Electronic transition LMCT

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). 

Figure 11.4 Energy level diagram for an octahedral transition metal complex showing the various kinds of electronic transition. MC = metal-centred, LC = ligand-centred, MLCT = metal-to-ligand charge transfer, LMCT = ligand-to metal-charge transfer.

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...

Electronic transitions and excited states of metal complexes are traditionally described in terms of text-book categories such as MLCT, LLCT (XLCT), IL, MC (=LF or dd), LMCT, etc. It was mentioned several times above that MLCT, LLCT, and IL characters in the case of [Re(L)(CO)3(N,N)] represent only limiting cases. In reality, electronic transitions and excited states have mixed character owing to two factors (1) delocalization of the optical orbitals (i.e., frontier orbitals involved in electronic transitions), and (2) combining several one-electron excitations in an electronic transition. 

Ligand to metal charge transfer (LMCT) transition An electronic transition in a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from a ligand to a metal center has occurred. 

For the sake of simplicity, electronic transitions in metal complexes are usually classified on the basis of the predominant localization, on the metal or on the ligand(s), of the molecular orbitals involved in the transition (4). This assumption leads to the well-known classification of the electronic excited states of metal complexes into three types, namely, metal-centered (MC), ligand-centered (LC), and charge-transfer (CT). The CT excited states can be further classified as ligand-to-metal charge-transfer (LMCT) and metal-to-ligand charge-transfer (MLCT). 

Electronic transitions in the absorption spectra of Pt(II) complexes include ligand-field (LF) and charge-transfer (CT) bands and perhaps 5d-6p transitions e.g., the absorption spectrum of aq [PtCl ] displays well-separated LF and CT transitions with a maximum at 480 nm (molar absorbtivity, e = 15 M cm ) assigned to a triplet LF absorption, two maxima at 394 nm (e = 57) and 337 nm (e = 62) assigned to singlet LF absorption, and intense bands 200-250 nm (e > 10 ) to LMCT transitions ". The luminescence of K lPtCl ] in ice occurs from the lowest LF-triplet state, and the molecular geometry of this excited state (ES) may be (distorted T j) rather than D41, (square planar), which is particularly relevant to the photochemistry of Pt(II) complexes. 

Fig. 3(A) shows the UV-Vis spectra of 0.1SnO2-MSM samples before and after calcination synthesized with varied periods of HT in comparison to that of commercial SnOa. Two absorption bands with maximum intensities at 220 and 270 nm were observed for the commercial SnOa. The former was assigned to the ligand to metal charge transfer (LMCT) transition from O to Sn, which was at octahedral (Oh) site. The latter band was the electron transition from the valance to conduction bands (band gap ca. 3.76 eV for bulk SnOa) [1]. Only one broad band of very weak intensity was observed in the spectra of the O.lSnOa-MSM samples before calcination, while two bands at 215 and 253 nm were seen for the samples after calcination. The broad band at ca. 205 210 nm for the uncalcined samples was assigned to the LMCT band of 0 to tetrahedral (Td) Sn ions in the silica lattice [17]. When the HT period was prolonged to 24 h, a shoulder appeared ca. 250 nm. These results imply that the Sn02 nanoparticles were probably formed after the HT treatment for 24 h.

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