Coordination complexes, ligands

This chapter is devoted to electrochemical processes in which chemical reactions accompany the initial transfer of one electron. This is actually a pretty common situation with organic reactants since the radical or ion-radical species resulting from this initial step is very often chemically unstable. Although less frequent, such reactions also occur with coordination complexes, ligand exchange being a typical example of reactions that may accompany a change in the metal oxidation number. 

There is currently a growing awareness amongst pharmacologists of the importance of stereochemistry, particularly of the chirality of drug molecules. These drugs may be coordination complexes, ligand molecules with potential for in vivo coordination, or molecules whose in vivo interactions are unknown. 

Mabbs and co-workers studied the single crystal polarized spectra of pentacoordinate VO " compounds at 298 and 5 K and of poly(dimethylsiloxane) mulls of six VO compounds (five-and six-coordinate complexes). Ligand-field strengths ranged from that of cyanide to that of bromide. Transitions I and II were assigned to and to 3d 3d ,3dyz. Spectra... 

Some of the oxidation states given above, especially the higher oxidation states (7, 6) and oxidation state 0, are found only when the metal atom or ion has attached to it certain groups or ligands. Indeed the chemistry of the transition elements is so dominated by their tendency to form coordination complexes that this aspect of their behaviour must be considered in some detail. 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Coordination Complexes. The coordination and organometaHic chemistry of thorium is dominated by the extremely stable tetravalent ion. Except in a few cases where large and stericaHy demanding ligands are used, lower thorium oxidation states are generally unstable. An example is the isolation of a molecular Th(III) complex [107040-62-0] Th[Tj-C H2(Si(CH2)3)2]3 (25). Reports (26) on the synthesis of soluble Th(II) complexes, such as... 

Phosphorus Donors. Phosphine coordination complexes of thorium are rare because the hard Th(IV) cation favors harder ligand donor types. The only stable thorium—phosphine coordination complexes isolated as of the mid-1990s contain the chelating ligand,... 

Many complexes have more than one coordination mode of BH4 featured in their structure, e.g. [U ()9 -BH4)()9 -BH4)2(dmpe)2]. Likewise, whereas [M(BH4)4] are monomeric 12-coordinate complexes for M = Zr, Hf, Np, Pu, they are polymeric for M = Th, Pa, U the coordination number rises to 14 and each metal centre is coordinated by two r) -BH4 and four bridging r) -BH4 groups. It is clear that among the factors which determine the mode adopted are the size of the metal atom and the steric requirements of the co-ligands. Many of the complexes... 

The molecule has an almost linear N3 group and an angle C-N-N of 112.4° (Fig. II.4a).( ) The (linear) azide ion, N3", is isoelectronic with N2O, CO2, OCN", etc. and forms numerous coordination complexes by standard ligand replacement reactions. Various coordination modes have been established, including end-on bridging... 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

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