Monodentate amines

Diamine chelate complexes are more stable than the monodentate amine heterocycles and, therefore, can be studied under physiological conditions. The imidazole complexes are unstable in aqueous solution and decompose rapidly to technetium oxide hydrate. Six-membered ring chelates are significantly less stable than five-membered ones. Lesser flexibility of the ligand, such as 1.2-diamino-cyclohexane, parallels somewhat lower stability of the complex [53] ... 

Effects of acid strength and anion concentration have been studied as well as the effect of monodentate amines.564,565 Phosphines, in particular dppp, have been used as favorable ligands,566,567 but it has been shown that dppp is oxidized by nitrobenzene under the prevailing conditions.568 Giant clusters and heteropolyanions have been used.569,570 The role of nitroso... 

Catalytic homogeneous hydrogenation of cyclohexene has been claimed for simple systems such as nickel(II) acetylacetonate [39] or a nickel-chloride complex with two monodentate amines [40]. The latter complex was used as comparison for a heterogeneous catalyst obtained by impregnation of the complex on y-alu-mina [40]. SCRs of 100 were used at 30 atm. H2 and temperatures up to 100°C, resulting in conversions of only 20-35% after 1 h. 

A number of chromium(III) complexes containing monodentate amine ligands are known and details of their electronic spectra are given in Table 48. 

Table 48 Electronic Spectra of Monodentate Amine Cbromium(III) Complexes...

Table 38 Selected Complexes with Ammonia and Monodentate Amines...

Diaminoethane (en), H2NCH2CH2NH2, forms tris- and bis-chelate complexes. The tris-chelate [Ni(en)3]2+ cation (94) has been known since 1899.658 It is considerably more stable than the six-coordinate complexes formed with monodentate amines as shown in Table 39 where the stability constants of some six-coordinate complexes are reported. 

In aqueous solution, silver does not appear to bind more than two monodentate amine ligands, which are predicted to give linear or almost linear structures, similar to that found for ammonia. 

Studies on bidentate and higher dentate amine ligands have indicated that complex formation with silver(I) was rather more complicated than for monodentate amines. Polynuclear, protonated and even hydroxo complexes have been postulated to occur in solution. 

Malhotra introduced monodentate amines derived from a-pinene as chiral lithium amide precursors118. Using 20 mol% of the base 105 with excess LDA resulted in 95% ee of the corresponding (R)-allylic alcohol (Scheme 76). 

Table 32 Electronic Spectra of some Monodentate Amine Complexes of...

Table 5 Properties of some simple monodentate amines...

However, the outcome of such reactions does depend on the entering ligand, as the above reaction performed with the aromatic monodentate amine pyridine instead of 1,2-ethanediamine leads to only four pyridines binding, with [V(py)4(S04)] as the product. 

Although such isomers have not been resolved for monodentate amines (suitable tertiary amines are not easily coordinated to inert metal centers), the phenomenon is quite widespread with coordinated aliphatic polyamines (20), (21)... 

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