Octahedral Substitution reaction way.ppt

octahedral

1. 1/10/2024 1
Stability of Coordination Compounds
• The kinetic stability depends on the activation energy (DG
‡
)
of the ligand substitution reaction, the thermodynamic stability
is given by the free energy change:

2. 1/10/2024 2
Mechanisms for Substitution Reactions
If rate determining step is:
a)breaking bond of leaving group –>
dissociative mechanism (D)
(this mechanism corresponds to the S
N1 reaction in
organic chemistry)
b) making bond of entering group –>
associative mechanism (A)
(this mechanism corresponds to the S
N2 reaction in
organic chemistry)
c) Interchange (I)
• Both, dissociative and associative reaction
mechanisms involve two-step pathways and an
intermediate:

3. 1/10/2024 3

4. 1/10/2024 4
There are several ways in which a substitution reaction can
take place:
1.Dissociative substitutioninvolves the reversible
dissociation of one ligand to form an intermediate complex
with one fewer ligand followed by the addition another ligand
to the metal.
(a) The rate is first order in ML
x
and zero order in L'.
(b) The rate doesn't depend on the nucleophilicity of L'.
(c) This is common for octahedral complexes and other
complexes with an 18 electron count.

5. 1/10/2024 5
2-Associative substitutionoccurs when the incoming
ligand first adds to the complex to form an intermediate
complex with one lignad more than the starting complex
followed by a loss of one ligand.
(a) The rate is first order in ML
xand first order in L'.
(b) The rate is directly proportional to the nucleophilicity of
L'.
(c) This is common for square planar complexes and other
complexes with less than an 18 electron count.

6. 1/10/2024 6
3-Interchange substitutionis intermediate between the two.
There is no intermediate complex formed. Instead bond
making to the new ligand occurs along with bond breaking to
the original ligand. This is very similar to S
N2 substitution in
organic chemistry.
(a) The rate is first order in ML
xand zero order in L'.
(b) The rate doesn't depend on the nucleophilicity of L'.
(c) The rate is inversely proportional to the strength of the
M-L (leaving group) bond.

7. 1/10/2024 7
Dissociative: activated state has lower coordination number due to
dissociation of the leaving group
Associative: activated state has a higher coordination number due to
bonding of the incoming group
Rate determining step:
dissociation of X (outgoing group) is slow
Rate determining step:
association of Y (incoming group) is slow

8. 1/10/2024 8
II.1TheDissociativeMechanism
•Inthismechanismtheintermediatehasalower
coordinationnumberthanthereactantandthis
intermediatestabilizesforlongenoughtoequilibratewith
itsenvironmentbeforecapturingtheenteringgroup.
•Theleavinggrouphasleftthereactioncentrebeforeany
interactionbetweenthemetalcentreandtheentering
grouphastakenplace.
•Therateofthereactionshouldthereforebeinsensitiveto
thenatureandconcentrationoftheenteringgroupbut
dependentonthenatureoftheleavinggroup.
•ReferringtoFigure4,thereactionprofileconsistsofa
singleintermediateandtwotransitionstates.

9. 1/10/2024 9
Figure 4 Activation profile for a dissociative mechanism.

10. 1/10/2024 10
II.2TheAssociativeMechanism
•IntheAmechanismtheintermediatehasalargercoordination
numberthanthereactant.
•ItcanbeseeninFigure5thatinthefirsttransitionstate
(bondmaking)thebondwiththeenteringgroupYislargely
establishedbeforethebondtowardstheleavingligandXis
weakened.
•Thereactionratethereforedependsstronglyonthenatureof
theenteringligandYsinceitparticipatesinthetransitionstate.
•Thisreactionprofileconsistsofasingleintermediate
(containingtheenteringandleavinggroupbondedtothe
reactioncentre)andtwotransitionstatesleadingtothe
formationanddecompositionoftheintermediate.

11. 1/10/2024 11
(a) the bond-breaking transition state at higher energy (b) the bond-
making transition state at higher energy. the deeper the energy well the
more stable the intermediate will be.

12. 1/10/2024 12
II.3TheInterchangeMechanism
•Inthismechanismnodetectableintermediatecanbe
foundandthereisonlyasingletransitionstate.
•Heretheactsofbondmakingandbreakingareeither
synchronousorelsetakeplacewithinapre-formed
aggregate.
•TheincominggroupYandtheleavinggroupXare
interchangedbetweentheinnerandoutercoordination
spheresofthemetal.
•Astheenteringgroupapproachesthereactioncentrethe
leavinggroupdeparts.
•I
DandI
Acanbeconsideredasextremebordercasesof
theImechanism.

13. 1/10/2024 13
III.FactorsAffectingRatesofReactions
1-EffectofTemperature
Theeffectoftemperatureontherateconstantofareaction(k)is
governedby
WhereKisBoltzman`sconstant
hisPlanck`sconstant
DH
≠
andDS
≠
arethestandardenthalpyandentropyofactivation,
respectively,andrepresentthechangeinenthalpyandentropy
respectively,intheformationoftheactivatedcomplex(transition
state)fromthereactant.
DH
≠
isrelatedtotheenergyrequiredinbringingreactantsupto
eachotherandenergyrequiredforreorganizationofbonds.
DS
≠
measuresthedegreeofdisorderontheformationofactivated
complex.
IfDS
≠
is+ve,reactiongoesbydissociativemechanism.
IfDS
≠
is-ve,reactiongoesbyassociativemechanism.kKThe e
HRTSR


 
(/)
/ /D D

14. 1/10/2024 14
2-EffectofExternalPressure
Theeffectofexternalpressureontherateofareactionis
expressedbytherelation
WhereDV
≠
(volumeofactivation)isthemolarvolumechange
thattakesplaceintheconversionofthereactantstothe
activatedcomplex;k
2andk
1aretherateconstantsatthe
pressuresP
2andP
1respectively,ataconstanttemperature,T.
SinceDV
≠
canhaveeitherpositiveornegativevalues,
reactionsmaybeacceleratedorretardedbyincreaseof
pressure.
DV
≠
isdiagnosticofthemechanism;asDV
≠
isrelatedtoDS
≠
.
Foraquaticreactionsofoctahedralcomplexes,thisfollowing
relationshipholdsD DV S
 
 104 44. . RT
PPV
k
k
T
303.2
)(
)log(
12
1
2 D



15. 1/10/2024 15
3-Effectofionicstrength
AccordingtoDebye-Huckeltheory,therateofa
reactionbetweenionsofchargeZ
AandZ
Bshouldvary
withtheionicstrength(I)ofthesolutioninthe
followingmanner
Wherek
1andk
oaretherateconstantsationicstrength
Iand0respectivelyandtheconstantQisproportional
tothesolventdielectricconstant.loglog
/
k kQZZI
o AB1
12
2 

16. 1/10/2024 16
4-InfluenceofSolvent
IftheinteractingionshaveZ
AandZ
Bandtheyareata
distanced
ABapartinthetransitionstate,thenitcanbe
shownthat
Whereeistheelectroniccharge,KtheBoltzaman`s
constant,Ttheabsolutetemperature,k
therate
constantinamediumofdielectricconstantandk
is
thevalueofrateconstantinamediumofinfinite
dielectricconstant.loglog
.
k k
ZZe
dKT
AB
AB


 

2
2303

17. 1/10/2024 17
Kinetic concepts:
Inert complex: An inertcomplex reacts slowly, even though the
reaction may lead to a more stable, thermodynamically favoured
product.
→ high energy of activation E
A
Labile complex: Fast reactions (reaction half-life < 1 min.)
→ low energy of activation E
A
• Generally complexes with high LFSE are inert.
Inert and Labile Complexes
Thermodynamic concepts:
Stable complex: Large complex formation
constant
Unstable complex: Small complex
formation constant
e.g. [Cu(OH
2)
6]
+
is stable in water but
unstable against the formation of
[Cu(NH
3)
4(H
2O)
2]
+
in the presence of NH
3
in aqueous solution.

18. 1/10/2024 18
Itwasassumedthatastablecomplexisinertandthatanunstable
complexislabile.Thisisnotalwaystrue,ascyanideionforms
verystablecomplexeswithmetalionssuchasNi
2+
.Thestability
indicatesthattheequilibrium(1)liesfartotherightandthatNi
2+
prefersCN
-
toH
2Oasaligand[Ni(H
2
O)
6
]
2+
+ 4CN
-
[Ni(CN)
4
]
2-
+ 6H
2
O

(1)
However, when
14
C-labeled cyanide ion is added to the solution, it
is almost instantaneously incorporated into the complex (2). Thus
the stability of this complex does not ensure internees[Ni(CN)
4
]
2-
+ 4
14
CN
-
[Ni(
14
CN
4
)]
2-
+ 4CN
-
(2)
It is important to know that the terms stability and lability relate to
different phenomena. The stability of a complex depends on the
difference in energy between reactants and products

19. 1/10/2024 19
A stable compound will be considerably lower in energy than
possible products. The lability of a compound depends on the
difference in energy between the compound and the activated
complex; i.e., if this activation energy is large, the reaction will be
slow.ReactantsEnergy
Products
Activated complex
Activation energy
Reaction energy
Scheme (2) The relative energies of reactants, activated complex and
products of a reaction

20. 1/10/2024 20
LabileComplexes
1-Allcomplexesinwhichthecentralmetalatomcontainsd
electronsine
gorbitals(thedx
2
-y
2
anddz
2
orbitalsthatpoint
towardthesixligands).
Forexample:
[Ga(C
2O
4)
3]
3-
,d
10
(t
2g
6
e
g
4
);[Co(NH
3)
6]
2+
,d
7
(t
2g
5
e
g
2
);
[Cu(H
2O)
6]
2+
,d
9
(t
2g
6
e
g
3
);[Ni(H
2O)
6]
2+
,d
8
(t
2g
6
e
g
2
);
[Fe(H
2O)
6]
2+
,d
5
(t
2g
3
e
g
2
).
2-Allcomplexesthatcontainlessthanthreedelectrons,for
example,
[Ti(H
2O)
6]
3+
,d
1
;[V(Phen)
3]
3+
,d
2
;[Ca(EDTA]
2-
,d
0

21. 1/10/2024 21
• For the main group elements, there appears to be a correlation
between the ion size/charge-density and exchange rate:
• For transition metal elements the ion-size/charge density is less
important and the d-electron configuration largely determines the
rate.
• However, trivalent ions still do react more slowly than the
divalent ones.

22. 1/10/2024 22
•Theratebehaviourofcomplexesisaffectedbychargeandsizeof
theircentralatoms.Smallhighlychargedionsformcomplexesthat
reactslowly.Thusthereisadecreaseinlabilitywithincreasing
chargeofthecentralatomfortheisoelectronicseries
[AlF
6]
3-
> [SiF
6]
2-
> [PF
6]
-
> SF
6
•Similarly,therateofwaterexchange(1)decreaseswithincreasing
cationicchargeintheorder:
[Na(H
2O)
n]
+
> [Mg(H
2O)
n]
2+
> [Al(H
2O)
n]
3+
•Complexeshavingcentralatomswithsmallionicradiireactmore
slowlythanthosehavinglargercentralions,forexample
[Mg(H
2O)
6]
2+
< [Ca(H
2O)
6]
2+
< [Sr(H
2O)
6]
2+
•Foraseriesofoctahedralmetalcomplexescontainingthesame
ligands,thecomplexeshavingcentralmetalionswiththelargest
charge-to-radiusratioswillreacttheslowest.[M(H
2
O)
6
]
n+
+ 6H
2
O
*
[M(H
2
O
*
)
6
]
n+
+ 6H
2
O

(1)

23. 1/10/2024 23
•Ingeneral,four-coordinatedcomplexes(bothtetrahedraland
squareplanarmolecules)reactmorerapidlythananalogoussix-
coordinatedsystems.Thegreaterrapidityofreactionsoffour-
coordinatedcomplexesmaybeduetothefactthatthereisenough
roomaroundthecentralionforafifthgrouptoenterthe
coordinationsphere.Thepresenceofanadditionalgroupwouldaid
inthereleaseofoneoftheoriginalligands.
•Forsquareplanarcomplexesitisnotpossibletoapply
successfullythecharge-to-radiusratiogeneralizationthatworks
wellforsix-coordinatedcomplexes.
•Therefore,therulesthatpredictratebehaviourforsix-coordinated
systemswilloftennotapplytocomplexeshavingsmaller
coordinationnumbers.Sinceratebehaviourisdependenton
mechanismandsincereactionsofmetalcomplexesareknownto
proceedbyavarietyofpaths,itisimpossibletomake
generalizationsthatapplytoallcomplexes.

24. 1/10/2024 24
IV.MechanismsofOctahedralSubstitution
Reactions
A)DissociativeMechanism
Step1.DissociationofXtoyielda5coordinateintermediate
ML
5X+Y→[ML
5Y]+X (1)
M-XbondisbrokenSlowandratedeterminingTherateofDis
onlydependsontheconc.ofML
5X
k
1
ML
5X → ML
5+X (2)
Trigonal Bipyramidal D
3h Square Pyramidal C
4h

25. 1/10/2024 25
B)AssociativeMechanism
Step1.CollisionofML
5XwithYtoyielda7-coordinate
intermediate.(slow)
k
1
ML
5X+Y→[ML
5XY](slow,ratedetermining)(3)
Capped Octahedron Pentagonal BipyramidL
L
L
L
X
M
Y
L M
L
L L
Y
X
L
L

26. 1/10/2024 26
IV.2ReplacementofCoordinatedWaterbyotherLigand
TherateofreplacementofcoordinatedwatermoleculebySO
4
2-
,
S
2O
3
2-
,EDTAandotherspecieshasbeenmeasuredforavarietyof
metalions(1)
Theratesofthesereactionsareindependentontheconcentrationof
theenteringligand,thatis,afirst-orderratelaw.Eq.2applies.In
manycases
therateofreaction(1)foragivenmetalionisindependentof
whetherH
2O,SO
4
2-
,S
2O
3
2-
orEDTAistheenteringligand(L).
Thisobservationandthefactthattheratelawdoesnotincludethe
enteringligandsuggestthatthesereactionsoccurbyamechanism
inwhichtheslowstepisthebreakingofabondbetweenthemetal
ionandwater.Theresultingspecieswouldthenbeexpectedto
coordinaterapidlywithanynearbyspecies.[()] [()]MHO MHOL O (1)
X X
n
2
2
2 1
2


 + L + H
2-
2 RatekMHO
X
n


[()
2] (2)

27. 1/10/2024 27
Itwasfoundthatthemorehighlychargedhydratedmetalionssuch
asAl
3+
andSc
3+
undergoH
2OexchangemoreslowlythanM
2+
ions.Thissuggeststhatbondbreakingisimportantintherate-
determiningstepofthesereactions.Thisevidencesuggeststhat
S
N1processesareimportantinsubstitutionreactionsofhydrated
metalions.
IV.3ReplacementofLigand(orAnion)byWaterMolecule
Thesesprocessesarecalledaquaticreactions.Ingeneral,ammonia
oraminescoordinatedtocobalt(III)areobservedtobereplacedso
slowlybywaterthatonlythereplacementofligandsotherthan
aminesisusuallyconsidered.Theratesofreactionsofthetype(1)
havebeenstudiedandfoundtobefirstorderin
thecobaltcomplex(Xcanbeanyofavarietyofamines).Sincein
aqueoussolutiontheconcentrationofH
2Oisalwaysabout55.6M,
theeffectofchangesinwaterconcentrationofthereactionratecan
notbedetermined.(1) X + ])[Co(NH H + ])([
-3
2532
2
53

 OHOXNHCo

28. 1/10/2024 28
Therefore,theratelawdoesnottelluswhetherH
2Oisinvolvedin
theratedeterminingstepofthereaction.Thedecisionastowhether
thesereactionsproceedbyanS
N2displacementofXbyH
2Oorby
S
N1dissociationfollowedbyadditionofwatermustbemadefrom
thefollowingevidences:
1-Therateofhydrolysis(replacementofonechloridebywater)of
trans-[Co(NH
3)
4Cl
2]
+
isapproximately10
3
timesfasterthanthatof
[Co(NH
3)
5Cl]
2+
.Increasedchargeonacomplexisexplainedto
strengthenmetal-ligandbondsandhenceretardmetal-ligandbond
cleavage.Itisalsoexpectedtoattractincomingligandsandaid
displacementreactions.Sinceadecreaseinrateisobservedasthe
chargeonthecomplexincreases,adissociative(S
N1)process
seemstooperative.
2-Anotherevidenceresultedfromthestudyofthehydrolysisofa
seriesofcomplexesrelatedtotrans-[Co(en)
2Cl
2].Inthese
complexestheethylenediamineisreplacedbysimilardiaminesin
whichHatomsonCreplacedbyCH
3groups.

29. 1/10/2024 29
Thecomplexescontainingthesubstituteddiaminesreactmorerapidly
thantheethylenediaminecomplex.ThereplacementofHbyCH
3
increasesthebulkoftheligands,consequentlythiswillmakeitmore
difficultforanattackingligandtoapproachthemetalatom.Thissteric
crowdingshouldretardanreaction.Bycrowdingthevicinityofthe
metalatomwithbulkyligands,oneenhancesadissociativeprocess,
sincetheremovalofoneligandreducesthecrowdnessaroundthemetal.
Theincreaseinrateobservedwhenthemorebulkyligandswereusedis
goodevidencefortheS
N1mechanism.
IV.4Replacementofanacidogroup(X
-
)inacobalt(III)
complexwithagroupotherthan(H
2O)
This reaction is illustrated by (1)
It has been observed that this reaction takes place by initial substitution
by solvent H
2O with subsequent replacement of water by the new group
Y (2)(1) X + ])[Co(NH Y + ])([
-2
53
-2
53

 YXNHCo [Co(NH
3
)
5
X]
2+
[Co(NH
3
)
5
(H
2
O)]
3+
H
2
O
slow
Y
fast
[Co(NH
3
)
5
Y)]
2+
(2)

30. 1/10/2024 30
Therefore, in a number of cobalt(III) reactions the rates of reaction (1)
are the same as the rate of hydrolysis.
Hydroxide ion is uniquely different from other reagents with respect to
its reactivity toward Co(III) amine complexes. It reacts very rapidly (as
much as 10
6
times faster than H
2O) with cobalt(III) amine complexes in
a base hydrolysis reaction (3).
In this reaction, the second order kinetics and the unusually rapid
reaction (3) suggest that OH
-
is
an exceptionally good nucleophilic reagent toward Co(III) and that the
reaction proceeds through an S
N2-type intermediate. However, an
alternative mechanism (5), (6), (7).[()] )]CoNHCl OH
35
2
5
2 
 + OH [Co(NH + Cl (3)
-
3
- (4) ][OH])([
-2
53

 ClNHCokRate [Co(NH
3
)
5
Cl]
2+
+ OH
-
[Co(NH
3
)
4
NH
2
Cl]
+
+ H
2
O (5)
fast
[Co(NH
3
)
4
NH
2
Cl] [Co(NH
3
)
4
NH
2
]
2+
+ Cl
-
(6)
slow
[Co(NH
3
)
4
NH
2
] + H
2
O [Co(NH
3
)
5
OH]
2+
(7)
fast

31. 1/10/2024 31
willalsoexplainthisbehaviour.Thereactionthenproceedsbyan
process(6)togiveafive-coordinatedintermediatewhichthen
reactswiththesolventmoleculetogivetheobservedproduct(7).
ThismechanismissupportedbyconsideringthatifnoN-H
hydrogenispresentinaCo(III)complex,thecomplexreacts
slowlywithOH
-
.Thiscertainlysuggeststhattheacid-base
propertiesofthecomplexaremoreimportanttotherateofreaction
thanthenucleophilicpropertiesofOH
-
.
Substitutionreactionsofawidevarietyofoctahedralcompounds
havenowbeenstudied;whereadissociativetypemechanism(S
N1)
hasmostfrequentlybeenpostulated.Thisresultshouldnotbe
surprising.Sincesixligandsaroundacentralatomleavelittleroom
foraddinganothergroup.Inaveryfewexamples,evidencefor
seven-coordinatedintermediatehasbeenpresented.Therefore,the
S
N2mechanismcannotbediscardedforoctahedralsubstitution