Download Substitution reactions

Inorganic reaction mechanisms
Substitution reactions
Thermodynamic factors
Kinetic factors
Energy profile
Activated Complex
Ea, ΔH#, ΔS#
Kinetics
Y + M―X
Reactants
Thermodynamics
Products
M―Y + X
Thermodynamics
Both kinetics and thermodynamics are important in the assesment of a
reaction.
This is because some thermodynamically feasible reactions may be
kinetically hindered
Ligand substitution reactions will be used to illustrate these concepts:
Consider the reaction: Y + M ― X → M ― Y + X
This is a typical substitution reaction
Y = entering group
X = leaving group
The simplest illustration is a complex formation reaction:
e.g. [Co(H2O)6]2+ + Cl- → [CoCl(H2O)5]+ + H2O
Aquo complexes
Fundamentally metal ions dissolved in water are complexed ÎAquo
complexes Î [M(H2O)x]n+
Formation of coordination compounds from an aqueous medium Î
requires displacement of aquo ligands by other ligands.
Î Several geometries are possible: Typical examples of aquo
compounds Î e.g. Co(II)
[Co(H2O)6]2+
Octahedral
[Co(H2O)4]2+
Terahedral
In reality the value of x varies Î due to extensive hydrogen bonding
around the coordination sphere of M
Spectra and magnetic properties Î similar to hydrated salts of noncoordinating anions Î Co(ClO4)2.6H2O; Co(NO3)2.6H2O
Aquo complexes
Predominantly [Co(H2O)6]2+ Îoctahedral
Also general for the 1st transition series Î [M(H2O)x]n+; n=+2 or +3
Note: Cr(II); Mn(III) and Cu(II) show distortions in the octahedral
geometry due to Jahn-Teller effects
2nd and 3rd TS are much less certain Î octahedral probably Î higher
coordination also possible
Most M(H2O)6 exchange rapidly Î can easily be demonstrated by
isotopic labeling with 18O enriched H2O.
Aqua ions are more or less acidic Î dissociate as follows:
[M(H2O)x]n+ → [M(H2O)x-1(OH)]n-1 + H+
e.g. [Co(NH3)5(H2O)]3+ → [M(NH3)5(OH)]2+ + H+
Formation constants
Formation constant: the strength of a ligand relative to the strength of
the solvent molecules (usually H2O) as a ligand.
[Fe(OH2)6]3+(aq) + SCN-(aq)
In dilute solutions [H2O] Î constant;
[Fe(SCN)(OH2)5]2+(aq) + H2O(l)
Kf
[Fe(SCN)(OH2)52+]
[Fe(OH2)6 3+][SCN-]
Step-wise formation constant: formation constant of each solvent
replacement stage.
Kf1 ; Kf2 ........Kfn
Overall formation constant: product of the step-wise formation
constants.
βn = Kf1Kf2........Kfn
Successive formation constants
[ML]
Kf1
M + L
ML
[M][L]
ML + L
ML2
MLn-1 + L
M + nL
MLn
MLn
Kf2
Kfn
βn
ML
[ML][L]
[MLn]
[MLn-1][L]
[MLn]
[M][L]n
The inverse of each Kf is the
dissociation constant Kd Î:
M + L
[ML2 ]
Kd1
[M][L]
1
[ML]
Kf1
Trends in successive formation constants
The general trend Î Kf1 > Kf2
Formation constants of Ni(II)
…… Kfn-1 > Kfn.
2+
ammines,
[Ni(NH
)
(H
O)
]
3
n
2
6-n
This trend is a result of the
n
Kf Kn/Kn-1 Kn/Kn-1
sequential decrease in number of
Exp
Stat
H2O to be replaced.
Situations do arise where Kfn > Kfn-1
1 525
Generally there are 2 reasons to
2 148
0.28
0.42
account for anomalies in the
3 45.7
0.31
0.53
trends of successive Kf values:
1 Î due to a major change in
4 13.2
0.29
0.56
electronic structure of the
4.7
0.35
0.53
complex, e.g. moving from a high 5
spin (due to weak field H2O) to a
6
1.1
0.20
0.42
low spin complex.
2 Î due to a major structural change e.g. from an octahedral to a
tetrahedral or square planar geometry Î characteristic of some
halo complexes
Q. Consider the formation constants of the following Fe(II)
complexes: Justify the observed trend?
[Fe(bipy)(H2O)4]2+:
[Fe(bipy)2(H2O)2]2+:
[Fe(bipy)3]2+:
Kf1: 4.2
Kf2: 3.7
Kf3: 9.3
Kf3: [Fe(bipy)3]2+ >> Kf2: [Fe(bipy)2(H2O)2]2+:
The reason is due to a major electronic shift Î
from a high spin (due to weak field H2O) t2g4eg2 (LFSE Δo = 0.4)
config Î
to a low spin t2g6 config (LFSE Δo = 2.4). Large increase Δo.
Therefore [Fe(bipy)3]2+ is more stable than [Fe(bipy)2(H2O)2]2+
Q. Justify the following observation in the successive formation
constants for complexes of cadmium with Br-:
[Cd(Br)(H2O)5]+:
Kf1: 36.3
[Cd(Br)2(H2O)4]:
Kf2: 3.47
[Cd(Br)3(H2O)3]-:
Kf3: 1.15
[Cd(Br)4]2-:
Kf4: 2.34
The anomaly here is that Kf4: > Kf3:
The reason may not be electronic since both H2O and Br- are
considered weak field ligands Î
The reason is due to a major structural shift from an octahedral
[Cd(Br)3(H2O)3]- configuration Î
to a tetrahedral [Cd(Br)4]2- geometry with the simultaneous
expulsion of 3 molecules of water from a restricted geometry
Most halo complexes of M2+ have tetrahedral or square geometry
Irving-Williams series
10
log Kf
The Irving-Williams series summarises the relative 8
stabilities of complexes formed by M2+ ions.
The series reflect a combination of electrostatic 6
effects and LFSE.
For the series Ba2+ < Sr2+ < Ca2+ < Mg2+ the 4
observed trend is purely electrostatic.
For the TMs in addition to electrostatic effects , the 2
high values of Kf is due to additional stability from
0
LFSE.
Ba Sr Ca Mg Mn Fe Co Ni Cu Zn
In terms of ionic radii Î
RECALL: Δo is
Mn2+ > Fe2+ > Co2+ > Ni2+ > Cu2+ > Zn2+.
highest at d3 and d8
But,
Generally for strong field ligands the observed order is:
Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+.
The additional stability of d9 Cu(II) is due to the influence of JahnTeller distortion Î results in the strong binding of the 4 planar ligands
in tetragonally distorted Cu(II) complexes Î higher Kf values.
Other thermodynamic factors
In addition to the foregone discussion the following thermodynamic
factors are also important to the stability of TM complexes:
The chelate effect: this is largely an entropy effect Î represents the
greater stability of a complex containing a chelated polydentate ligand
compared with the equivalent complex with an analogous monodentate
ligands.
H O
H2O
N
N
N
N
Ru
N
2
OH2
Ru
N
H2O
H2O
OH2
Steric Effect: The size of ligands are important in determining the most
stable configuration Î e.g. octahedral vs. tetrahedral
Electron delocalisation: This is important in complexes containing
chelated ring structures. The empty ring π* orbitals act as electron sinks
that drain electrons from the full metal t2g non-bonding orbitals.
Labile and non-labile complexes
The rate at which one complex inter-converts into another is determined
by the height of the activation barrier between the two:
Labile complexes: A complex with a half life of the order of
milliseconds; e.g. [Ni(H2O)6]2+ is said to be labile
Non-labile or inert complex has half-life of the order of minutes; e.g
[Co(HH3)5(H2O)]3+
Factors affecting lability of complexes:
Complexes with no stabilising LFSE or chelate effects are very labile
Complexes of small metal ions are less labile due to greater M-L bonds
Complexes of M(III) ions are less labile than those of M(II) ions
Complexes of d3 and low spin d6 config are non-labile Î high LFSE
Chelate complexes of metal ions with high LFSE (e.g. [Fe(phen)3]2+) are
very stable
Complexes of d10 ions (zero LFSE: Zn2+, Cd2+, Hg2+) are generally labile
Classification of
mechanisms
Associative (A);
Dissociative (D);
Interchange (Ia or Id)
Classification of mechanisms
Dissociative:
5-coordinate intermediate
D
Interchange
Id or Ia
MLnX + Y
Associative:
7-coordinate intermediate
A
MLnY + X
MLn+ X + Y
MLnX + Y
MLnXY
MLnY+X
MLnX + Y
MLnY+X
Energy profile for Associative
Energy profile for Dissociative
mechanism: Intermediate has lower mechanism: Intermediate has higher
coordination number than reactant. coordination number than reactant.
Classification of mechanisms
No intermediate
With intermediate
Intermediate Î occurs at a local E minimum Î may be isolated
Transition state occurs at an E maximum Î cannot be isolated
Interchange I
In most TM substitution reactions Î bond formation between M + Y
happens concurrently with bond cleavage between M + X Î
Interchange mechanism:
MLnX + Y
Y……MLn…….X
MLnY + X
For an interchange mechanism Î there is no intermediate formed,
but various TS may be possible:
Hence there are 2 types of I:
‰ Dissociative Id: bond breakage dominates over formation and
‰Associative Ia: bond formation dominates over breakage
‰ It is often difficult to distinguish between:
‰ A and Ia
‰ D and Id
‰ Ia and Id
Substitution reactions
in square planar
complexes
‰ Referring to complexes of d8 ion TM
‰ Under a large crystal field:
Rh(I) Ir(I) Pt(II)
Pd(II) Au(III)
Steric Effects
Remember 4-coord. Ni(II) can be tetrahedral or sq planar (why?)
Nucleophilic subst. in sq. planar complexes Î usually A or Ia Î
the rate of the reaction is dependent on the nature of the entering
group Y
But steric crowding at the reaction centre by bulky groupsÎ
usually inhibits A or Ia reactions Î results in D or Id mechanism
k values for cis-[PtClL(PEt3)2] + H2O → cis-[Pt(H2O)L(PEt3)2]
L=
k/s-1
pyridine
8x
10-2
2-Mepy
2x
10-4
2,6-diMepy
1x
10-6
The rate ↓ with ↑ in steric bulk around N→M bond
Î The Me groups hinder the attack by H2O
Me
PEt3
M
Cl
PEt3
Stereochemistry
Substitution reactions at square complexes are stereospecific Î the
original geometry of the complex is preserved:
‰ cis reactants lead to cis products
‰ trans reactants lead to trans products
‰ most of these are 16 electron complexes Î useful in catalysis
Î oxidative addition-reductive elimination (16 e↔18 e) chemistry
Y
trans reactant
X
5 coord TS via A mech
trans product
The kinetic trans-effect and thermodynamic trans-influence
Since the substitution reactions of sq. planar TM complexes are usually A
or Ia, what factors determine the nature of the TS and
Why a 5 coordinate (trigonal bipyramidal or square pyramidal) TS?
Which ligand leaves is determined by the ligand trans to it Î
trans influence Î trans effect
σ-donor ligands Î Ligands capable of contributing more electron
density to the shared orbital between itself and the trans ligand, thereby
weakening the bond to the leaving group Î trans influence
π-acceptor ligands Î drain away electrons in A or Ia substitution
reactions Î TS characterised by high electron density at the metal
center due to a 5-coordinate intermediate Î trans effect
The kinetic trans-effect and thermodynamic trans-influence
trans influence and trans-effect are related Î weakening the
bond to the trans-ligand and increasing the rate of substitution,
Note: there is NO close correlation between the relative
magnitudes of the two phenomena
Why a 5 coordinate (trigonal bipyramidal or square pyramidal) TS?
L2
The leaving group (X), the entering
group (Y) and the trans ligand L3 all lie
in the same plane. The TS is
stabilised if L3 is a good π acceptor L3
dxy
ligand such as CO. The three are able
to communicate electronically via
L1
the π-bonding orbitals.
good π acceptor ligand e.g. CO
Remember the influence of π bonding
on the crystal field splitting Δ
X
Y
The kinetic trans-effect and thermodynamic trans-influence
The trans effect
The trans effect states that the bond holding a group trans to an
electronegative group is weakened. This trans group is the first to
be removed in a substitution reaction
Kinetic Studies to determine trans-effect of ligands
L
k (s-1)
C2H4, CO
too fast
P(OMe)3
10.3
PEt3
6.6
PPh3
3.1
Me2SO
0.0082
Et2S
0.0024
Me2S
0.0015
NH3
6.3x10-6
H2O
8.0x10-8
Trans-influence from x-ray crystallography
Trans-influence from vibrational spectroscopy
L
ν Pt-Cl
(cm-1)
L
ν Pt-Cl
(cm-1)
L
ν Pd-Cl
(cm-1)
CO
322
py
336
py
342
SMe2
310
SMe2
336
Cl
330
C2H4
309
COD
327
NH3
327
SEt2
307
SEt2
324
EtSCH2CH2SEt
323
PPh3
279
NH3
321
H2NCH2CH2NH2
307
PEt3
271
PPh3
305
PCH3
297
PEt3
294
❑ Ligands with high trans-influence have high trans-effect.
❑ Exceptions to this rule include CO, C2H4, and DMSO.
❑ Frequency order for Rh(III)-Cl: H- >PR3 >Me- >CO >I- >Br- >Cl-.
❑ For octahedral complexes, trans-influence parallels trans-effect.
Trans effect vs. trans influence
❑ The trans effect is the influence of a ligand (L) on the rate of
substitution of the ligand trans to it (X). It is a kinetic effect (ground
and transition states)
H- = CH3- = CN- = C2H4 = CO >> PR3 = SR2 > NO2 = SCN- =
I-> Br-> Cl-> py > RNH2 = NH3 > OH- > H2O
❑ The trans influence is the extent to which a ligand (L) weakens the
bond that is trans to itself. It is a thermodynamic effect (ground state).
Apply primarily to the leaving group
R3Si- = R- = H- > PEt3 > PMe2Ph > PPh3 > P(OPh)3 = CN- >
SEt2> Et2NH > py > OSMe2 = C2H4 = CO > Cl❑ The trans effect is often the manifestation of the trans
influence although there are some ligands (DMSO, CO, C2H4) which
do not show significant trans-influences yet show strong effects
Theory on trans influence
❑ Grinberg Polarization Theory (1935): M→ L and then L→ M induced
dipole results in repulsion of electrons in X Î weakening of M-X
bond.
❑ Chatt/Orgel theory of back-bonding: Stabilisation of a 5 coordinate
(trigonal bipyramidal) activated complex.
Y
X
Q. Does back-bonding generally indicate stronger trans effect?
YES……….
……………and NO
Q. Given Pt(NH3)4]2+, [PtCl4]2-, HCl and NH3 suggest synthetic
routes to
a) cis-[PtCl2(NH3)2]
b) trans-[PtCl2(NH3)2]
H3N
NH3
HCl
Pt
NH3
H3N
Cl
Cl
NH3
Cl
NH3
Cl
HCl
Pt
H3N
Cl
Pt
Cl
H3N
NH3
Pt
Cl
Cl
H3N
Cl
Pt
Cl
NH3
Cl
NH3
NH3
Pt
Cl
NH3