Bromo complexes stability

The products obtained from the Pt(PR3)2X2-CNCH3 reactions were dependent on the nature of the platinum species. Five-coordinate adducts, [Pt(PR3)2(CNCH3)2X]X, were isolated for the iodo and bromo complexes (R=Ph), although the latter was unstable and slowly lost isocyanide. The observation of five-coordination here is somewhat unusual, but since this report, it was also observed in a different situation (85), mentioned above. The more common observation was the isolation of four-coordinate species, implying the low stability of most five-coordinate complexes. Data on these reactions are summarized below [Eqs. (33, 34)]. 

Overall stability constants for the formation of lanthanide chloro- and bromo-complexes in anhydrous methanol, ethanol, and propanol, as determined by spectrophotometry. 

These values reflect moderate interaction between Ln(III) and the chloride ion. The overall stability constants for the formation of chloro and bromo complexes of lanthanides (y6(MX)2+ and (MXj)) are given in Table 4.7. 

The data for bromo complexes were obtained in aqueous methanolic solutions and outer-sphere bromo complexes with K = 1.3-1.9 were obtained for Pr, Nd, Sm, which are larger than the values of Ho (0.97) and Er (0.70). Chloro and bromo complex formation in dimethyl formamide studied by titration calorimetry [122] showed the evidence for MC12+, MCC. MCI3, and MCI4 species in solutions. In the case of bromide, monobromo and dibromo inner sphere complexes have been detected. The stepwise formation constants could not be determined for iodo complexes due to the small value of enthalpies of reaction. The stability constants data obtained in DMF are given in Table 4.8. 

The bromides and iodides are made by direct combination of the elements. Some are coloured TiBr4 is yellow and TH4 red-brown, in accordance with the position of the ligands in the spectrochemical series (p. 134). They are solids of low m.p. the crystals have a cubic lattice and contain tetrahedra molecules. Some bromo-complexes have been made, for instance NH4)2TiBrg.2H20, but they are much less stable than the fluoro-compounds iodo-complexes are unknown the stability falls rapidly with the more easily polarisable halogens. 

An attempt to determine the stability constants of the nickel bromo complexes was complicated by the slow formation of bromine from oxidation of bromide by the nitrate ion. However, the results indicated that the bromo and chloro complexes of nickel(II) have similar stability. 

In keeping with the hard or class a nature of the Fe " ion the most stable complexes are formed with the small non-polarizable F ion. Consecutive formation constants for some fluoro and chloro complexes in aqueous solution may be found in ref. 322. Bromo complexes are even less stable than chloro complexes while with the easily oxidizable 1 ion no stable simple iodides have been prepared although a very few complexes containing Fe "—I bonds are known in combination with other ligands. Consistent with the stability order F > CL > Br is the occurrence of higher coordination number Fe complexes with F than with Br . Thus, while F readily forms hexacoordinate [FeFfi] complex ions and no tetrafluoro complex ions, CL forms both [FeClgf and [FeCl4] while Br apparently does not form a stable hexabromo complex [FeBr ] ". 

The chloride ion is a stronger ligand towards class (a) acceptors than the bromide ion. Thus chloro-complexes will be stable in certain solvents, in which bromo-complexes cannot be obtained due to the higher stabilities of solvent-coordinated species. For example cobalt(II) bromide is completely ionized in... 

The interaction between bromide and lanthanide ions was also studied, but experimental data are available for aqueous methanol solutions only (Kozachenko et al. 1973). Using a spectrophotometric method, the formation of rather weak outer-sphere bromo complexes was evidenced, and their stability constants for Pr, Nd, Sm, and Ho were determined in water and in 50% and 90% methanol (table 3). For solutions in 50% methanol, the stability of the outer-sphere bromo complexes is larger for Pr, Nd, and Sm Ki = 1.3—1.9) compared to Ho (0.97) and Er (0.70). Kozachenko et al. (1973) explained this behaviour as reflecting a higher stability for the solvates of the heavier lanthanide ions. A similar trend was observed in the stability constants of the chloro complexes in absolute methanol vide supra). Finally, the stability of the bromo complexes of the lanthanides increases as the dielectric constant of the medium is reduced. 

Stability constants and Aff, and Affj values for the formation of chloro- and bromo-complexes in DMA at... 

Dimethylacetamide. The stability constants and enthalpy values for lanthanide-nitrate interactions in anhydrous DMA (R=La, Nd) have been determined by means of calorimetric titration (Airoldi et al. 1982). The experimental results support the existence of the iimer-sphere mono- and dinitrato complexes with the following formation constants A i = 17,. 2= 16 for lanthanum and K =25, K2 = 92 for neodymium (cf. table 7). The enthalpies of formation 1 (La 48.2, Nd l.OkJmol" ) and AH2 (La 23.1, Nd 8.3kJmor ) are all endothermic, although rather small. Finally, mononitrate complexes appear to be less stable for both La(III) and Nd(III) ions compared to the corresponding chloro- and bromo complexes (cf. tables 6, 7). 

In water, no interaction is detected between perchlorate and Er(III) or Nd(III), but the addition of methanol causes complexation with Er(III) (Silber and Strozier 1987). The presence of water restricts chloride complexation to the monochloro complexes and the stability of this species diminishes regularly with increasing water content (Hamze et al. 1986). However, the electronic spectra of the Nd(III) solvates are nearly independent of water content (0-10%). Similar results are reported for the complex formation between Pr, Nd, Sm, Ho, Er and bromide in water and in 50% and 90% methanol (Kozachenko et al. 1973). The stability of the bromo complexes increases as the dielectric constant of the medium is reduced. In either water or aqueous methanol, bromo complexes are less stable than chloride complexes. 

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