wk12a complex stability

8/14/2019 Wk12a Complex Stability

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omp ex a yomp ex a y

To understand the solution chemistry of metals, wemust know the nature and stability of the complexes

stability constant factors that influence complex stability

determination of stability constant


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STABILITY CONSTANTS -MEASURE THE STRENGTH OF COMPLEXATION

][LL

MLK

n

n=Stepwise constantsMLn-1 + L MLn

Cumulative constants

M + nL MLn nn

n

LM ]][[=

n = K1K2K3Kn


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For a protonated ligand we have:Stepwise complexation

+]][[* HML

K n

+

=n-1 n

Cumulative com lexation]][[ 1

HLLn

M + nHL MLn + nH+

n

n

n

HLM ]][[

* =

The larger the value of the stability constant, the more

stable the complex, and the greater the proportion ofthe complex formed relative to the simple ion.


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Stability constantsStability constantsoror po ynuc earpo ynuc ear comp excomp ex

m M + n L MmLnnm

nmnm

LM

L

][][][=

m M + n HL M L + n H+n

nm HLM ]][[*+

=nmnm

HL][][

m = , e secon su scr p on nm s om eand the expression simplifies to the previous

.


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e reng o oor na on one reng o oor na on on

Coordination bonds tend to be stronger, theCoordination bonds tend to be stronger, themore rea y e onor a om can supp ymore rea y e onor a om can supp yelectrons, and the more effective theelectrons, and the more effective the


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++

--

filledfilled unfilledunfilled

NH


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NH


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H N


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DONOR ATOM

++ --

e a y o a onor a om o urn san electrons pair to an acceptor atom

de ends on the artial char e of thedonor atom.

e more nega ve y c arge e onore more nega ve y c arge e onoratom, the more effective the coordinationatom, the more effective the coordination


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3 VS 3 3

-- --

++ ++

HH33

NN:: (CH(CH33))NN::


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3 VS 3 3

--

-- ++

FF33

NN:: (CF(CF33))NN::


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1.1. The Bond MultiplicityThe Bond Multiplicity

-2.2. The Groups attached toThe Groups attached to

the Donor Atomthe Donor Atom

3.3. Po ar za ty o t e DonorPo ar za ty o t e Donor

PARTIAL CHARGE OFPARTIAL CHARGE OFTHE DONOR ATOMTHE DONOR ATOM

..Donor Electron PairDonor Electron Pair


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1. The Bond Multi licit1. The Bond Multi licit

CH3 SiH3

3 : 3 :

N- CH N SiH

3 3

More effective Less effective


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Factors : N(CH3)3 N(SiH3)3

AA -- BBSmallerSmaller LargerLarger

on o ar yon o ar y owerower g erg er

Negative PartialNegative PartialCharge on NCharge on N

LowerLower HigherHigher

The Donor atom N in N(CHThe Donor atom N in N(CH33))33 should donateshould donateelectron less readily than that in N(SiHelectron less readily than that in N(SiH33))33


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H C N: H3Si N:

In C : All of the available

orbitals are fully occupied

In Si : All of the available

d orbitals are unoccupied

N and Si (NOT between N and C)N and Si (NOT between N and C)


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2. The groups attached to the2. The groups attached to the

onor a omonor a om

PH3 < P(CH3)3 < P(C2H5)3Due to the presence of electron donating

groups

NH3 > NF3 > NBr3Due to the presence of electron withdrawingrou s


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..

- < - < r- < -

Due to the difference in polarizability


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..Electron PairElectron Pair

CH3-O-CH3 MoreMoreeffectiveeffective

CH3-CO-CH3

CH -CO-OCHLesseffective

Due to the difference in Concentration of theElectron Pair


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ACCEPTOR ATOM

BF3


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1. Partial1. Partial Positive Char ePositive Char e

. acan. acan r a sr a s va a eva a e

3.3. lnvolvementlnvolvement of the Vacantof the VacantOrbital in Partial MultipleOrbital in Partial MultipleBondinBondin

ACCEPTOR ATOM


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Partial Charge. If other factors beingequal, one would expect an acceptor toa rac an e ec ron pa r more s rong y, ehigher its partial positive charge.

Vacant Orbitals Available. Acceptor

atoms may be able to form more stablecoor na on on s w en ey ave morethan one vacant orbital available to be usedin bonding to the same donor, as in transition

meta atoms aving severa avai a eorbitals.


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nvo vemen o e acan r a nPartial Multiple Bonding. Acceptoratoms are less e ective when the "vacant"orbital can be at least partly used in multiple

bond resonance within the molecule.


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PEARSON HARDPEARSON HARD--SOFT ACIDSOFT ACID--BASEBASE

small highly charged

large low charge

d0 electron

configuration

d10 electron

configuration e ec ron c ou s no

easily deformed

e ec ron c ou s eas y

deformed

bonds

bonds


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Pearsons Principle - In a competitive situation,hard acids tend to form complexes with hard

bases, and soft acids tend to form complexes with

soft bases.

In other words :

form complexes with ligands that tend to bond

covalentl , and similarl , metals that tend to bond

electrostatically preferentially form complexeswith ligands that tend to bond electrostatically.


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Classification of me tals and ligand s in term s o f Pearson s (196 3) H SA B principle.

H ard B orderline Soft

A c id s A c id s A c id s

H +

Li+> N a+> K+> R b+> C s+

B e2+> M g 2+> C a2+> S r2+> B a2+

Al3+> G a3+

3+ 3+ 3+ 3+ 3+

Fe2+, M n2+, C o 2+, N i2+,

C u 2+, Zn2+, Pb2+, Sn2+,

As3+, Sb3+, B i3+

A u + > A g+ > C u +

H g 2+> C d2+

Pt2+> Pd2+

other PG E 2+

3+ +

C e4+; Sn4+

T i4+> T i3+, Zr4+ H f4+

C r6+> C r3+; M o 6+> M o 5+>4+ 6+ > 4+ 5+ 5+

,

R e7+> R e6+> R e4+; V 6+> V 5+>

V 4+; M n4+; Fe3+; C o 3+; As5+; Sb5+

T h4+; U 6+> U 4+

PG E 6+> PGE 4+, etc. (Ru, Ir, Os)

B a s e s

F -; H 2O, OH-, O 2-; N H 3; N O 3

-;

C O 32-> H C O 3

-; SO 42-> H S O 4

-;3- 2- -

B a s e s

C l-

B a s e s

I -> B r-; C N -; CO ;

S 2-> H S -> H 2S;

4 4 2 4carboxylates (i.e., acetate,oxa late, etc.);

M o O 42-; W O 4

2-

3o rganic thio ls (RP );

po lysulfide (S nS2-),

thiosulfate (S 2O 32-),

su lfite S O2-

H Se-, Se

2-, HTe

-, T e

2-;

A sS 2-; SbS 2

-


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u en a e gan s are muc s ronger

complex formers than monodentate

gan s.

Chelates remain stable even at very diluteconcentrations, whereas monodentate

complexes tend to dissociate.


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Gro = Hr

o - TSr0

For many ligands, Hr

o is about the same in multi-

and mono-dentate complexes, but there is a larger

en ropy ncrease upon c e a on

u 2 4 + 3 u 3 4 + 2

Cu(H2O)42+ + N4 Cu(N4)

2+ + 4H2O

The second reaction results in a greater increase

n r .


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nature of metal and ligand

size and charge.

a metal ion, the more stable are the metal

com lexes. Thus stabilit is favored b a lar echarge-to-radius ratio of the metal ion

Crystal field effect

Class A and class B metals


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Li and Base strengthThe greater the base strength of a ligand toward H+, the

complexes with class a metals Chelate effect

the stability of a metal chelate is greater than that of ananalogous nonchelated metal complexes. The more

extensive the chelation, the more stable the s stem. Chelate ring size. The most stable metal chelates

contain saturated ligands that form five-membered-

membered rings Steric strain. Bulky ligands form less stable complexes


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Determination of stabilit constant:

- spectroscopy method- electroanalysis

wk12a complex stability

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