V,/V-tri s(6-me thy 1-2-pyridy line thy Famine) complex. 

Intraligand (IL) excited states of coordination compounds arise from electronic transitions between molecular orbitals primarily localized on a coordinated ligand. It is difficult, a priori, to predict the reactivity of this type of state. While it is logical to expect ligand-centered reactions, the influence of the metal on such processes can be substantial and result in net photochemistry which differs from that of the free ligand. A few examples should serve to illustrate the range of IL photoreactions reported to date. 

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal-ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. 

Neutral 17-electron complexes tend to undergo metal-centered reactions, while reactivity of 19-electron complexes are typically ligand-centered. Scheme 4 displays an example of these reactivity differences within the same system. Photolysis of the iron dimer results in homolysis of the metal-metal bond to form the neutral 17-electron species (22), whereas in the presence of PPh3, the intermediate 19-electron radical (23) is produced, which then undergoes ligand oxidation to yield a 17-electron final product. Selected... 

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