Coordination geometries, common

The most common oxidation states and the corresponding electronic configuration of mthenium are +2 and +3 (t5 ). Compounds are usually octahedral. Compounds in oxidations states from —2 and 0 (t5 ) to +8 have various coordination geometries. Important appHcations of mthenium compounds include oxidation of organic compounds and use in dimensionally stable anodes (DSA). 

The most common oxidation states and the corresponding electronic configurations of osmium ate +2 and + (t5 ), which ate usually octahedral. Stable oxidation states that have various coordination geometries include —2 and 0 to +8 (P] The single most important appHcation is OsO oxidation of olefins to diols. Enantioselective oxidations have also been demonstrated. 

The most common oxidation states, corresponding electronic configurations, and coordination geometries of iridium are +1 (t5 ) usually square plane although some five-coordinate complexes are known, and +3 (t7 ) and +4 (t5 ), both octahedral. Compounds ia every oxidation state between —1 and +6 (<5 ) are known. Iridium compounds are used primarily to model more active rhodium catalysts. 

Table 1. Polyhedral Symbols for Common Coordination Geometries ...

Macrocycles may also promote the formation of less common coordination geometries for particular metal ions because of increased macrocyclic ring strain on coordination. Such an effect is illustrated by the variation in the structures of the nickel complexes of the 14-, 16-, 18-, and 20-mem-bered tropocoronand macrocycles of type (14) (Imajo, Nakanishi,... 

The metal complexes discussed thus far bear little resemblance to the vast majority of common transition-metal complexes. Transition-metal chemistry is dominated by octahedral, square-planar, and tetrahedral coordination geometries, mixed ligand sets, and adherence to the 18-electron rule. The following three sections introduce donor-acceptor interactions that, although not unique to bonding in the d block, make the chemistry of the transition metals so distinctive. 

Table 3.4. Suggested notations for common coordination geometries (from Lima de Faria et al. 1990).

Figure 13 Common transition metal coordination geometries.

As is common with the higher coordination geometries, the interconversion of Dzh Cav niay be easily achieved. The monocapped square antiprism (C4 ) can be generated from the s-tricapped trigonal prism by raising the points a and b (Fig. 27) and lowering the two caps c and d so that the four form the basal plane of the square antiprism. 

The spirocyclic structures of 8b, 9b and 10 consist of two zincacyclopentane (8b and 10) or two zincacyclohexane (9b) rings, having the zinc atom in common. Each of the lithium atoms is bonded to the a-carbon atoms of the two metallacycles, while a tetrahedral coordination geometry at lithium is reached by bidentate N—Ei coordination... 

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