Ligand field excited

A pulsed-laser photolysis study of [Cr(en)3]3+ illustrates quite dramatically the enhancement in reactivity that can result upon populating a ligand field excited state.26 Thus a significant fraction of the primary photoproduct, [Cr(en)2(enH)(OH2)]4+, is formed within the 20 ns duration of the laser pulse and is thought to arise from reaction of the lowest excited quartet state, Q° (see Figure 6). This observation establishes that the pseudo-first-order rate constant for this excited state... 

Mixed-ligand Crm complexes have a particularly rich substitutional photochemistry in that two (or more) reaction modes are normally observed. Data for the well-studied class of acidoamine complexes are presented in Table 2. The dominant photochemical reaction for [CrX(NH3)5]2+ complexes in aqueous solution is NH3 aquation, with X- aquation occurring to a lesser extent (equation 31). In contrast, the latter pathway is the favored thermal reaction of these compounds. Such behavior again illustrates that the reactivity of ligand field excited states can differ sharply from that of the ground state. 

Quantum yields have been measured for the photoaquation of a large range of substituted pyridine complexes of the type [Ru(NH3)5(pyX)]2+.53 The marked dependence of quantum yields on the nature of X indicates that metal-to-ligand-charge-transfer (MLCT) excited states are not involved in the photosubstitution. Presumably, a ligand-field excited state is responsible. Evidence has been reported for a simple outer-sphere reduction of cytochrome c by [Ru(NH3)6]2+.54 Such a... 

In the tetrahedral Ni(CO)4 complex we have a formal d10 system and there is no CO to Ni a donation. We therefore need no CO a orbitals in the active space. Instead we add empty orbitals of the same symmetry as the 3d orbitals, e and t2. These orbital will turn out to be a mixture of CO tt orbitals and Cr 3d and thus include the double shell effect. The lOinlO active space turns out to be quite general and can be used for many transition metal complexes. This active space will allow studies of the ground state and ligand field excited states. If charge transfer states are considered, one has to extend the active space with the appropriate ligand orbitals. 

The first quantitative photochemical study of a Rh111 amine was reported by Moggi,8 who noted that both 254 nm (LMCT) and 365 nm (ligand field) excitation of [Rh(NH3)5Cl]2+ caused chloride labilization (equation 131). Other early reports include Basolo s study of the photoinduced stereo-retentive halide aquation from [M(en)2X2]+ (M = Rh, Ir X = Cl, Br, I), and Broomhead s observation of chloride aquation from [RhCl2(phen)2]+.726 While halide labilization dominates upon photolysis of [Rh(NH3)5Cl]2+, both bromo and ammine loss occur upon photolysis of the bromo analog (equation 132)685,707 and ammine is labilized from the iodo analog (equation 133).70 Biacetyl sensitization of the bromo complex quenches the biacetyl phosphorescence, but not the fluorescence,707 consistent with a photoreactive triplet state. 

Lazarides, T., Davies, G.M., Adams, H., et al. (2007) Ligand-field excited states of hexacyanochromate and hexacyanocobaltate as sensitisers for near-infrared luminescence from Nd(III) and Yb(III) in cyanide-bridged d-f assemblies. Photochemistry and Photobiological Sciences, 6, 1152. 

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