Photochemistry and Photophysics of Coordination Compounds

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6 V. Balzani et al. Fig. 3 Molecular orbital diagram for an octahedral complex of a transition metal. The arrows indicate the four types of transitions based on localized MO configurations. For more details, see text contributions as: (1) strongly bonding, predominantly ligand centered σL orbitals; (2) bonding, predominantly ligand-centered πL orbitals; (3) essentially nonbonding, metal-centered πM orbitals of t2g symmetry; (4) antibonding, predominantly metal-centered σ ∗ M orbitals of eg symmetry; (5) antibonding, predominantly ligand-centered π∗ L orbitals; and (6) strongly antibonding, predominantly metal-centered σ ∗ M orbitals. In the ground electronic configuration of an octahedral complex of a d n metalion,orbitalsoftypes1and2are completely filled, while n electrons reside in the orbitals of types 3 and 4. As for organic molecules, excited configurations can be obtained from the ground configuration by promoting one electron from occupied to vacant MOs. At relatively low energies, one expects to find electronic transitions of the following types (Fig. 3): metal-centered (MC) transitions from orbitals of type 3 to orbitals of type 4; ligand-centered (LC) transitions of type 2 →5; ligand-to-metal charge-transfer (LMCT) transitions, e.g., of type 2 →4; and metal-to-ligand charge-transfer (MLCT) transitions, e.g., of type 3 →5. The relative energy ordering of the resulting excited electronic configurations depends on the nature of metal and ligands in more or less predictable ways. Low-energy metal-centered transitions are expected for metals of the first transition row, low-energy ligand-to-metal charge-transfer transitions are expected when at least one of the ligands is easy to oxidize and the metal is easy to reduce, low-energy metal-to-ligand charge-transfer transitions are expected when the metal is easy to oxidize and a ligand is easy to reduce,
Photochemistry and Photophysics of Coordination Compounds 7 and low-energy ligand-centered transitions are expected for aromatic ligands with extended π and π ∗ orbitals (vide infra). The step from configurations to states is conceptually less simple than for organic molecules because coordination compounds may have high symmetry (i.e., degenerate MOs) and open-shell ground configurations (i.e., partially occupied HOMOs). For octahedral complexes of Co(III), Ru(II), and the other d 6 metal ions, the σL and πL orbitals are fully occupied and the ground-state configuration is closed-shell since the HOMO, πM(t2g) 6 , is also completely occupied. The ground state is therefore a singlet, and the excited states are either singlets or triplets, as in the case of formaldehyde. In octahedral symmetry, the ground-state configuration gives rise to the state 1 A1g. Inthecaseof [M(NH3)6] n+ complexes (e.g., M = Co or Ru), whose ligands do not possess orbitals, the lowest-energy transition is metal centered and the re- πL and π∗ L sulting πM(t2g) 5σ ∗ M (eg) configuration gives rise to the singlet states 1S1g and 1S2g and the corresponding triplets 3T1g and 3T2g. The energy level diagram (at low energies) for [Ru(NH3)6] 2+ is shown in Fig. 4 (the triplet-state energy has been obtained by comparison with the analogous Ir(III) complex [21]). In the case of [M(bpy)3] 2+ (M = Ru or Os), however, since the M(II) metal is easy to oxidize and the 2,2 ′ -bipyridine ligands are easy to reduce, the lowest triplet and singlet excited states are metal-to-ligand charge-transfer in character (Fig. 4). For the corresponding [M(bpy)3] 3+ complexes, the lowest triplet and singlet excited states are ligand-to-metal charge-transfer in character [22], since the M(III) metal can be easily reduced and the 2,2 ′ -bipyridine ligands are not too difficult to oxidize (Fig. 4). In Cr(III) complexes (d3 metal ion), there are three electrons in the HOMO πM(t2g) orbitals. Therefore, these complexes exhibit an open-shell groundstate configuration, πM(t2g) 3 , that splits into quartet and doublet states Fig. 4 Schematic energy level diagrams for [Ru(NH3)6] 2+ , [Ru(bpy)3] 2+ ,and[Ru(bpy)3] 3+
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Photochemistry and Photophysics of Coordination Compounds

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