Ammine complex, formation constant

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

TABLE 11.35 Cumulative Formation Constants of Ammine Complexes at 20°C, Ionic Strength 0.1 ... 

Figure 2. Catalytic activity for aniline formation as a function of electronegativity and formaiion constant of ammine complex...

With this situation in mind, the formation constant (ft) of the first ammonia molecule making the coordination bond with metal cations was taken as the measure of the ease of ammonia adsorption on metals. The values of (ft) were taken from Ringbom s table (H). The plot of the catalytic activity against formation constants of ammine complexes is shown in Figure 2. The correlation is good except for CdY. Again, the metals with lower formation constant (Na+, Ca+, Mg2+) have no activity for chlorobenzene reaction. 

As described above, the catalytic activity of metal ion-exchanged zeolites for aniline formation has a good correlation with electronegativity and with the formation constant of ammine complexes of metal cations. The order of the activity agrees with the Irving-Williams order. These facts give irrefutable evidence that the transition metal cations are the active centers of the reaction. 

The good correlation of catalytic activity and the formation constant of the ammine complex or the electronegativity of the metal cation could... 

In addition to chelate complexes, the cyclic amine 1,4,7-triazacyclononane will complex to platinum(II) and (IV). The hexacoordinate platinum(IV) complexes are bonded to two molecules of the tridentate ligand, but platinum(II) complexes with the ligand monodentate and bidentate.988 Also the formation of platinum(II) ammine complexes from chloride complexes is a reversible process. The rate constants decrease as the basicity of the leaving amine increases.989... 

Table 19.2 Comparison of Formation Constants for Ammine and Ethylenediamine Complexes...

The larger the value of the constant is, the more stable the complex (Table 12.11). The metal-ammine formation constants Ksl and Kst are known as stepwise formation constants. Stepwise formation constants1 could be used to estimate overall formation constants. For example,... 

Replacing the metal-ammine complexes in the denominator as a function of NH3, M2+, and overall formation constants, as demonstrated in Equation 12.24, gives... 

The curves for calcium ions differ from those for nickel in two ways, (i) Before the end point the curves are essentially independent of pH because calcium does not form ammine complexes. (2) After the end point the pM value is smaller than for nickel because of the smaller value of the formation constant Kc y-- For the same reason, at low pH values, ay and are so small that no pM break occurs at high pH, ay approaches unity, so it is advantageous to perform calcium titrations at pH 10 to 12. 

The value of aM can be expressed readily in terms of the ammonia concentration and the formation constants for the various ammine complexes, as described for a general metal-ligand reaction in Feature 17-1. The result is an equation analogous to Equation 17-9 ... 

In the course of a study of nickel ammine complexes, in two experiments turbidity occurred, which was interpreted as an indication of formation of Ni(OH)2. When the solubility product (log,p = -13.8, 25°C) is extrapolated to / = 0 by means of the Davies equation the following constants were obtained (log, X°g= -14.64, log, X" = 13.36) [62DAV]. These values are in line with others which refer to freshly precipitated probably amorphous Ni(OH)2, see Table V-6 [380KA], [54FEI/HAR]. 

Since it is known that OH does not in fact attack co-ordinated ammonia, a rapid dissociation of the Co ammine complex followed by attack by this radical on the ammonia released would then account for the formation of NHa radicals. The reaction products are NH4+, Ng, N2O, and a relatively small amount of Oj. The yields of N2 and NjO are close to those expected if NH2OH were the intermediate giving rise to these gases. An interesting overlap on these studies is provided by data on the rate of detachment of NH3 from cobaIt(in) complexes reduced by pulse radiolysis. The rate constant Atq = 8 x lO 1 moI s for the process... 

LMCT excitation of Co (NH3)5X2 with X = halide or pseudohalide yields the complex fragment Co (NH3)52+ which is kinetically very labile [2,5,94]. Co(II)-ammine complexes decompose in aqueous solution to Co + and ammonia with a rate constant k > lO s [95]. The LMCT photolysis of PtCl52- and other Pt(IV) complexes leads to the formation of Pt L5 intermediates [111-113] which are also kinetically labile and undergo facile ligand substitutions. Since these Pt(III) radicals can efficiently exchange electrons with Pt(IV) they catalyze the substitution of inert Pt(IV) complexes in a chain reaction [2,10]. The overall quantum yield for photosubstitution of PtClg - was shown to exceed unity. 

To obtain satisfactory yields, the reaction must be carried out at pH > 9 (see Sect. 25.4). Hence, the ammonia buffer is used. In these conditions and due to the fact that a buffer must be used in a rather important concentration, the following ammine complexes form [Ni(NH3)] +, [Ni(NH3)2] +,. .., [Ni(NH3)6] +, whose overall formation constants are Pj, P2, , Pe- As a result, the formation reactions of the complex of interest must be written as... 

Let s begin by recalling the successive formation constants of some ammine-metal complexes (Table 35.1). They govern the following equilibria ... 

---