Some Monodentate Ligands

bromide Br, iodide I, azide N3, isocyanate NCO , hydroxide OH , nitrite NO2, nitrate NO3, isothiocyanate NCS , sulfide S , carbonate C03 , sulfate S04, and thiosulfate ions  

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Although some monodentate ligands achieve significant asymmetric inductions [34, 35], intrinsically bulkier bidentate chelates have a better starting position. The chiral bidentate ligands L3-L12 (cf. Scheme 3) discussed here may be split into two classes, namely Cp and C2-symmetric ones. The Ci-symmetric BINAPHOS L3, presented first in 1993 [36], is still the only ligand which combines ee values of about 90 % with a broad variety of substrates. Unfortunately, the reaction conditions for L3 are not satisfactory, so that it remains without industrial application. 

These authors found [178] that the Cu(II)-catalyzed oxidation of o-phenylene-diamine by O2 is strongly influenced by organic and inorganic ligands. While chelating agents such as ethylenediamine and EDTA inhibit catalysis, reaction is accelerated under the influence of some monodentate ligands, e.g. Cl", Br", NH3, pyridine and imidazole. 

Table 1-3. Some typical monodentate ligands and representative complexes that they form.

There is one striking group of exceptions to the otherwise almost unbroken success of Kepert s approach. No model predicated solely upon the repulsions between monodentate ligands (or between bonds) can account for the planarity of some four-coordinate complexes. Yet hundreds of planar (f complexes like [Ni(CN)4] or [PtCl4] are known. Clearly, Kepert s model is to be augmented and we discuss this matter further in Chapter 7. 

Analogous to the dithiophosphate complexes, cis octahedral complexes are formed with bident-ate ligands like phenanthroline, 2,2 -bipyridine, or tmeda and trans octahedral complexes are formed with monodentate ligands like pyridine and its derivatives. Some examples are mentioned with linear bidentate ligands like 4,4 -bipyridine, which build up ID chains by bridging the [NiL2] molecules.926-933... 

For such a mechanism, the overall second-order formation rate constant is given by the product of the first-order constant ktx and the equilibrium constant Kos. The characteristic solvent exchange rates are thus often useful for estimating the rates of formation of complexes of simple monodentate ligands but, as mentioned already, in some cases the situation for macrocyclic and other polydentate ligands is not so straightforward. 

In spite of the interesting results obtained for organometallic dithiophos-phates, similar studies with diselenophosphates have not been performed for comparison. Only recently some organogermanium, -tin, and -lead derivatives of the type Ph3MSe(Se)P(OR)2 (M = Ge, Sn, Pb R = Et, Pr, Pr ), containing monodentate ligands have been reported.446... 

N-Diazeniumdiolates can also undergo some reactions other than those directly associated with NO release. They act as bidentate chelators and form complexes with a variety of transition metal ions. Suitable choice of the ligands can lead to the formation of complexes in which the diazeniumdiolates act as a monodentate ligand coordinated to Cu(II) at 02 only [223]. 

Gentile and Shankoff (28) isolated several adducts, or complexes, of simple salts with monodentate ligands. They also found that diacetamide, CH3CONHCOCH3, formed 1 2 complexes with some but not all MX,... 

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