Ligand centered

Metal-centered emission is found in several rare-earth complexes, for example europium diketone chelates [29]. These compounds emit relatively narrow spectra associated with the d-f transitions of the metal ion. Relatively few comprehensive studies of this class have been carried out to date, but the available evidence [30] seems to indicate that the photoluminescent efficiencies are somewhat lower than the ligand-centered emitters. 

The maximum zero-field splitting in the ligand-centered luminescence of [Ir(5,6-Me2-phen)2-Cl2]+ in glycerol is 3 0.5cm-1, compared to clcnO1 for [Ir(phen)3]3+.151... 

The terdentate cyclometalated complexes [Ir(L)(L )]2+ and [Ir(L )2]1, L = 2,6-bis(7 -methyl-4 -phenyl-2 -quinoyl)pyridine (233), L = monoanion of L (234), luminesce at 77 K in MeOH/EtOH (lmax = 592 nm, r =20 ps) and at room temperature in deoxygenated acetonitrile (imax 620 nm, r = 325 ns).405 Both compounds undergo four reversible, ligand-centered, one-electron reduction processes. 

The Ni complex of corrole (octaalkyl derivative (71)) is a S= 1/2 low-spin Ni11 species with a ligand-centered 7r-cation radical, as evidenced by the isoptropic EPR signal at g 2.01.291,292... 

Complexes (690) undergo two one-electron reversible reductions and two oxidations, all of which appeared ligand centered. Thus, these ligands behave electrochemically much like... 

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). 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

Spectral modulation in the NIR region has been reported for the complex [Ru(L8)3]2+ which undergoes six ligand-centered reductions, two per ligand.17 It initially shows no absorption... 

The first reduction in the cobalt-based polymer is metal-centered, resulting in the appearance of a new MLCT transition, with the second reduction being ligand-centered. For the nickel-based polymer, in contrast, both redox processes are ligand-based. 

Figure 3 Ligand-centered redox interconversions of [ Ru(bipy)2 3(/i-L22)]n+ ( = 3-6) (potentials vs. SCE), and the resulting electrochromic behavior. Spectra were measured at 243 K in MeCN.192...

The dinuclear [Fe(C5H5)(C0)2(CgH4S8-C8H4Sg)Fe(C5H5)C02] organometallic complex exhibits ligand-centered oxidation at a low potential [94]. The crystalline material that forms upon chemical oxidation with iodine contains both I3 and I5 anions and has a room temperature compressed pellet conductivity of 1.7 x 1CT4 S cm-1. 

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... 

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