Stability of coordination compounds
The statement that a compound is stable is rather loose, for several different interpretations may be placed upon it. Used without qualification it means that the compound exists and, under suitable conditions, may be stored for a long period of time. How
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.... (BF2 )N(SiH 3 ) 2 + SiH 3 F
The complex [BF3 (N(SiH 3 h)] can be prepared and stored at low temperatures ( ~ -80 oq since the decomposition then proceeds very slowly-at this temperature the complex is kinetically fairly stable. At room temperature the complex is kinetically unstable and the rate of decomposition is much greater. This is the key distinction made in Chapter 4 between kinetically inert and kinetically labile complexes. There it was pointed out that the species which crystallizes from a solution of a mixture of related labile complexes depends not only on the cation and ligand concentration but also on the solvent and crystallization temperature. Although it may be a relatively minor component in the solution, the least soluble complex is probably the one which crystallizes. In the solution there is a series of equilibria such that,
S. F. A. Kettle, Physical Inorganic Chemistry © S. F. A. Kettle 1996
74
1
Stability of coordination compounds
if one component crystallizes, the concentrations of the others also change. This chapter is devoted to a discussion of the stability constants which characterize such equilibria.
5.2
Stability constants
When a complex is formed by the reaction 1 M + L.,tML
the equilibrium constant K 1 for the complex containing a single ligand will be K1
=
[ML] [M][L]
where, for the moment, activity coefficients of unity have been assumed. If ML adds a further molecule of L, ML + L+t ML,
then the equilibrium constant for the complex containing two ligand molecules is
In general, the equilibrium constant for the formation of the complex ML. from ML._ 1 will be Kn =
[Ml.,] [Mt.,_ 1 )[L]
The equilibrium constants Kt> K 2 , ... , K. are known as stepwise formation constants. Alternatively, one may consider the equilibrium constant for the overall reaction as [Ml.,]
Pn = [M][L]"
Pn is known as the nth overall formation constant. Pn is related to the stepwise formation constants Kt> ... , K. by Pn = K1 That is
X
K,
X · · • X
Kn
1 Throughout this chapter we shall often not specify the charges on the species in reactions or equilibria. Square brackets are used to indicate both the concentrations of complex species and the species themselves. It will be clear from the context which is intended.
Determination of stability constants
1
75
Table 5.1 Typical stability constant data for monodentate ligands. All values are logarithmic so, for the Sn 2 + fCI- system, logK1 = 1.51 Metallon
Ugand
Stability constants
Sn 2 +
c;c;-
K1 = 1.51 K1 = 6.1 K1 = 2.67
Pd2 + Ni2 + Cu 2 + Mn 2 +
NH 3 (30 'C) NH 3 (30 'C) F-
Pb2 +
;-
Fe 2 +
CW
Fe3 +
cw
K2 K2 K2 K2 {J,
K1 = 3.99 K1 = 5.52 K1 = 1.98 {J6 =24 {36 = 31
p,
= = = = = =
0.73 4.6 2.12 3.34 9.04 3.15
K3 K3 K3 K3 {J, {J,
= = = = = =
-0.21 2.4 1.61 2.73 11.64 3.81
K4 K4 K4 K4
p. fJ.
= = = = = =
-0.55 2.6 1.07 1.97 13.4 3.75
K5 = -2.1 K5 = 0.63
K6 = -2.1 K6 = -0·09
{J5 = 14.7 {J5 = 3.81
{36 = 15.5
{36 = 8.01
These data refer to 25 'C unless otherwise stated and to zero ionic strength. As a compa
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