Transitions in metal complexes

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). 

As described above, CPL spectroscopy is particularly suited for the study of forbidden transitions, so the potential for applying this technique to intraconfigu-rational transitions in metal complexes is very high. In the case of transition metals, however, applications of CPL have been quite limited. The reasons for this are many. Certainly, the preparation of resolved chiral transition metal complexes is quite difficult. 

Among the J ,J -DBFOX/Ph-transition(II) metal complex catalysts examined in nitrone cydoadditions, the anhydrous J ,J -DBFOX/Ph complex catalyst prepared from Ni(C104)2 or Fe(C104)2 provided equally excellent results. For example, in the presence of 10 mol% of the anhydrous nickel(II) complex catalyst R,R-DBFOX/Ph-Ni(C104)2, which was prepared in-situ from J ,J -DBFOX/Ph ligand, NiBr2, and 2 equimolar amounts of AgC104 in dichloromethane, the reaction of 3-crotonoyl-2-oxazolidinone with N-benzylidenemethylamine N-oxide at room temperature produced the 3,4-trans-isoxazolidine (63% yield) in near perfect endo selectivity (endo/exo=99 l) and enantioselectivity in favor for the 3S,4J ,5S enantiomer (>99% ee for the endo isomer. Scheme 7.21). The copper(II) perchlorate complex showed no catalytic activity, however, whereas the ytterbium(III) triflate complex led to the formation of racemic cycloadducts. 

Introduction 231 Fundamental concepts 233 Electronic structure of transition-metal ions 235 Structural characteristics necessary for complex formation 240 Preparation of metal-complex colorants 248 Isomerism in metal-complex dyes 260 Stability of metal-complex dyes 261 Chromium-related problems in the mordant dyeing of wool 268 References 277... 

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

Historically, crystal field theory was the first theoretical model (11, 86, 101, 123) used to explain d-d transition energies in metal complexes. Its usefulness is restricted to those complexes whose bonding is largely ionic, and its mqjor deficiency arises from its inability to account for charge transfer transitions. The iterative extended Hiickel and the ab initio, limited basis set, Hartree-Fock calculations are capable of de-... 

Coordination Chemistry of Thioethers, Selenoethers, and Telluroethers in Transition- 1654 Metal Complexes... 

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