Cyanide MLCT absorption

All the complexes of this series contain the -Ru(bpy)2- chromophore, which has a typical absorption spectrum, dominated by intense metal-to-ligand charge transfer (MLCT) transitions in the visible. When further metal centers are attached to this chromophore by means of bridging cyanides, the MLCT transitions are still present, but their energy is somewhat perturbed. Some relevant examples [65,67-70,73,74] are collected in Table I, where Ru(III)/Ru(II) redox potentials are also given in addition to MLCT absorption maxima. It is seen that the extent of intercomponent perturbation depends on several factors (i) presence of the additional metal center (see. 

The demetalation process was followed by absorption spectrophotometry (measurement of the decay, as a function of time and cyanide concentration, of the di-copper(I) complexes characteristic metal-to-ligand-charge-transfer (MLCT) bands in the visible region [111]) which gave access to the kinetic parameters, in particular to the second-order dissociation rate constants CN given in Table 1. 

The most spectacular effect resulting from the highly rigid and compact structure of Cu2(K-84)p+ is undoubtedly its extraordinary kinetic inertness in the cyanide demetalation process. Measurement of the absorbance decay of its MLCT band in the visible region (A = 524 nm) could be performed by classical absorption spectrophotometry (whereas stopped-flow techniques were required for the methylene-bridged knots) and allowed to demonstrate that its demetalation implies two rate-limiting steps, well resolved in time, as schematically represented in Figure 27. 

In this complex, in spite of the binding mode of the bridging cyanide, the Re-based MLCT states are at considerably higher energies than the Ru-based ones, so that distinct MLCT bands are present in the absorption spectrum, selective (or predominant) excitation of the Ru-based and Re-based chromophores is possible, and a substantial driving force for energy transfer is present. As a matter of fact, as shown by emission-excitation studies, and by TR experiments, very efficient energy transfer takes place in this system from the Re-based to the Ru-based MLCT state [94]. 

The Ru(II)-Rh(III) cyanide-bridged complexes show absorptions that are characteristic for ruthenium and rhodium polyazine complexes. The electronic absorption spectra of a series of cyanide-bridged Ru(H)-Rh(ni) complexes recorded in DMSO-H2O solutions correlate well with the electrochemical properties. Coordination of a second Rh(III) shifts the lowest energy transition, which is Ru bpy MLCT in nature, to higher energy in [(bpy)2Ru (CN)Rh(NH3)5 2] relative to [(bpy)2(CN)Ru(CN)Rh(NH3)5] . This positive shift is consistent with the stabilization of the Ru(djt) orbitals of [(bpy)2Ru (CN)Rh(NH3)5 2] and correlates well with the electrochemistry where the Ru° oxidation moves to more positive potential with Rhflll) addition. The of... 

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