estimates relying on metal-metal distance and size or alternancy of the bridging ligand.5b The contributions from very different kinds of metal fragments to the overlap with one specific ligand bridge are certainly more difficult to predict; however, intensities of pertinent charge-transfer transitions can serve as useful guidelines.25b This approach has triggered the successful search for organometallic d1 2345/d6 7(µ-pyrazine) analogues of the Creutz-Taube ion that display more intense charge-transfer transitions and much larger values of Kc than the inorganic parent system.25 Considering these more practically oriented rules for electronic coupling in addition to the well-known statistical and solvational contributions to Kc, it should thus be possible for experimentally working chemists to rationally design and synthesize new mixed-valence systems with very large stability constants.7,25 Acknowledgment. This work was generously supported by the Deutsche Forschungsgemeinschaft and Stiftung Volkswagenwerk.We also thank Degussa AG for donations of precious metal chemicals and Dr. Sylvia Ernst (Frankfurt) for preliminary experiments.
We measured the electronic absorption and emission properties of [M(COD)(p-L)]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene; µ-L = hp (2-hydroxypyridinate), mhp (6-methyl-2-hydroxypyridinate), chp (6-chloro-2-hydroxypyridinate), 2hq (2-hydroxyquinolate), pz (pyrazolate)) complexes and the deactivation parameters of the 3B state of [Ir(COD)(u-L)]2 in MTHF (MTHF = 2-methyltetrqhydrofuran). Each Rh and Ir complex exhibits an intense absorption band around 420 and 490 nm, respectively, that is assigned to the *A -* *B component of the d * -transition. The Ir complexes exhibit temperature-dependent phosphorescence bands ( > 720 nm) and temperature-independent fluorescence bands (Xmax = 610 nm). The temperature dependence of the phosphorescence intensity was fit to the following expression: 1 /0obad = 1 / 0 + A' txp(-Ea/kBT).
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