Ligand ratio

The change in colour when one ligand is replaced by another can be used to determine the coordination number thus if the colour change is measured in a colorimeter as the new ligand is added, the intensity of new colour reaches a maximum When the metal/ligand ratio is that in the new complex. 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

The ultraviolet spectrum of compound 2 in DMF did not exhibit an absorption maximum above 300 nm. Formation of such a complex was evident from the changing color of solution into brown and the appearance of an absorption maximum at 534 nm. The metal-ligand ratio determined at this wavelength by the Job method was 1 1. 

Similar studies on the Bu2Sn(IV) complexes formed with the same ligands were performed. The synthetic procedure resulted in the formation of Bu2Sn(IV) compounds with a 1 2 metal-to-ligand ratio for pyridinemonocarboxylic acids. 

In the same context, Kang et al. have examined several bidentate thiol derivatives as ligands in the same reaction to that described above. A nickel complex derived from the p-amino thiol depicted in Scheme 2.31 catalysed the reaction in a useful asymmetric level of 74% ee, whereas the use of the thiol phosphine ligand depicted in Scheme 2.31 gave a lower enantioselectivity of 27% ee. In this study, the authors found that the enantioselectivity was strongly dependent on the ligand concentration and on the nickel-to-ligand ratio. 

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Another key factor is the choice of reactant proportion. We found that it is necessary to maintain a high metal-to-ligand ratio because a low ratio (such as Ln Cu gly Ac = 1 4 2 2) will lead to the final product being contaminated by a small number of light-blue crystals of CudglyutCICUh. 

Chances are, as long as there are two strong Co—P bonds existing at all times, that the requirements for selectivity will be satisfied. The use of two bisphosphines (32) or one trisphosphine (35) is merely to assure the presence of these two Co—P bonds. Thus, with a very good chelating ligand such as the bis(diphenylphosphino)propane 33, Hata (66) showed that only a 1 1 Co/ligand ratio was needed for best selectivity. 

Fig. 36.8 ee-values obtained in the Rh-catalyzed hydrogenation of methyl-2-acetamidocinnamate, plotted against the ligand ratio (the ratio (L1 + L2)/Rh always remains 2). 

Various chromium-complex dyes were prepared recently by reacting ammonium chromium sulphate with a series of chelatable o,o -dihydroxyazopyridone structures (Scheme 5.9). Elemental analyses corresponded to a 1 2 metal-dye ligand ratio (5.35). The... 

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