Metal complexes isomeric structures

In order to provide a detailed discussion on isomerism among metal complexes, the isomers are first broadly classified on the basis of topology as structural isomerism and stereoisomerism, as shown in Figure 1. The stereoisomers have the same topology it is the arrangements of the ligands in the space that make them different. The structural isomerism, also known as constitutional isomerism, includes all other types of isomerism shown by the metal complexes. The structural isomers have different topology. 

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... 

Contents Introduction. - X-Ray Difraction. -Conformational Analysis. - Structure and Isomerism of Optically Active Complexes. - Electron-Density Distribution in Transition Metal Complexes. - Circular Dichroism. - References. 

The primary allene adducts of general structure 99 as introduced in Section 8.2.2.1 generally are able to undergo (Baldwin disfavored) S-endo-trig cyclizations to furnish products of type 100. These isomerizations are either promoted by bases or catalyzed by Lewis acids or transition metal complexes. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

CgQ with this Zr complex, a red solution is formed, unlike the green solution ofr transition metal complexes of Cjq. The structure of the air-sensitive Cp2ZrClC5oH was confirmed by NMR spectroscopy. The hydrogen transferred from the Zr to CgQ resonates at 5 = 6.09, a typical value for fullerenyl protons [83]. Hydrolysis of Cp2ZrClC5oH with aqueous HCl provides access to the simplest hydrocarbon C5QH2 (30, Scheme 7.14). Spectroscopic characterization of CggH2 showed that the compound is the isomerically pure 1,2-addition product. 

In contrast, chromium)III) and cobalt(IH) form 2 1 dye metal complexes that have nonplanar structures. Geometrical isomerism exists. 

The crystal and molecular structures of isomeric cluster complexes, HFe3(MeC=NH)(CO)9 and HFe3(N=CHMe)(CO)9, derived from reduction of acetonitrile have been reported.149,150 In addition to these, a wide variety of complexes with ligands containing azomethine groups (>C=N—) along with other functional groups (e.g. metal complexes of salicylaldiimine) are known and these will be discussed in Chapter 20. 

In many complexes, mixed oxidation states for the transition metals are found. Complex (41) is a diamagnetic compound with the actual composition (42).68 Copper dithiocarbamates are synthesized not only with copper in the oxidation states I, II or III but also with the metal in mixed oxidation states III and I (43, 44, 45), II and I (46, 47) and even III and I [48, 49).56 Complex (50), which has two isomeric structures a and / , has Ru either in a trapped oxidation state II,in or in a delocalized oxidation state of 2.5. The /J-isomer is the most stable.69 Slow, controlled potential electrochemical oxidation of (51) yields (52), in which the metal has a trapped oxidation state II,HI.70... 

Na—Np isomerism Perhaps the most immediately obvious source of isomerism in metal complexes of tridentate azo compounds is the involvement of different nitrogen atoms of the azo group in coordination to the metal. This is not relevant in the case of symmetrical azo compounds but two structurally isomeric metal complexes (79) and (80) are conceivable for unsymmetrical azo compounds. These have been designated by Pfitzner78 as Na and Np isomers. 

In contrast, chromium(m) and cobalt(m) form 2 1 dye metal complexes that have nonplanar structures. Geometrical isomerism exists. The o.o -dihvdroxyazo dyes (75) form the Drew-Pfitzner or mer type (76) (A = C = O), whereas o-hydroxy-o -carboxyazo dyes (77) form the Pfeififer-Schetty or fac type (78) (A = CO 2 and C = O). 

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