Stability of Coordination Compounds

STABILITY of COORDINATION

COMPOUND

AMARIA

Jurusan Kimia FMIPA

Sub-CLO (Course Learning Outcome)

 Students are able to explain the stability of coordination compounds and the factors that influence

Indicator

1. Students are able to explain the stability constants 2. Students are able to explain the thermodynamic

parameters of complex compounds

3. Students are able to explain the factors that affect the stability of complex compounds

4. Students are able to predict the stability of complex compounds based on their reaction data

CoCl₃.6NH₃ CoCl₃.5NH₃

Write complexes in square brackets, with charge on outside

Ex: Cu²⁺(aq) + 4NH₃ (aq) → [Cu(NH₃)₄]²⁺ (aq)

H |

Cu²⁺ (aq) + 4 :N ─ H (aq) → Cu | H

:NH 3

H³N: :NH³ :NH

3

2+

Oxidation Numbers

Knowing the charge on a complex ion and the charge on each ligand, one can determine the oxidation number for the metal.

[ Cu(NH

₃

)

₄

]

²⁺

+2 +4(0) = +2

Thermodynamic Stability

Stability of a complex in solution (in Lab)

refers to the degree of association between the two species involved in the state of equilibrium

the greater the association, the greater the stability of the compound

 The factor which actually determines whether a chemical process can take place spontaneously is the change in free energy (G) of system

 A reaction can proceed spontaneously in the direction corresponding to a decrease in the free energy of the system (negative value of G)

 The more greater negative G is the greater the tendency of the reaction to occur and the greater the capacity of the system to do useful work when undergoing the reaction

G = 0, The system is in a state of equilibrium

At the constant temperature and pressure the change in free energy, accompanying a reaction at absolut T is equal to the change in enthalpy H-T.S

The entropy S is a measure of disorder of system.

The change in entropy S is the increase (S positif) or decrease (S negatif) in the disorder of the system accompanying of reaction.

Example : an increase a number particles of system

The magnitude of the (stability or formation) equilibrium constant for the association, quantitatively expresses the stability.

 Thus, if we have a reaction of the type:

M + 4L → ML₄

Then the larger the stability constant, the higher the proportion of ML₄ that exists in the solution.

Free metal ions rarely exist in solution so that M, will usually be surrounded by solvent molecules which will compete with the ligand molecules, L, and be successively replaced by them

1. M + L → ML K1 = [ML] / [M] [L]

2. ML + L → ML2 K2 = [ML2] / [ML] [L]

3. ML2 + L → ML3 K3 = [ML3] / [ML2] [L]

4. ML3 + L → ML4 K4 = [ML4] / [ML3] [L]

where K1, K2 etc are referred to as stepwise stability constants. Alternatively, we can write the "Overall Stability Constant" thus:

M + 4L → ML4 β4 = [ML4]/ [M] [L]4

The stepwise and overall stability constants are therefore related as follows:

β4 =K1.K2.K3.K4 or more generally, βn =K1.K2.K3.K4 ...Kn

For simplicity, we generally ignore these solvent molecules and write four stability constants as follows:

Formation of complex in water does not take place by the simultaneous formation of all metal-to ligand bonds, but the ligand replace the H₂O molecules in successive step

For formation of complex [MLn] we can write a series of stepwise equilibrium reaction:

[M(H₂O)₆]^z+ (aq) + L(aq) [M(H₂O)₅L]^z+ (aq) + H₂O (l) (1)

[M(H₂O)₅L]^z+ (aq) + L (aq) [M(H₂O)₄L₂]^z+ (aq) + H₂O (l) (2) [M(H₂O)₄L₂]^z+(aq) + L (aq) [M(H₂O)₃L₃]^z+(aq) + H₂O (l) (3) [M(H₂O)₃L₃]^z+(aq) + L (aq) [M(H₂O)₂L₄]^z+(aq) + H₂O (l) (4)

[M(H₂O)₂L₄]^z+(aq) + L(aq) [M(H₂O)L₅]^z+(aq) + H₂O (l) (5) [M(H₂O)L₅]^z+(aq) + L(aq) [ML₆]^z+ (aq) + H₂O (l) (6)

1. Please write of stepwise stability of constant of K1 until K6

2. Please write of formula for β6

TASK 1

The addition of the four ammine groups to copper shows a pattern found for most

formation constants, in that the successive stability constants decrease.

The four constants are:

logK₁ = 4.0, logK₂ = 3.2, logK₃ = 2.7, logK₄ = 2.0 or logβ4 = 11.9

ΔG° = -RTLnβ

ΔG° = -2.303 RTLog10β ΔG° = ΔH° - TΔS°

A number of texts refer to the instability constant or the dissociation constant of coordination complexes. This value corresponds to the reciprocal of the formation constant, since the reactions referred to are those where fully formed complexes break down to the aqua

ion and free ligands.

Worked Example

Formation of [Ni(H₂O)_{6 – x} (NH)_x]²⁺

Results of pH study using a glass electrode (in 2M NH₄NO₃ aqueous solution) give values of the stepwise stability constant (at 303 K) of [Ni(H₂O)_6-x(NH₃)_x]²⁺

(x = 1- 6) as: log K₁ = 2.79; log K₂ = 2.26; log K₃ = 1.69;

log K₄ = 1.25; log K₅ = 0.74; log K₆ = 0.03.

Calculate (a) β ₆ for [Ni(NH₃)₆]²⁺, and (b) ΔG°₁ (303 K). (c) if the value of ΔH°₁ (303 K) = -16.8 kJ mol^-1, calculate ΔS°₁ (303 K). (R = 8.314 J K^-1 mol^-1)

(b) ΔG°₁ (303 K) refers to the stepwise formation of [Ni(H₂O)₅(NH₃)]²⁺

A negative free energy indicates spontaneity

(c) ΔS°₁ (303 K)

A negative entropy indicates the surrounding water is becoming more ordered

Metal complex formation is governed by thermodynamic processes in aqueous solution. Free energy change (G^o) is negative if the process is spontaneous and favored--- which is based on enthalpy and entropy.

G^o = H^o - TS^o

When complex formation occurs between charged cations and anions, with a resulting partial or total cancellation of charge, the changes in enthalpy for these processes are significantly negative and usually the predominant factor in complex formation.

However, the accompanying changes in entropy are significantly more positive because less order is imposed on the H₂O molecules around the uncomplexed metal cations and anionic ligands.

The corresponding values for G^o are substantially negative indicating that very stable complexes are formed.

FACTORS INFLUENCING THE STABILITY CONSTANTS OF COORDINATION

COMPOUND

 The infuence of Oxidation state of metal

The influence of the Size of metal ion

The influence of the electronic configuration of metal

Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu (II) > Zn(II) disebut deret Irving Wiliam

Task 2

1. (a) Write all the stepwise (K’s) and overall (β’s) stability constants for the following reactions

(b) Estimate which of all the stepwise constants will be the greatest

(c) Predict which of all the stepwise constants will be the smallest

2. The value for formation constants for each step in the formation of [Ni(en)3]²⁺ are log K1 = 7.52, log K2 = 6.28, and log K3 = 4.26 at 30^oC in 1.0 M KCl. (a) What is log β3 for overall formation from Ni²⁺ and 3 en?, (b) Why do value of K decrease with K₁ > K₂ > K₃ ?

Kinetic stability

 Inert: not easily replaced by other ligands

 labile: easily replaced by other ligands

Influenced by external factors, such as ligands Read text books of Fred Basolo p. 97-106, or Quagliano, J. V. and Vallarino, L.M. P. 80-90