Anionic monodentate ligands

Highly sensitive ammonium and methylammonium tetraazidoaurates(III) have been prepared according to different procedures depending on the ammonium counter ion [37] (Equations 2.2 and 2.3). 

A few stable gold(III) imidate complexes have been authenticated by X-ray structure determinations. Most of the claimed gold(III) imidate complexes including those containing succinimide, phthalimide, saccharin or N-methylidantoin derivatives [40] were subsequently reformulated as gold(I) species [41[. 

The tveak and reversible binding of these complexes to calf-thymus DNA (ct DNA) suggests a dominant electrostatic mode of interaction nevertheless, relevant conformational distortions of the double helix are caused [50]. A multinuclear NMR study of the reactivity of [Au(en)Cl2]Cl and [Au(en)2]Cl3 vith guanosine 5 -monopho-sphate (5 -GMP) reveals that in an aqueous solution only [Au(en)Cl2]Cl binds very weakly to 5 -GMP via N(7) to give a 1 1 adduct [48]. 

In the solid state, the [Au(4,4 -tBu2bipy)(tdt)] cations are arranged in stacks with alternating intermolecular Au Au separations of 360 and 375 pm. 

Formation of a 2 1 L-histidine Au(III) complex is suggested on the basis of multiinstrumental techniques the N1 of the imidazole ring and the nitrogen of the a-amino group are likely involved in the coordination [66]. 

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The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

Fig. 9. Eight-coordinate transition state in an interchange associative mechanism of substitution reactions on a pentagonal-bipyramidal complex (S=solvent molecule, A =anionic monodentate ligand).

It was found that the 1,3-diphenyl-1,3-propanedionate anion behaved as a bidentate ligand whereas the benzoate acted as a monodentate ligand. The Ag-C bonds in 21a and 21b were more or less similar to that found in 18 (21a 2.064(6) A, 21b 2.085(10) A). It is interesting to note that 21b crystallizes as dimers with the presence of short intermolecular Ag-Ag distance of 3.000(10) A, in contrast to the monomeric nature of 21a with a longer Ag-Ag distance of 3.2177(13) A. 

Published equilibrium constants for monocarboxylato complexes are summarized in Table VII. All that can be deduced with certainty from these data is that the anions derived from monocarboxylic acids form rather weak complexes with beryllium. In all probability they act as monodentate ligands. The possibility of bidentate chelation using both carboxylate oxygen atoms can be ruled out on the grounds... 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

In the available literature, only six PBP structures of Ni(II) have been reported so far (34,38,40,45,48,50) and four of them include acyclic pentadentates (XVII-XX). These pentadentates are equato-rially coordinated in their neutral form with weak conformational flexibility. Apical positions are always occupied by water molecules, while nitrate anions always represent counter ions in the structure. The change of any of the mentioned structural elements (polydentate, axial ligands, or counter ion) affords complexes of which the structure has not been determined (52,54,56,58). These are complexes with the same type of ligands XX-XXIII, containing Cl- ions and water molecules, but it is still unknown which of these two monodentate ligands occupies the apical positions. It is questioned whether both monodentates actually occur in the coordination sphere, and because of that it is impossible to discuss the geometry of these complexes in more detail. 

When nickel salts of coordinating anions are used, for example Ni(NCS)2 and Ni(N03)2, complexes with a number of coordinated monodentate ligand molecules less than six are obtained.1888,1889,1892 Assignment of IR spectra of the Ni(DMSO)2+ cation using lsO isotope shifts has been reported.1893... 

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