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CHAPTSR - I
INTRODUCTION
1
Sulphur and selenium being members of the
same group of periodic table resemble In most of the
properties and form structurally similar compounds.
Both elements are donors in a variety of ligands which
form complexes with metal ions. The complexlng ability
of donor and acceptor el an eats depends upon the nature
of the respective elements.
Ligands in which sulphur or selenium act as
donors have been classified as class ' b* donors by
Arhland, Chatt and Davies (1) and soft bases by
Pearson (2 ). 3uch ligands In which sulphur or selenium
atom is a doaor form more stable complexes with metal
ions which have low or aero positive charge, large size
and several easily excitable outer electrons and such
metal ions have been classified as d a 3S * b* acceptors( 1)
or soft acids (2 ).
Metal ions with d*° configuration, viz. Copper(l),
3ilver( I ) , Zinc( I I ) , Gactaium(Il) and mercury( II) and d^
metal ions namely nickel(Il), Palladium( II) and
i^latinumC II) show very high formation constants
particularly with sulphur ligands.Being class ' b‘ metal
ions they tend to fom strong sigaa bonds with these
ligands due to polarization of sulphur atom ( 3 ) or due
to greater covalent interaction (4 ) . A d* - d* bond
by back donation of electrons to vacant 3d orbitals of
sulphur atom is one of the reasons for the greater
stability of such complexes.
The sulphur and selenium ligands can be
broadly classified into two groups; inorganic and
organic compounds.
Amongst the important inorganic ligands are:-
(i) Julphidee and selenides.
(ii ) Trlthiocarbonates and t ri s eleno carbon at e 3 .
(i i i ) Thiocynates and selenocynates*
(iv) Thiosulphates and selenosulphates.
(v) Sulphite? and selenites.
The organic sulphur and selenium compounds
are of more varied type - the simple ones are
/ \ 0(i) Thils and selenols - one co-ordinating site.
(ii ) Thioethers and alkyl selenides- one coordinating site.
(i i i ) Mercaptoacids and selenol - acids, - two
bonding site^*
3
( iv) Thioketones and selenoketones, - thioacety-
acetone and selenoacetylacetone each have two
bonding sites.
The other class of compounds is represented by
thioureas and selenoureas, thiocarbamates and seleno-
carbamates; these ligands have each two bonding sites
nitrogen, sulphur or selenium act as donating atoms in
these ligands.
The formation of sulphur or selenium ligand
complexes of typical class * b* metal ions like
mercury(ll) and rsincCll) with hi^ti stabilities is quite
obvious.
The organic multidentate ligands like 2-mercapto-
propanoic acid and >* me reap to propanoic acid which are
isomers contain oxygen as well aB sulphur based
ft actional groups act as excellet chelating agents for
all * a' , * b' and borderline metal ions.
Similarly thioureas and selenoureas having
sulphur, nitrogen and selenium, nitrogen respectively
as donors form a variety of complexes with class • b’
and borderline metal ions.
4
The Important aspects of the studies on the
complexation are * (i ) physioo - chemical investigation
leading to understanding of nature of complex species
and their equilibria, (i i ) synthesis and determination
of structure of metal complexes (il i ) the analytical
applications. A review of literature reveals an intense
interest in all the three directions.
1. EQUILIBRIUM STUDIES ON MERGAPTOCAi-BOXYLATE
M3T,,L COMPLEXES •
Fernando aad Preiser (5) studied the chelating
properties of 3~mercaptopropanoic acid and determined
stepwise formation constants of Hickel(ll) and &Lnc(ll)
with the ligand and observed that Nickel(Il) complex is
less stable than zinc(ll) complex. With mercaptoacetic
acid as chelating agent they have determined the
relative stability of five and six membered ring chelates
using the relation ^ l o g K1 / / \ ^ aVt ^\log ILj is
decrease in value of first formation constant of the
chelates in going from five to six membered rings and
A p Kav is increase iQ arithmetic mean of negative
logarithm of acid dissociation constants of ligand
molecules, they found five membered rings are more
stable than six membered rings in each complex.
Leussing (6) reported the stability constants
of mangaese(ll), iron(ll), nickel(ll), and zinc(ll)
with thioglycolllc acid; only the formation of 1:1
and 1:2 (metal:ligand) complexes were reported and
zinc complex was found to be more stable than nickel
complex. The formation of polynuclear complexes between
nickel(ll) and thiogLycollic acid ions have been
described by Leussing and co-workers(7), they have
reported the formation of 2:2, 3:4, 4*6 (metal: ligand)
complexes with the metal ion.
Bear et al (8) have investigated potentiometrio-
all y the formation constants of lanthanide metal
complexes with 2-, and 3- mereaptopropanoic acids, they
have reported that both the acid anions function as
monodentate ligands. Sigam et al (9) have potentiometrically
studied metal complex of uranyl(VI) with 3-mercapto-
propanoic acid, i'he workers (10) have reported stability
constants of 1:1 complexes of copper(Il), cobalt(ll),
raercury(ll) and 1:2 con pi exes of nickel(ll), zinc(ll),
<LTi
palladium(ll), iron(lll), uranyl(Vl), zirconium(iv)
and thorium(iv) with thiol acetic acid using
potentiometric technique. In case of palladium, nickel,
zinc, the values of stability constants follow the
order Pd Hi Za. In case of eopper(ll) the value
is low* Saxena and Gupta (11) reported the composition
and stability constants of 3-mercaptopropaaoic acid
with 2inc(ll); 1:1 and 1*2 (metal:ligand ) complexes,
overall stability constant of 12.65 at 30°C has
been observed. Ihe workers (12) have investigated
potentiometrically the stability and composition of
thallium(l) with 3-merceotopropanoic add acid, 1*1
(metal;ligand) complex with logk value of 2.85 at 50°
is reported. Similar studies wer conducted with
nickel(ll) and 1*1 and 1:2 (a«tal*ligand) complexes
with the sme ligand with overall stability constant
value of 10.74 at 50°o were reported (13). Polarographic
studies of the same ligand with cadmiwa(ll) revealed
fo eh at ion of four complexes of 1*1, 1*2, 1*3 and 1*4
(metal*ligand) stoichiometry with overall stability
value of 17.19 at 30°C (14).
Bhattacharya sod Reddy (15) have investigated
the nickel (ll) and thallium(l) complex formation with
7
lactic acid and thiolactic acid by pH metric technique
and have reported that nickel(ll)-thiolactate complexes
are more stable than the lactate complexes and reverse
was the case with thallium(l). They have supported the
observations on the assumption that 3d Pi orbitals in
sulphur atom render metal to ligand Pi interactions
possible in case of nickel-thiolactate complex, and
such interactions were not existing in other complexes,
Bhattacharya and co-workers( 1 6) have also reported the
stepwise formation constants of complexes of
thioglycollic acid and thiolactic acid with 3e (Il),
Zn(ll) and Cd(ll), they have observed the stability
order : Gd ^>Zn }>Be and thlolacctic acid forms stronger
complexes than thioglycollic acid with these metal ions.
The stability of chelates is diseussed in terms of
metal - ligand Pi interactions, Nitriloacetic acid as
primary lirand and thioglycollic acid or thiol ®Aic acid
and thlomalie acid as secondary ligands with zlno(ll)
have also been studied by Bhattacharya et al (17).
They (18) also studied the mixed ligand systems of
copper(ll) and nlckel(ll) with nitrlloacetic acid as
primary ligand and a merceptoI'cid or an amino acid as
secondary ligand. Similarly phen ant hro line as primary
ligand and mereeptoacids as secondary ligands have
also been studied by Bhattacharya and co-workers (19).
The objective of these studies was to find out the effect
of Pi bonding, they concluded that the metal sulphur
covalent bond is strengthened and effect of Pi interaction
is not significant.
Tanaka and co-workers (20) have studied
potentiometrlcally the complex formation between phenyl
and methyl mercury and pencillamine and amino acids like
cysteine and reported a 1s2 (metal: ligand) complex
with the ligands. They have found that stability order
of the bonds decreased in the order:
MeHg -S >PhHg > PhHg -3 > MeHg - N
^>PhHg - 0.
They have also evaluated stability constants of
complexes of thiolactic acid and pencillamine with
Gr(lll) which foms 1:1 , 1*2 and 1:3 monomeric complexes
with thiolactic acid and 1:3 complexes are of regular
octahedral geometry. The complexes of pencillamine are
more stable than complexes of thiolactic a d d (21).
Van Pouke et a l .(22) have conducted
potentiometric and colorimetric studies of Mi(ll)
and 2n(Il) with thiolactic add and reported the
formation of mononuclear and polynuclear species of
the composition 1 :2 , 2:2, 3*4 (metal;ligand) with
stability constants 14.33* 17.29 and 34.92 respectively
The non-existence of 4 :6 complex is explained due to
presence of ateric hinderance by o£-methyl group
in thiolactic acid as the similar complexes are
existing in case of tfi(ll) and zn(ll) thioglycollic
acid. A similar study has been extended to the systems
of N i(Il) and sal(11) and 3-merceptopropanoic acid (23)
with 2n(ll) a number of species with 1:2, 3:4 , 4 :6
(metal:ligand ) stoichiometries have been observed and
in case of Hl(Il) complexes of 5:10, 6:9 and 6:12
(metal:ligand) stoichiometric ratios have been reported
2he absence of steric hinderance in 3-mercaptopropanoic
acid being responsible for the foumation of complexes
with such hi^i ligand ratios.
Sarin and Munshi (24) have investigated
potentiometric ally the complex formation of In (IIl)
with several mere ap to acids snd have reported that
stability of complexes with various li ands decrease
in order:
Thiolactic acid Thioglycollic add^>
Lactic acid Glycollic acid Alanine and
Glycine.
They have correlated these values with
stepwise formation constants of In(lIl)-3-mercapto-
propanoic acid complexes and have found these to b@
less stable than those of thiolactic acid and
thioglycollic acid and values of formation constants
decrease with increase in temperature. Stepwise
formation conatants of Ga(lII) complexes with thiolactic
acid, thiomalie a d d , thioglyoollic acid, lactic acid,
malic a d d and glycollic acid increase with increase
in temperature, and Ga(IIl) forms 1:1, 11 2, 1*3
(metal*ligand) complexes with thiolactic a d d . Ga(lll)-
thioglycollic acid complexes are more stable than those
of thiolactic a d d (25).
Cassasas and co-workers (26) have investigated
the Cd(II)-thiolactic system potentiometrlcally and
reported the existence of polynuclear species of the
composition,1 * 2, 2*3 and 3*4 (metal:ligand) with overall
formation constant, values of 15.04, 28.45 and 40.80
respectively. The acid sulph-hydryl group tends to
bridge metal ions in these complexes.
11
2. SOLID CGMPLiSXJS OP M̂ HCAP'JX) CA3BOXYLIC
ACIDS.
Fernando and Jfrleser (5) have reported complexes
of >-mercaptopropanoic a d d of ?b (ll), Cu(ll) and Ag(l),
the composition of these complexes reported is*
Pb(SCHgCHgCOO) f A d SCHgOHgCOO ) H^O,
Cu( SCHgCHgCOO ) and Cu( ESCHgCHgCOO),
Copper formed two types of complexes.
Shindo and Broun (27) have reported complexes
of L-cysteine, 3-methyl-L- cysteine and 3-mercapto-
propanoic acid with in(ll), Cd(ll), Hg(ll) and Pb(ll),
they have observed that Hg(ll) forms a number of
complexes with stoichiometrics -
Hg2 [SCH2CH(MH2)C02] 2 ,
Hg2 JTsCH gCH(HH 2 ) CO 2J 2 HCl,
Hg [SCH2 CH(*TH2)C02i 2 and
Hg5 [SCH2CH(NH^COgSj C l* ZHgO
12
Zinc and ca&nium form r-
C^4]2+M , M « 2n or Cd
L « S- CH2CH( IIHj) CO 2 # X * Cl~
The metal is hound to sulphur and earboxylate
oxygen, and sulphur atoms bridge the three metal atoms,
the other types of complexes reported are
The metal atom is bound to sulphur atom of sulphhydryl
group and nitrogen atom of amino group,
Mishra and STigera (9) have Isolated aid
characterized a uranyl complex of 3-mere apt op ropanoic
acid, its composition is reported to be *-
0oL2(H20 )2 (1 * -BCH2CH2C02 ) with octahedral
geometry has been reported (2S)| sulphur atom of -311
group and oxygen atom of -COOH group is involved in
bonding with Os>(ll) ion. Hi gam and co-workers (29) have
2-
[uOgLg ] 2H20 j L - -SCHgCHgCOgH.
A cobalt complex of the composition
isolated complexes of Ag(l), Cd(ll), Hg(Il) and
U02(VI) of 5-mercaptopropanoic a d d , the reported
composition of the complexes is
AgCC^OgSj.fflgO, CdtC^OgSj.aigO,
HgCC^OgSjg and UOgCG^Og.ffigO.
With Ag(l), Hg(ll) and Uranyl ions the ligand acts as
mono-negative bident ate ligand, co-ordinating through
oxygen atom of -COOH and sulphur atom of -SH group.
In case of cadmium the ligand acts in a binegative
bicentate fashion. Srlvastava and Higsra ( 30) have also
isolated the c cm pi exes of thiolactic a d d with Ag(l),
Cd(ll), Hg(ll) and Pb (II) with compositionsi-
H [lg(KL)] H20, Gd |hl] . 2H20
Sa2 [cdl2 ] .2 H 20, K2 QigL2 ] and
PbL.2H20 (L » C ^ O g S )
They have found that thiolactic a d d acts as bidentate
ligand co-ordinating through sulphur of -SH and oxygen
of -COOH groups. Complexes of Cr(IIl), Iln(ll), Co(ll),
and 'li(ll) with thiolactic a d d have been reported ( 3> ),
their compositions found are*-
14
H3 [cr ( C ^ 402S)3 ] , Mn [ (O ^ O g S ) (H ^O )^ 2H20
Ha2 [bo (C ^ O g S jg (H20 )2^|t
£»i (C ^ O g S ) (H20)4 ] and
Ua2 £ H tCC^OgSjg]
In these oomplexes the H a n d acts as bi dent ate
chelating agent co-ordinating throu^x sulphur of -3H
and oxygen of -COOH groups* They have suggested Mn(ll)
complex a square plannar geometry with ma&ietic moment
Of 4.51 B.M.
A trlnuclear complex of thiolactic acid with
Gu(ll) having composition-
CUj (GgH^OgS)^ 4.5 HgO has also been reported (32).
Mehrotra and co-workers (33) have reported
various complexes of Ti(lV) and Zr(lV) lsopropoxides
with 2-merc apto ethanol, thiolactic a d d and 3-mercap-
topropanolo acid.
Panchal and Bhattacharya (16) have prepared
complexes of Zn(Il), Cd(Il), Hg(ll) and Ag(l) with
thiolactic ad d and thioglycollio acid and compositions
of the complex©a is represented as i
2a( ZnI»2)2H20t Cd(0dL2), Hg( Hgl»2) and
Ag^( AgLg) t
L » C ^ O g S and -SCgHgOg
Horl (34) has reported the preparation of
mixed ligand complexes of 2-, and 3-mer captopropanoic
acids and ethylenediamine with Co(lll). The reported
composition of the complexes <S s-
Jco( SCHgCHgCOO) en2] x
[oo(CH3.S.CHOOO) en2 ] x
( en « ethylenedi amine, x * cl~ or 1“ )
complexes are mononuclear and the meroaptocarboxyllc
acids function as hi dentate ligands.
Coleman et al (35) have preoared the dialkyl-tin
complexes of merc&ptocarboxylic acids* the reported
composition of these complexes is t
(0H3) 2 Sal2.H20
3> -SCHgCOOH, -SOHgCHgCOgH and -SCH^CHCO^
16
From IE and i'fMR data they have suggested that tin
binds two groups of two ligand molecules, two alkyl
groups and a water molecule resulting in a penta-
coordinate tin atom.
3. SOLID COMPLEXES OP SELMO-, AND
THIOUREAS.
Podder et al (36) reported complexes of
mercury(ll) with guianyl-1hiourea, In which they have
shown co-ordination of the ligands throu$i nitrogen
and sulphur.
*- disubstituted selenoureas have been shown
to function as unidentate, Se-bonded ligands in Pd(ll)
and Pt(Il) complexes which are similar to corresponding
thiourea complexes but show an increased tendency to
form five co-ordinate complexes ( 37,38).
The selenourea complex of cadmium (I I ) has
analogous structure to its thiourea complex (39) and
its composition has been found to be s-
Cd( Su) 2 Clg ( Su « Selenourea )
Goddard et al (40,41 ) have determined the
stability constants of metal oomplexes of selenourea
and aelenosemicarbazides as well as of thiourea and
thiosemicarbazides with class 1 b' metal ions and have
reported the order of stability with
Be > S > 0
Domiano et al (42) have inferred from X-ray
powder data that bis ( selenourea) metal(ll) thiocynates
M(Su ) g(^CS)g (where II *» Co,Ni,Cd) are isoatructural
with corresponding thio urea complexes, having polymeric
structure consisting of chains of octahedra Joined by
two selenourea molecules which fora bridges between two
metal atoms. Selenourea complexes of Mo(V) have been
studied by e .s .r . Spectra (43).
Adducts of thiourea (l ) and selenourea ( I * ) to
1 , 6-dichlorobis ( ethylenediaaine) cobait( III) halides
have been characterized (44). 2he adducts bear following
compositions:
1,6- £coX2-en2 ’]x .n .L , i 1,6- [ 0oX2en2 "]x.1.5 I 1.5 l'
(X * 01, Br, n = 1 ,3 , en *= ethylene diamine)
In these complexes sulphur or selenium are not bonded
to cobalt atom.
Daraatelli and chlarl (45*46) have synthesized
polymethylene bis( phenylthiourea) and polymethylene
bis( phenylselenourea). On the basis of electronic spectra
and magnetic moments these complexes have been
characterized and bear the compositions -
M iL2X2*
L * polymethylene bie( phenyl thiourea), X * Cl,Br
IfiL Cl2 | L * trimethylene bis (phenylselenourea)
ITilg Br2 ; 1 «* octa©ethylene bis (phenylselenourea).
NiLj (C104 >4 ; L » tetramet> ylene bis( phenylselenourea)
In case of thiourea the HiS^ is square plannar and in
case of selenourSa both square plannar and octahedral
complexes have been found.
Various selenourea complexes of class ' b' metals
have been prepared (47), the reported composition of the
complexes ia
(GoL4 ) (C104 ) 2 , CoL5 (S04 )
and Hg 1 2 X2 ? ^ “ Selenourea, X *» Cl,Br.
19
3in^a and Gupta (48) have reported synthesis
and characterization of mixed ligand complexes of
LI,2J - dime tbylselenourea and pyridine with potassium
hexaisothiocynatoatannate (IV) and potassium hexai30thi0-
cynato tit annate (IV ), these complexes have coeapositiom
M * Co, Zn, Hg j L a ‘J , dimethyl-selenourea or
pyrldene.
H,H dibenzyl selenourea)
electronic spectra of these complexes has also been
reported,
Kubicki et al ( 50) have reported crystal and
molecular structure of tetrakis (selenourea) Pt(ll)
chloride- ( Pt (btO^Clg). Compound is monochinic,
In a el enourea complexes selenium is bonded to metal m '.
Rivero (49) has reported complexes of Co(Il)» »
■with - dibenzylthiourea and - dibenzyl selenourea
the composition of the complexes is -
CoL2C12, CoL2Br2 and CoL2(C104 )2
(L » £!,&' , dibeazyltbiourea or
selenourea Is bonded to platinum through, selenium;
PtSe^ is plannar with Pt-Se distance 2.439 %.
Proskina. (44) have reported mixed co-ordination
compounds of copper with heterocyclic amines acid
selenourea. The composition of complexes reported iss-
[cuL2q 1 (010^) 2 } L=2,2 “ dipyrldyl Q * Selenourea
[cuL Q( 010^ ) J t L o 1 ,10-phenanthroline
Co-ordination number is five in first oomplex with
trigond blpyranu'del geometry and later has co-ordination
number of four. Analogous thiourea derivatives have
also been reported.
4 . analytical applications of
mekoaptocarboxtlic a c id s .
Formation of metal complexes have been utilized
in qualitative and quantitative analysis of various
metal ions. Buscarons and Casaasas (51) have used
thiolactic ad d for detection of Pd and >ln. Solubilization
of hydroxides and sulphides of heavy cations with
thiolactic acid (52) as masking agent in -the
precipitation reactions of metallic hydroxides and
sulphides was used for separation of the cations -
As, Sb, Sn,Mo, Se, Te, Au, Pt, Mn, Mi and Co, which
are so stable that their sulphides do not precipitate.
They have used mereaptoethanol ; 2 ,3-dlmercaptopropanol
and mereaptosuccinic acid for such separations.
Busev and Baou (53) have suggested a photometric
method of determination of Palladium with
1-naphthylamide, 2-naphthylaraide $ o ,p - phenetidi de
derivatives of 1 -mereaptopropaaoic acid; ?d reacts
with the reagent in the ratio of 1:2 in presence of
large anount of (1000 fold) ?t(IV), Rh(IIl), Ir( I?)
and Os(IY) which do not interfere with the estimation
of Pd. The workers (54) have also determined Mo(V) and
Ho(Vl) with 1-mercaptopropanoic acid and its amide
derivatives like o,p - phenetidide, o,p-anisidide and
o,p-toluidide. Ho(V) and Mo(Vl) react with the reagents
in 1 :2 ratio over a wide pH range. Mo-1-mercaptopropionato-
phenetidide is extracted with 1 :1-isosmylalcohol: CHCl^
in presence of diphenylguaaidinium or benzylthiouronlum
in aqueous phase. Extraction photometric method was
developed to determine Mo in presence of W ,Cr(IIl)t
Ti,Co,Mi, 2n, Al,Fe( III) and V. Absorbance is measured
at 350-65 nm, in presence of tun gs ton at 430 run.
Shi geo Kara (55) determined cobalt colorime
tric ally with 3-ra eroap to propanoic acid at pH 9,0 and
at 370 run. Beer* s law is obeyed, a 1*3 (metal:ligand)
complex is formed, interference of cooper is decreased
by adding potassium cynide, iron and nickel interfere.
Prlbil and Vesely (56) have determined zinc and
cadmium in presence of copper with >~m ereap to propanoic
acid as masking agent followed by direct titration of
zinc with t ri ethyl en e-tet ram in e hexar-acetic acid. After
addition of dianinocyclohexane tetra-acetic acid
cadmium is determined in directly by back titration with
2n(tf0j)2 solution, all the titrations are carried in
presence of xylenol orange as indicator in acidic medium,
iacu and coworkers (57) have used thiolactic acid for
gravimetric estimation of zirconium. Thi©lacotic acid
is added to zirconium salt at pH 4.5* Zr02 is
precipitated and dried. Sn( I I ), Hg(Il), Zn(Il), Gd(Il),
Ti(IV), Th(IV), Cu(Il), Pb(Il), Iii(II) and Ca(ll)
interfere in the estimation. Fritz and Beuuaan (58)
su©jested a rapid selective Spectrophotometric determination
of molybdeaura( XV) which is based on the formation
of a yellow molybdenum - thiolactic acid complex at
pH 1 .0-1 .6 and measuring absorbance at 365 nm.
In view of the literature cited it seems that
mercury(II) complexes of 3-, and 3-mercaptopropanoic
acids need further investigation. Besides this, it is
worthwhile to study the complexes of group IIB metals
with ligands having sulphur or selenium donors like
selenoureas and thioureas.
Therefore, the following studies have been
undertaken in the present work:-
(i) The physico-chemical investigation of mercury( I I )
meroaptopxopanoic acid systems leading to
calculation of stability constants and other
related parameters.
(ii ) Preparation and elucidation of structures of
anionic complexes of mercury( II) with a mercapto-
propanoic acids.
(iii) Determination of degree of dissociation of
soluble complexes and solubilities of less
soluble complex species.
24
(It ) Use of mercaptooarboxylie aoids as masking
agents for group I IB metal ions.
(▼) Preparation sod elucidation of structures of
complexes of N»N*-disubstituted seleno-, and
thioureas with mercury( II) and zLno(Il).
The sulphur and selenium containing ligands
whose ccmplexatlon with various metal ions is being
reported here, are t
(l) 2-Mercaptopropanoie aoid or
thiolactio a d d (abbreviated as HgTLA)
(i i ) 5-Meroaptopropanoio acldi
thiohydracrylic acid (abbreviated as H^MPA)
CHjCH COOH
SH
HSCHgCHgGOOH
( i i i ) JI - S TH r»V> Mtrl iial
( iv) HtN*«Dlbensyl Selenourea!
(abbreviated as DBSU) OgHgCHg.NH. C.SH.CHgCgF^
Se
or;r*J W
(v) N,M -Dibenzyl thiourea*
(abbreviated as DBTU)
5 . STABILITY COHSTASTTS
Stability of a complex refers to the extent of
dissociation of a complex species in solution form or
it is a measure of the extent to which a complex species
will form, or will be transformed into another species
under certain conditions whoa the system has reached
to equilibrium. A complex molecule is considered to
be formed from the free ligand (L) and a metal
ion (M)s-
, M + L = ML j ML + L - MLg and
ML + La m MLn+j | when the concentration of
metal ions and free ligand la considered and
the logarithmic ratios of products to
reactants give the stepwise stability
constants,
1 0 gK, . log
1 o #j5>I« « lo g
2 m s
. . ( i . i )
. . . ( 1 .2 )
0 6H5 0112#HE *G ,HH # GH2° 6H5
S
2G
log Kn - log(1 .3)
K-i] M• • •
Summation of log Kj, log Kg - - - log KQ values give
the overall stability constant log of a complex
species which can be evaluated directly also -
M is a metal ion and L may be either a charged ion
stability constant.
The logarithmic values of overall stability
constants at various ionic strengths extrapolated to
zero ionic strength give thermodynamic stability
constants, which are relted to free energy change,
enthalpy change and entropy change by the equation:-
• • • (1 .4 )
, e.f • •
A g° a -RTlnyg a * A h° - I As0 . . . . . ( 1 . 5 )
where T * absolute temperature.
R “ gas constant (1.987 aal.deg:1 mole”1 )
P * stability constant at zero ionic strength.
A g ° = Standard free energy change.
A h0 * standard enthalpy change
Atf° = standard entropy change.
The determination of enthalpy change and other
thermodynamic parameters from the stability constants
are helpful for thorough understanding©f the factors
influencing the complex formation (59). A number of
methods can be used for determination of stability
constants viz.,
(i) Potentiometry (pH metric method)
(il) Measurement of B.M.I.
( i i i ) Spectrophotometry.
( iv) Polaro graph y.
In the prescent work potentiometric technique
of measuring xS.M.F. of mercury ( I I ) ions in presence
of large excess of ligand has been used for determining
stability constants. A potentiometer with mercury
and saturated calomel electrodes is used to measure
of the concentration cell which contained
mercuric(Il) ions and large excess of ligand. The 3.1-!.?,
in a concentration cell arises due to the cell
reaction*-
Hg / 11 g If Hg2^2^ ^ • • • ( I • 6)
ro
The J3.M.F. of the concentration cell is
related to free Hg(ll) ion concentration by the relation*
0 + 2.292 RT to h _2+ _IS.M.F. - ^ l0* 3 . G . .
. . . ( 1 .7 )
where 2°jI^ Hg2+ » standard electrode potential of
mercury at 25°C (0.854 volts).
n ,, * Standard electrode potential of saturatedC* • v/ • 2i +calomel electrode at 25°C ( 0.242 volts).
lig4" » Free mercuric ( I I ) ion concentration or
activity of mercurie(ll) ions.
K - Gas constant ( 8 .3 U J molt1 )
F = Faraday constant (96,500 coulombs)
n « Valency ( 2 in case of mercury(II))
At temperatures other than 25°G , the relation
(1 .7 ) is modified (60) as*
S.M.F. - B ^ Hg2++ log Hg2+ - e“ . c. b/°-00076
(t- 25). . .(1 .8 )
t m temperature °G .
n.i;
0.03057 at 35°C (61)
•— « 0.02958 at 25°C and
In presence of large excess of ligand
mercury(ll) ione are supposed to exist as the hi^aest
co-ordinated species IIgLn . The pH of the reaction
mixture is adjusted above the pKn (dissociation
constant) of the ligand so that all the ligand exist
in completely dissociated fom ( HnL ^ nll+ + Ln'"‘ ) .
The overall stability constant was evaluated
with the help of exparimantally determined concentration
2+of free Hg ions using the relation: -
logjS Q= log -HeLnl
[Hg2+][x ,]n. . . (1 .9 )
The free ligand concentration La under the experimental
conditions is approximately equal to total ligand
concentration.
6. SOLID MI3R0APT0CAHB0XTLATE COMPLEXES OF Hg( I I ) .
(i ) Preparation aid opposition:
Sodium bis( 3-mercaptopropionato)-mercurate
and potassium bisthiolacetatomercurate were crystallized
9 ft«J
with ethanol from a solution of mercuric oxide and
respective mercaptoacid. Their other metal derivatives
were prepared from the solutions of sodium
bis(3-mercaptopropionato)-mercurate( I I ) and potassium
bls-thiolaotato mercurate( II) and respective salt
solutions of various metals.
The complexes were analysed for the metal,
sulphur and mercury content and thus composition was
established.
(ii) Structurest The infrared spectra of metal -
bis(mercaptooarbosylat3)iiicrcurates were reoorded between
4-000 cm"” ̂ - 600 using potassium bromide on
Pye unicam IR Spectrophotometer. Magnetic moments of
some complexes were measured at room temperature using
G-ouy balance. Proton magnetic resonance spectrum of
sodium-bis( j5-mercaptopropionato)-mercurat e( II) complex
was recorded on M-390 , 90 MHz Yarian LfMR Spectrometer.
XR, aagaetic moments and M R data were used to assign
structures to metal complexes.
(iii) Solubility : Com pi exam etri c and conductors etric
technique was used to determine solubility of less
soluble complexes in their saturated solutions at
different temperatures.
31
( iv) Other Investigations: The apparent molecular
weight and degree of dissociation of potassium
bisthlolaotatomercurate( II) and sodium bis(3-mercapto-
propionato)-mercurate( I I ) was determined by cryoscopic
method (62). The ionic mobilities of the anions -
[n g( TLa) 2 and [k g (MPA)2 "J 2 ware determined by
conductometric method.
(v) Analytical Applications* Use of thiolactic
acid and 3-mercaptopropanoic acid as masking agent
for group IIB metal ions in the estimation of
mag»esiua( II) has been studied.
7. 3TUBIBS OH Jm '-DISUBSTITUTSD SELM0-,
A'ID THIOUHBA COMPLiSXKS OF ZIHC(ll) A20
MSK,UHY(II).
( i) Preparation and composition* Complexes of• t- diphenyl selenourea, 31,51 -dibenzylselenourea and
1U,U - di benzyl thiourea have been prepared and their
composition established on the basis of elemental analysis.
(ii ) Structure lelucldatlon* The infrared spectra of
the complexes have been used in assigning the structures
for the complexes.
The work presented in this thesis comprises of
seven ohapters.
First chapter deals with the review of literature
and introduce the topics of the thesis.
Second chapter incorporates the studies of Hg( II)-
TLA and Hg( I I )—MPA systems leading to determination of
stabilities of Hg( I I ) complexes and evaluation of
relat ed thermodynamic parameters.
Third chapter includes the preparation of metal-
bis(mercaptocarboxylato)-mercuratee and chloro-
oomplexes of Hg( II) with 2-, and 5- mereaptopropanoic
acids, date m l nation of composition and structure of
these complexes*
Fourth chapter includes determination of degree
of dissociation and ionic mobilities and protonation
constants of alkali metal-bisthiolactato-mercurate and
bis( 5-meroaptopropionato)-mercrurate and solubilities
of other less soluble metal derivatives.
Fifth chapt er deals with analytical applications
of 2—, and >»mercapto propanoic acids.
Sixth ohapter incorporates preparation of Hg( II)
and Zn( II) complexes of BPSU, DBSU and DBTU, determination
of their composition and structure.
Seventh ohapter gives a summary of entire work.
3 ro
33
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