20. Formation Constant
In general, chemical equilibrium is reached when the
forward reaction rate is equal to the reverse reaction rate
and can be described using an equilibrium constant, K.
Complex ion equilibria have their own unique equilibrium
constant.
formation constant, Kf, describes the formation of a complex
ion from its central ion and attached ligands.
You may also see this constant called a stability constant or
association constant;
• the units depend on the specific reaction it is describing.
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
20
21. the larger the Kf value of a complex ion,
the more stable it is
log Kf
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
21
22. Stepwise Equilibria
the formation of tetraamminecuprate(II) ion in solution :
You may notice that
each stepwise formation constant
is smaller than the one before it.
K1= 1.9 x 104
K2 = 3.9 x 103
K3 = 1.0 x 103
K4 = 1.5 x 102
Kf = β4 =1.1 x 1013
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
22
23. This decreasing trend is due to
the effects of entropy,
causing each step to be
progressively less likely to occur.
You can think of this in the
following way, continuing with
the previous example:
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
23
24. As always though, there are exceptions to this rule. If the
values do not continually decrease then the structure of the
complex ion likely changed during one of the steps.
When the first ammine ligand goes
to displace an aqua ligand it has
four sites from which to choose
from, making it "easier" to displace
one.
Yet
with every step the number of sites
decreases making it increasingly
more difficult.
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
24
25. Chelation Effect
Generally,
complex ions with polydentate ligands have
much higher formation constants
than those with monodentate ligands.
Complex Ion Kf
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
25
26. ∆G = –RT lnβn = ∆H – T∆S
Entropy plays an important role.
• The more positive ∆ S is the more negative ∆ G will be and the greater
βn will be.
• Entropy is largely responsible for the greater stability constants
observed for chelates, compared to complexes of unidentate ligands
with the same metal ion.
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
26
33. Here B is at lower energy than A so that ΔG is negative.
The reaction should therefore proceed spontaneously and
B is the more thermodynamically stable species.
The reaction as shown though has a barrier to the progress of the
reaction called the Activation Barrier (Ea) and so
the reaction may proceed very slowly.
The thermodynamics describes only the starting
and ending position of the reaction and not the
intermediate or transition state.
If the kinetics is slow, A is described as being inert while
if it proceeds quickly then A is described as being labile.
http://wwwchem.uwimona.edu.jm/courses/IC10Kstability.html
تفاعالت ميكانيكية العيان أسامة الدكتور محاضرات
غيرعضوية
33
38. 3المتراكب نوع لتحديد طرقاذاكانInert or Labile
1- Valence Bond Argument (Taube, 1953).
2- Crystal Field Activation Energy Approach (Basolo and Pearson, 1958).
3- Molecular Orbital Approaches
نفسرفىهذةالمحاضرةوبالتفصيلالطريقة(2)والتىتعتمدعلىحسابات
CFSEللمتراكبفىGround stateثمفىTransition stateوبعدها
نحسبCFAE(crystal filed activation Energy)كما
بالمعادلة:
LFAE = LFSE(sq pyr) - LFSE(oct)
تفاعالت ميكانيكية العيان أسامة الدكتور محاضراتغيرعضوية 38
39. General Approaches to Rationalize Ligand Substitution Reactions at Oh complexes
• 1. Valence Bond Argument (Taube, 1953).
– d0, d1, d2 complexes are co-ordinatively unsaturated therefore could
form complexes of co-ordination numbers (C.N) > 6 i.e. they are labile
by means of an associative mechanism. d4, d5, d6 (high spin), d7-10
labile by means of a dissociative mechanism. d3-6 (low spin) are inert.
• 2. Crystal Field Activation Energy Approach (Basolo and
Pearson, 1958).
– What determines the substitution mechanism adopted by a Oh
complex is the difference between the CFSE of the ground and
transition states, the so-called CFAE (crystal field activation
energy).
• 3. Molecular Orbital Approaches
– Evaluate the extent to which ligand-based orbitals are stabilized and
metal-based orbital are destabilised as a result of overlap for starting
and transition state complexes. محاضراتتفاعالت ميكانيكية العيان أسامة الدكتور
غيرعضوية 39
40. L
M
L L
L
L
X
L
M
L L
L
L
X
L
M
L L
L
L
G
Ea
Labile or inert?
LFAE = LFSE(sq pyr) - LFSE(oct)
تفاعالت ميكانيكية العيان أسامة الدكتور محاضراتغيرعضوية 40