Metal-to-ligand charge transfer MLCT

Another large family of d complexes with MLCT absorptions which, however, appear in the UV region comprises cyano complexes [1] (e.g. [M(CN)6]4- with M = Fe, Ru and Os and metal carbonyls [1,4,43] e.g. M(C0)6 with M = Cr, Mo, W). These MLCT bands may obscure less intense bands of different origin such as LF absorption. A characteristic long-wavelength MLCT band was also identified in the abs. spectrum of the carbene complex Mn(C5H5)(CO)2(CPh2) (X iax = 380 nm. Fig. 5) [44]. 

Generally, d complexes occur in square-planar or trigonal-bipyramidal structures. Reducing d metals include Fe(0), Ru(0), 0s(0), and Pt(II). Pt  

In addition to CT bands ds (or dp) absorptions can appear in the spectra of d O complexes. The spectrum of Ni(CO)4 is a case in question. Although it has been suggested that ds bands occur [4] it seems more likely that these absorptions are of the MLCT type [50,51]. The long wavelength absorption of the isoelectronic anion [Fe(CO)4]2- has been also assigned to a MLCT transition [52]. Well-documented are the MLCT spectra of Cu(I) complexes with polypyridyl [9] (e.g. [Cu(o-phen)(PPh3)2] (X ax = 365 nm [53]) as well as those of Au(I) complexes such as as [Au(CN)2] (X ax = 240 nm [54]). 

Metal ions with an s configuration are frequently reducing (e.g. T1+, Sn2+, Sb3+). Accordingly, long wavelength absorptions of the compounds M(bipy)X3 with M = Sb, Bi and X = Cl, Br, I were assigned to MLCT transitions [55]. However, these compounds exist only as solids and complications by solid state effects can presently not be excluded. 

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Effects of spacer groups on the formation and properties of the mixed-valence states of conjugated ferrocene dimers have been extensively studied by both electrochemical and spectroscopic methods. It should be noted that a characteristic feature in the electronic spectra of ferrocene dimers with conjugated spacer groups is the appearance of metal-to-ligand charge transfer (MLCT) bands in the neutral form as well as IT bands in the mixed-valence state. The dimer Fc — CH=CH — Fc... 

In order to illustrate the approach suggested above, it is of value to consider a specific case. Visible or near-UV excitation of the complex RuCbpy results in excitation and formation of the well-characterized metal to ligand charge transfer (MLCT) excited state Ru(bpy)32+. The consequences of optical excitation in the Ru-bpy system in terms of energetics are well established, and are summarized in eq. 1 in a Latimer type diagram where the potentials are versus the normal hydrogen electrode (NHE) and are... 

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

The kinetics of this reaction were followed by tracking the appearance of Cu(dmp)J at 455 nm, the A,max of the metal to ligand charge transfer (MLCT) absorption band. At a fixed pH, the kinetics in aqueous solution followed the rate law... 

Metal-to-ligand charge transfer (MLCT) transitions between the nonbonding metal-centred MOs and antibonding ligand-centred MOs. Such transitions are found where a metal is easily oxidised and the ligand is easily reduced. 

Figure 2.16 Structure of the octahedral ruthenium (II) trisbipyridyl complex. The orange colour of this complex results from metal-to-ligand charge-transfer (MLCT) transitions...

Charge transfer excited states internal charge transfer (ICT) states, metal-to-ligand charge transfer (MLCT) and twisted internal charge transfer (TICT) states... 

The versatility of the structural types possible for 1,10-phenanthroline ligands is exemplified in a review article on metal-to-ligand charge-transfer (MLCT) excited states of copper(ll) bis-phenanthroline coordination compounds, where 14 different 1,10-phenanthroline-based ligands were discussed (Figure 2) <2000CCR243>. 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Aqueous [Ru"(bpy)3]2+ is a model system for Metal-to-Ligand Charge Transfer (MLCT) reactions. Its excited state properties have been readily studied with optical spectroscopies [15,16]. However, little is known about its excited state structure, which we investigated via time-resolved x-ray absorption spectroscopy. The reaction cycle is described by Fig. 3 (where the superscripts on the left hand side of the ground and excited state compounds denote the... 

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 D, A or PS units may be metal coordination centres. Metal to ligand charge transfer (MLCT) in metal complexes (such as Ru(n) or Re(l)-diimine centres) has been extensively used for generating PeT processes [8.64-8.68, A. 10, A.20]. Our own work has been concerned with photoinduced charge separation in macropoly-cyclic coreceptors containing both a photosensitive porphyrin group and binding sites for silver(i) ions as acceptor centres. Thus, complexation of silver ions by the... 

The low-energy metal to ligand charge-transfer (MLCT) excited state has been extensively studied and involves low-lying ft -acceptor orbitals of the pyridine ligands ( ). ... 

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