Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

Palladium-catalyzed bond-forming processes (e.g., C-C, C-X, C-Y; X ) F; Y ) NR2, OR, SR, etc.) represent essential tools for the synthetic chemist. A fascinating myriad of adventurous and unique Pd-catalyzed transformations are routinely found as key steps in target-oriented syntheses, affording complex natural products, functional advanced materials, fluorescent compounds, pharmaceutical lead compounds, and other high-value commercial products. Innovative Pd catalyst design, the identification of new synthetic methodologies, and the acquirement of detailed mechanistic insight, spanning both homogeneous and heterogeneous fields, underpin the numerous developments seen in this area over the past 40 years. Most commonly, Pd-catalyzed bond-forming processes involve Pd0/PdII complexes as intermediates. In recent times, the involvement of PdIV complexes have been implicated in many new synthetic methodologies, for which important advances have been made in the last 5 years or so. While observing the emergence of catalytic PdIV chemistry, particularly in organic synthesis, we identified the need to comprehensively review this area, which draws on aspects from both inorganic (organometallic) and organic chemistry fields. The historical background to organopalladium(IV) chemistry is therefore detailed. We have selected a wide range of diverse transformations where PdIV complexes are believed to act as key intermediates. It is clear that catalytic reaction manifolds involving PdIV intermediates offer new * Corresponding author. E-mail: ijsf1@york.ac.uk. Telephone: +44 (0)1904 434091. Fax: +44 (0)1904 432516. Chem. Rev. 2010, 110, 824–889 824

Emergence of palladium(IV) chemistry in synthesis and catalysis.

Publisher Summary This chapter discusses crystal structures of metal–peptide complexes. Most of the crystal-structure analyses of metal–amino acid and metal–peptide complexes have been carried out on the assumption that such complexes act as models for the metal-binding sites on proteins. Crystal-structure analyses show that the geometrical features of metal complexes of amino acids, peptides and imidazole are related in systematic ways to the chemical structure, which the complexes have in the crystalline state. Moreover, the transfer of geometrical information from crystal-structure analyses to species that exist in solution depends on the assumption that the complexes found in crystals are present also in the solutions from which the crystals grow. This chapter illustrates that, the existence of cis-trans isomers coordination isomers, optical isomers, and dimeric species in the crystalline complexes emphasizes the variety of species, which must be considered when equations are written to represent metal–peptide equilibria in solution.

Crystal structures of metal-peptide complexes.

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

William Wardlaw

Papers

258. The supposed isomerism among the palladodiammines

178. The stereochemistry of quadricovalent atoms : copper and silver

CCCXCVII.—The complex cyanides of molybdenum

211. Isomerides of quadricovalent palladium and platinum

99. The planar configuration for quadricovalent nickel, palladium, and platinum