Localized Ligands

Rh(bpyL3+ is an example of a complex that exhibits an almost pure n-n phosphorescence and demonstrates one of the limitations of nearly pure ligand localized emissions. At 77K, the complex is highly emissive with a beautifully structured blue ligand phosphorescence (Amax = 446 nmfor the first peak) having at in the tens of msec,(17) but it has no detectable room temperature emission. It is this very long radiative lifetime that causes the absence of room temperature emission. The radiative decay is so slow that it cannot compete effectively against inter- and intramolecular radiationless decay at room temperature. 

Distinct peaks at the positive (+) and negative (-) regions of the voltammogram (Fig. 17.4), are observed. The cyclic voltammogram of [Re(CO)3Cl]jtpbq is dominated by metal-localized oxidation and ligand-localized reductions, as observed in analogous complexes. Reduction in the complexes may be attributed to the BL/BL... 

D. The Atom-plus-Ligand Local-Term Approximation... 

The atom-plus-ligand local-term approximation has been justified a posteriori for 30) and 57) shielding, since certain expected relationships of chemical shifts, such as additivity of substituent effects, were found to be absent for the observed shifts, but were fulfilled for i.e., after correction for the atom-plus-ligand diamagnetic term. Some examples are given in the following sections. 

A question of particular interest is whether or not the redox orbitals are essentially metal or ligand based and how this varies as one traverses the series. For example, [Ru(bipy)3]2+ (bipy = 2,2 -bipyridyl), a complex much studied for its photoredox chemistry, has been shown by electrochemical and spectroelectrochemical studies to undergo a series of reversible, ligand-localized electron-transfer reactions about the reversible metal-centred Ru VRu111 couple (reaction 2).9 10... 

A schematic diagram of the ligand localized and Eu3+ excited energy levels are shown in fig. 16. The ligand levels are denoted with S and T for the singlet and triplet states, respec-... 

E.p.r. studies459 indicate the formation of sandwich-type dimer complexes of V02+ and etioporphyrin (EP) in light petroleum at 77 K the e.p.r. data are consistent with an EP-EP plane distance of ca. 3.5 A. A planar ligand local environment for Vlv, possibly provided by a phthalocyanine- or porphyrin-type ligand, has been proposed460 to account for the axial microsymmetry of the V17 centre and the absence of zero-field splitting indicated by e.p.r. studies of vanadium in mineral oil. 

Beyond the primary coordination sphere, electrostatic interactions serve to modulate the redox potential set by the heme type and axial ligand. Local charges as well as solvent-exposure of heme have been correlated with redox potential in artificial and natural heme proteins. Since 1978, Stellwagen observed an inverse dependence between the redox potential and the exposure of heme to aqueous solvent [28]. On the other hand, the influence of charged residues near the heme group has been analyzed in artificial proteins for example, the role of negatively charged residues... 

The photochemistry of Re-bound L occurs from a ligand-localized triplet excited state (3nn or im ). This contrasts the photochemistry of free species L, which react from the corresponding singlet states. Binding L to Re in [Re(L) (CO)3(N,N)]+ or [Re(Cl)(CO)3(L)2] thus presents an excellent opportunity to generate the lowest triplet state of L in high yields and study its spectroscopic properties and reactivity. This is very difficult to accomplish for free L. 

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