Ligand-to-metal charge transfer LMCT

Low-energy LMCT transitions occur if the metal is oxidizing and the ligand reducing. LMCT absorptions determine the color of many coordination compounds. LMCT bands can be classified according to the electron configuration at the metal. The most important configurations are included in our discussion. 

Complexes with a d configuration contain a transition metal in the highest possible oxidation state. Since no valence electrons at the metal are left only LMCT transitions may occur (with the exception of ligand centered (LC) bands of complexes such as metalloporphyrins, see chapter 7 of this book). Typical examples are the tetrahedral oxo anions M04 and their thio 

Octahedral low-spin d complexes show LMCT bands at relatively short wavelength since the acceptor orbital occurs at rather high energies and has a large influence on the metal-ligand bonding. The reorganizational 

Since metals such as Au(I) and Hg(II) have completely filled d shells LMCT transitions must terminate at the next higher s orbital of the metal. Au(I) complexes of the type [AUX2] with X = halide show such LMCT bands at energies comparable to those of MC ds transitions [26]. The d O orbitals of mercury are much more stable and do not participate in low-energy electronic transitions. Consequently, the absorption spectra of simple Hg + complexes contain only LMCT bands, e.g.[Hg(N3)3] (X ax = 246 nm. Fig. 3 [27]). 

AU+ and Hg + may be also classified as s metals. However, in order to distinguish between d-block and main group metals the designation s is reserved for main group elements in their highest oxidation state. Owing to the absence of valence electrons at the metal, s complexes such as [TlCm (Xmax = 247 nm [28], Fig. 4) show only LMCT bands [29]. A few other typical examples are given in Table 3 [28-31]. 

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

The lowest transition energies of permanganate and dichromate will be ligand to metal charge transfer (LMCT) in nature. From symmetry arguments, it can be shown that the transition described by the 4—> e one-electron transition of a tetrahedral d° complex will have an Aj/Dj ratio of -0.5 (108,109). 

Fig. 1. Schematic orbital energy diagram representing various types of electronic transitions in octahedral complexes. A line connects an atomic orbital to that molecular orbital in which it has the greatest participation. 1 metal centered (MC) transitions 2 ligand centered (LC) transitions 3a ligand-to-metal charge transfer (LMCT) transitions 3b metal-to-ligand charge transfer (MLCT) transitions...

Transitions with a fairly large radial charge transfer ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge-transfer (MLCT) transitions. 

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