Ligand-centered states

Ligand localized 7r r ES are common to complexes having large aromatic ligands. Absorption and emission bands attributed to nn states shift to the red with increasing P [6]. For example, fluorescence maxima from the ligand centered states of the tetraphenyl porphyrin complexes Mg(TPP) and Zn(TPP) display shifts ranging from —34 cm VlOO MPa in methanol to —94 cm /fOO MPa in chloroform with... 

Since most Ni1 species with simple N-donor ligands are prone to disproportionation into Ni° and Ni11, relatively few Ni1 complexes with nonmacrocyclic N-donor ligands have been reported. Formation of Ni1 species is in most cases proposed on the basis of electrochemical data, although ligand-centered redox processes have to be considered. The ligands usually contain imine donor atoms or aromatic N-heterocycles, which because of their 7r-acceptor ability favor stabilization of lower oxidation states. 

Colombo, M. G., Hauser, A., and Giidel, H. U. Competition Between Ligand Centered and Charge Transfer Lowest Excited States in bis Cyclometalated Rh3+ and Ir3+ Complexes. 171, 143-172(1994). 

A redox activation mechanism usually involves a change in oxidation state of the metal (but could also be ligand-centered). Typically, the oxidation state changes from a state in which... 

The coordination chemistry of (phenoxyl)manganese complexes is rather more complicated because both metal- and ligand-centered electron-transfer processes are accessible in the normal potential range. The phenolato precursors are known to exist with manganesc(II), (III), and even (IV). In fact, three phenolato groups strongly stabilize the Mn(IV) oxidation state. 

It is remarkable that the oxidized states of the cytochromes cdi from P pantotrophus and P. aeruginosa have different structures. It is not clear at present whether one of these structures is superior for catalyzing nitrite reduction. Certainly in the P. pantotrophus enzyme the ligand switching at both ligand centers upon changing oxidation state is... 

The synthesis and properties of [ Os(bpy)2 3(/r-181)] have been reported the formal oxidation state of the Os centers is +3. In contrast, [ Ru py)2 3(/i-181)] + contains Ru° with the bridging ligand possessing three semiquinone domains. The complex undergoes three reversible ligand-centered oxidations taking it ultimately to [ Ru(bpy)2 3(/i-181)]°" with (181) in a tris(quinone) form. The wavelength of the intense NIR absorption (MLCT) that the complex exhibits varies with oxidation state. " ... 

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