CHAPTER I

INTRODUCTION TO THE CHEMISTRY OF THE RARE EARTHS

The 'Rare Earths' consists of the seventeen elements-

scandium (atomic number, Z = 21), yttrium (Z = 3 9 ) and

lanthanum to lutetium ( 2 = 57 to 71). However, promethium

( Z = 61), a fission reaction product, is excluded from

7 this group since it has not been found in nature . Scandium is quite common and is not readily available,

owing partly to the lack of rich sources and partly due to

the difficulty of separation. Also, the chemistry of

scandium is intermediate between that of aluminium and

lanthanides. The remaining fifteen elements of this group

generally occur together in nature and it is hard to

separate them from each other. These fifteen elements are

grouped into two classes - 'light' (or the cerium) group

and 'heavy' (or the yttrium) group. The cerium group

consists of La, Ce,,Pr, Nd, Sm, Eu and Gd. The yttrium

group consists of Y, Tb, Dy, Ho, Er, Tm, Yb and Lu. The

element yttrium shows close similarity with the

lanthanides and is classed along with the group even

though it possesses low atomic number.

The Chemistry of Yttrium

Yttrium (Z = 39) being the first member of the 4d

transition series, shows remarkable similarities with the

lanthanides and is often discussed along with them. This

similarity is apparent even with regard to the occurrence

in nature; yttrium is found along with the lanthanides in

ores. The close similarity of yttrium with the

lanthanides is a result of the fact that its atomic and

ionic radii are in between that of Ho and Er. For this

reason, Y shows greater similarity to the heavier

lanthanides rather than to the lighter ones. The

resulting similarity in size coupled with equality in

ionic charge, accounts reasonably for the invariably

natural occurrence of yttrium with the heavier

L lanthanides .

The coordination chemistry of yttrium has also

followed the course of development as that of the

lanthanides. The Complexes of Y differ from those of the

lanthanides only in those properties that depend upon the

presence of incompletely occupied 4f orbitals, i.e., in

magnetic properties, in light absorption and in energy

absorption - emission phenomenon.

The Chemistry of Lanthanides

The 'lanthanides' are fifteen elements from

lanthanum (Z = 57) to lutetium (Z = 71). The general

electronic configuration of lanthanide(II1) ions is

2 6 0 [pd]4fn5s 5p 5d , i.e., the deep seated 4f orbital of

2 6 lanthanide ions are effectively shielded by the 5s 5p

octet. The gradual filling of this subshells

characterises the fourteen elements of the lanthanide

series following lanthanum. The decrease in both the

potential energy and spatial extension of the 4f orbitals,

immediately after lanthanum, allow the preferential

8 occupancy of these orbitals . Thus, for instance, the

binding energy of a single 4f-electron drops from -0.95 eV

for the lanthanum atom to -5.0 eV for the neodymium atom.

In the occupancy of orbitals, the ground state electronic

configurations of neutral lanthanide atoms and cationic

species do not show any absolute regularity. This is due

to the similar energies of 5d and 4f orbitals. However,

among the highly charged Ln(II1) species, complete

regularity is achieved.

In lanthanides, the filling of the 4f orbital causes

a regular decrease in the size of the atoms and ions from

La to Lu and is called lanthanide contraction. This

contraction is appreciably greater than the corresponding

contraction occurs when d-orbital is being filled. The

lanthanide contraction is a result of imperfect shielding

9 of one 4f-electron by another . As the nuclear charge

increases, the imperfect shielding causes each 4f-electron

to experience added electrostatic attraction by the

nucleus. The reduction in size is best shown by the ionic

radii of lanthanides from 1.06 AO for La(II1) to 0.85 AO

for Lu(II1). The inclusion of Y(II1) in the heavy

lanthanide series is one of the consequences of the

2 lanthanide contraction .

The characteristic oxidation state of lanthanides may

be + 3 and the chemistry of lanthanides is defined by this

state. The stability of the tripositive ion is not due to

any particular electronic configuration, but due to the

2 unusual combination of ionisation and hydration energies . Also +2 and +4 states are common, but they are unstable in

aqueous solution except1' for Ce ( IV ) . Samarium, europium

and ytterbium exhibit +2 oxidation state in solution and

are sufficiently strong reducing agents. Pr, Nd, Tb and

Dy exhibit +4 oxidation state only in the crystalline

state. The existence of oxidation states other than +3 is

explained in some cases by the special stability of fO, f 7

and f l4 configurations . Also some thermodynamic and

kinetic factors play an important role in determining the

stability of +2 and +4 oxidation states.

1.1 Coordination Chemistry of lanthanides: General

The coordination chemistry of lanthanides has

expanded rapidly in the last few years. The majority of

complexes studied have been derived from strongly

chelating anionic or neutral ligands containing highly

1 electronegative donor atoms (eg., oxygen) . Before 1964,

nitrogen coordination was well characterised only in

association with oxygen donors. The past several years

have witnessed a boost in the study of lanthanide

complexes.

In contrast to the d-block transition elements, the

coordination chemistry of lanthanides is of recent origin.

The coordination chemistry of lanthanides has found

application in the isolation and separation of the members

of the series by fractional crystallisation and

precipitation. Only in the last quarter of the century,

synthesis of a wide variety of coordination compounds have

been made and their structures studied in detail.

The trivalent lanthanide ions behave as hard acids

(Pearson's classification) or 'a' type acceptors

(Arhland's classification) and have small ionic radii, low

polarisabilities and high oxidation states. Since hard

acids or 'a' type acceptors are expected to interact

strongly with hard bases or 'a' type donors, the

lanthanide ions preferentially form stronger complexes

with ligands having 0 and N-donor atoms. Forsberg 3

suggested that LnA>O interactions are stronger than

Ln->N interactions, since no cationic complexes derived

from N-donors are isolated from aqueous media. Majority

of the coordination compounds prepared from oxygen donor

ligands are from anionic (chelating) ligands, but recently

a number of lanthanide complexes of neutral oxygen and

11 nitrogen donor ligands have been prepared .

Since the 4f orbitals of lanthanide ions are well

shielded by the 5s2Sp6 octet, the chance of existence of

strong covalent metal-ligand interactions in lanthanide

complexes is impossible. Thus metal-ligand bonding in

lanthanide complexes is substantially electrostatic in

nature which is corroborated by magnetic, spectral and

11 kinetic data In a given series of ligands, increasing

covalency is expected with decrease in size (La to Lu) of

the lanthanides. The ligands encounter a stiff

competition from water molecules for coordination sites on

the metal ion, if the experiment is conducted in aqueous

solution. The weakly basic N-donor coordination could

occur only by the displacement of strongly bonded water

molecules, which is rather difficult. However, strongly

basic N-donor ligands, which might form strong complexes

with lanthanides, generate a sufficient concentration of

hydroxide ion by interaction with water to precipitate the

lanthanide hydroxides. Therefore complication arises in

the synthesis of complexes in aqueous media and thus

lanthanide complexes with N-donors are best prepared in

non-aqueous media.

The ligand field stabilisation effects are minimum in

the lanthanide complexes due to the absence of extensive

interaction with 4f orbitals. The lack of LFSE reduces

overall stability but on the other hand provides a greater

flexibility in geometry and coordination number. Also,

the lanthanide complexes are stable in solid state and

tend to be labile in solution. The tendency to exhibit

increased coordination numbers by the lanthanide complexes

distinguishes the lanthanide ions from those of the

d-transition metal ions. Coordination numbers of

lanthanides in complexes are usually greater than six and

vary from six to twelve. This variation of coordination

numbers in lanthanide complexes is well established and

may be attributed to steric factors and electrostatic

forces of attraction and repulsion rather than to the

directional orientation of bonds by the deep-seated

4f orbitals of the metal ion. Thus, it is expected that

the large lanthanide ions have a tendency to provide

accommodation to more than six donor atoms in the

coordination sphere. However, lanthanide ions show

a surprisingly low coordination number, eg., in air-

sensitive monomeric tris-N-bis(trimethylsilyl)lanthanide,

where the coordination number is only three 12,13 and

several other lanthanide complexes with five or six

coordination numbers have been reported elsewhere 14,15

The difference between the 3d transition metal ions

16 . and the lanthanide ions are summarised in Table 1.1.

The complex formation in most of the transition metals is

accompanied by a change in colour of the solution.

However, in the case of the lanthanides, no such colour

change takes place and whether complex formation has

occured or not is not immediately obvious. Also, the rate

of formation of lanthanide complexes is slower than the

corresponding complexes of transition metal ions. Inspite

of these difficulties, the coordination chemistry of

lanthanides continues to be an interesting and novel field

of investigation.

Table 1.1 Comparison of transition metal ions and lanthanide ions

First series Lanthanide ion transition metal

ion

1. Metal orbitals 4 f 3d

2. Ionic radii 1.06 - 0.85 AO 0.75 - 0.60 AO

3. Common coordin- 6,7,8,9,10 ation polyhedra

4. ~ypical coordi- Trigonal prism Square planner, nation polyhedra square antiprism, tetrahedron,

dodecahedron octahedron

5. Bonding Little metal-ligand Strong metal- orbital interaction ligand orbital

interaction

6. Bond direction Little preference Strong prefer- in bond direction ence in bond

direction

7. Bond strengths Ligand bond in order Bond strength of - electronegativity - determined by F OH-, H20, NO3 , orbital inter- C 1 action normally

in the following orher CE , N H ~ , H20, OH , F

8. Solution complexes Ionic, rapid, Often covalent ligand exchange complexes, may

exchange slowly

1.2 Coordination Number and Molecular Geometry

The complexes of the lanthanides, unlike those of the

transition metals, are characterised by high coordination

numbers. The high coordination numbers encountered is a

consequence of the predominant electrostatic nature of

the bonding between the metal ion and the ligand and the

large size of the lanthanide ions. As the 4f orbitals are

sufficiently shielded by the 5s25p6 octet as to be largely

unavailable for bonding with the ligand orbitals, they do

not have any bond orienting character. As a result, the

structure of a lanthanide complex is dictated by the

17 following considerations .

(a) Preserving a spherical symmetry for the central

metal ion

(b) Minimizing ligand-ligand and metal-metal

repulsions, and

(c) Steric requirements of the ligands.

Also it is generally observed that due to the

decrease in the cationic size along the lanthanide series

the coordination number also decreases for the complexes

with the same ligand. However, a few examples are known

where the coordination number increases along the

18 series .

Ideal polyhedra which describe high coordination ...- .

number are given below: , <--

'-< 1

Coordination Polyhedron -", . . number I'

6 Octahedron (Oh), Trigonal prism (D3h)

Pentagonal bipyramid (D ) Monocapped trigonal prism5h ( c ~ ~ ) Monocapped octahedron (C3")

8 Triangular faced dodecahedron (DZd) Square antiprism (D4d)

Tricapped trigonal prism (D ) Monocapped square antiprism 3h

10 4,4'-Bicapped square antiprism (D4d) Bicapped dodecahedron

Pentacapped trigonal prism Decahexahedron

12 Icosahedron (Ih)

It may be noted that the assignment of a cartesian

polyhedron to represent the atoms coordinating with the

lanthanides should be viewed only as an approximation.

The atom coordinating with an ion are best represented as

points on the surface of a sphere which will minimize the

repulsion between the coordinating ligands and provide a

spherical shielding for the ion.

Octahedron ~onocapped trigonal Monocapped prism

Pentagonal Square antiprism Triangular dodecahedron

\ f

Tricapped Monocapped square trigonal prism

antiprism Bicapped square

antiprism

-. .

Bicapped Icosahedron dodecahedron

Figure 1.1. Ideal polyhedra for various high coordination numbers.

The lanthanides exhibit mixed coordination number and

mixed geometries in certain complexes, eg., in the

orthorhombic Eu2(mal) 8H 0 complex, (where ma1 = malonate) 3 2

there are two different europium ions19. The exhibition

of mixed coordination numbers and mixed coordination

geometries is probably due to a result of compromise to

avoid steric crowding and minimizing the ligand-ligand

contact distances and still maintaining a well defined

geometry for the coordination polyhedra.

The complexes formed with high coordination numbers

from similar ligands can give isomers with different

17 geometries . For an eight coordinate complex the

symmetry transformation scheme that one can think of from

purely qualitative consideration is:

As the energy difference between these structures is very

small, there is a possibility that the structure of the

complex can be changed fast from one geometry to another

in solution. This results in the potential

'stereochemical non rigidity' of the lanthanide complexes

in solution.

Since the size of the tripositive ion decreases on

going from La to Lu, the ligand-ligand repulsion becomes

more important for heavy lanthanide complexes. Thus there

is a chance for change in coordination number and geometry

for a series of complexes of the same ligand. Also the

stereochemistry of a lanthanide complex is affected by the -

anions present in the complex. The anions such as NO3 , -

~ 1 - , Br , SCN- and SO4 2- compete with the ligand for the

coordination sites on the metal ion. The anions like

perchlorate, iodide, tetraphenylborate are not generally

coordinated to the metal ion and the complexes with

maximum ligands are obtained when these are used as

counter anions. When the anion is bidentate with small -

size (eg., NO3 ion), no appreciable change in the

stoichiometry has been observed. By changing the bulk of

the ligand, however, changes in coordination number cannot

be ruled out as nitrate may act as monodentate or bridging

anionic ligand.

1.3 Bonding in Lanthanide Complexes

The bonding in lanthanide complexes has been a

subject of much debate for quite some time and was not

well understood. The problem before the investigators

is, whether there is any appreciable covalent metal-

ligand interaction in the lanthanide complexes and if so,

whether the 4f orbitals are involved in such interactions.

It has been argued that the bonding in these complexes is

substantially electrostatic and uncertainty exists with

regard to the covalent interactions in the lanthanide-

ligand bond. The type of orbitals involved in covalent

bonding, if present, is not known. This is to be expected

since lanthanides are classified as hard acids in

Pearson's classification. The absence of extensive

covalent bonding in these complexes is detected both by

the electronic configurations of the cations and by their

large size. However, evidence for at least minor covalent

interactions is given by the existance of a derived

nephelauxetic series of ligands for the Pr(II1) and

Nd(II1) ions closely comparable with that for the

d-transition metal ions2'. Further, the evidence for

covalent interactions is provided by a study of the NMR

spectra of lanthanide complexes. For the diamagnetic

lanthanide complexes, down field shifts of the protons are

generally observed because of the deshielding of the

protons due to the drainage of electron density from the

ligand to the metal ions. This suggests covalent

interaction in these complexes. Further, the isotropic

shifts observed for the paramagnetic complexes have been

shown to involve contributions from a contact term, which

would arise only if there is covalent bonding in the

complexes. For example, the data for the interaction of

the complexes of substituted pyridine molecules with Pr

and Nd perchlorates in acetonitrile have been interpreted

in terms of contact and pseudocontact contributions of

2 1 similar magnitude . In this case the magnitude of

contact shifts shows only very small covalent interactions

but these may be more significant in determining the

nature of the bond.

Even if a small amount of covalency exists, the

question with regard to the orbital taking part in the

bond formation naturally arises. In the lanthanide ions,

the 4f orbitals are unavailable for covalent bonding

because they are deeply buried. For covalent bonding the

orbitals should not only be energetically suitable but

also be spatially favourable and hence the f orbitals of

lanthanides are not available for covalent bonding.

Experimental findings also substantiate the above

arguments. Most lanthanide complexes have colours which

resemble the colour of the free lanthanide ion. Since the

colours of the lanthanide ions are due to the f-electrons,

the above facts suggest non involvement of the

f-electrons in bonding. Small changes in the electronic

spectra of the lanthanide ions on complexation are

accounted for in terms of alterations in crystal field

symmetry and do not suggest covalent interaction involving

f-electrons 22'23. The magnetic moments of the lanthanide

complexes are almost similar to the free ion values and

show no change. This suggests non-involvement of

f-electrons in bonding.

From the above observations it can be inferred that

the f-electrons, although energetically suitable, are

spatially less available for bonding and hence involvement

of f orbitals in bonding, if any, must be minimum.

Therefore the covalent contributions to the lanthanide-

ligand bond must involve 5d or higher orbitals that are

normaly unoccupied in the lanthanide ions. These

orbitals, though energetically less suitable, are

spatially more favourable. It may be pointed out that the

analysis of 170 NMR shifts of aqueous solution of all

tripositive rare earth ions except Pm suggests the

involvement of 6s orbitals in covalent bonding with water

2 4 molecule .

1.4 Applications of Lanthanide Complexes

The lanthanide complexes find application in the

separation of individual lanthanides by ion exchange,

solvent extraction and more recently by gas

25 chromatography . The lanthanide complexes have been

widely used as NMR shift reagents for simplifying the

spectra of many complex organic molecules and in assigning

2 6 their conformations . The complexes of lanthanides with

optically active (3-diketones have been used to determine

2 7 the purity of optical isomers . Recently some

lanthanide ij-diketonates have been shown to act as

antiknock additive for gasoline and are good replacements

27 for tetraethyl lead . One of the largest single use of

rare earths is as catalyst for fluid catalytic cracking

28 process . With the help of these catalysts the heavier

fractions of crude oil is converted to valuable products

such as gasoline, LPG etc. The rare earth coordination

compounds combined with triethyl aluminium are used

as Ziegler-~atta catalysts for 1,4-stereospecific

29 polymerisation of butadiene . Some rare earth

carboxylates find application as antioxidants and in

"chelation therapy" for the removal of 144~s

30 contamination in the body . The Schiff bases possess

antifungal activity. It has been found that lanthanide

chelates of type LnL2 and LnLt2 (Ln = La, Pr, Sm, Ho, Er;

L = acetyl acetone thiosemicarbazide and L' = benzoyl

acetone thiosemicarbazide) exhibit antifungal activities

and it has been found that toxicity decreases with

31 chelation .

1.5 Schiff Base Complexes of Yttrium and Lanthanides

The metal complexes of Schiff bases have played a

major role in the development of coordination chemistry.

This is shown by a large number of publications ranging

from purely synthetic and modern physicochemical studies

to biochemically relevant investigations. Metal complexes

4 derived from Schiff bases have been known for over one

hundred years. In 1869 H. Schiff prepared the first

5 Schiff base comp1.e~ of copper using the ligand obtained

from salicylaldehyde and aniline. In this complex 1:2

metal-ligand stoichiometry was established.

The Schiff bases are those compounds containing the

azomethine group (-CH=N-) and are usually formed by the

condensation of a primary amine with an active carbonyl

compound. The Schiff bases which are effective as

coordinating ligands bear a functional group, usually -OH,

>C=O, -0-, -N- sufficiently near the site of condensation

so that a five or six membered chelate ring can be formed

upon interaction with a metal ion.

The transition metal complexes of the following

32 ligand systems have been excellently reviewed upto the

period covering 1964: (i) bi and polydentate

salicylaldinines (ii) bi and polydentate p-keto amines

(iii) aliphatic Schiff bases and (iv) Schiff bases

resulting from the condensation of functionally

substituted primary amines with 1,2-dicarbonyl compounds.

Some of the important solid lanthanide complexes of

Schiff bases reported are reviewed here.

The rare earth nitrate complexes of the ~chiff base

acetylacetone-4-aminoantipyrine ( Acp ) have been

synthesised 33 and have the general formula

[L~(ACP)~I(NO~)~. The Schiff base acts as neutral

tridentate ligand giving a coordination number of six to

the central ion leaving all the nitrate groups ionic.

Kuma and Yamada reported34 the complexes of the Schiff

base N-acetylsalicylideneimine (HL) with yttrium chloride

of type [Y(HL)~C~]C~~ with a coordination number of eight.

Uninesative tridentate "ONO" donor Schiff base derived

from salicylaldehyde and anthranilic acid (H2L) with

35 lanthanide chlorides were reported . They have the

general formula [Ln(HL)2(H20)C1] with a coordination

number of eight. Bis(vanil1in)benzidine (H2L) complexes

with lanthanide chlorides of type [Ln(L)2H20]C1.H20,

having binegative quadridentate ligand were synthesised

3 6 with a coordination number of six .

The halo complexes of lanthanides with the Schiff

bases N,N'-ethylenebis(salicylideneimine), N,N'-propylene-

bis(salicy1ideneimine). N,N'-propane-1-diylbis(salicy1-

ideneimine) and N,N1-hexane-1,6-diylbis(salicy1ideneimine)

37 have been reported by Bullock and Heider-Ali . These

complexes contain coordinated halide ions. Also

ethylenebis(salicy1ideneimine) complexes with rare earth

38 nitrates of type [Ln(L2)(N03)3].nH20 have been reported .

Here both the ligand and the nitrate group act as

bidentate giving a coordination number of ten. Rare earth

complexes of N,N'-ethylenebis(t-butylsalicylideneimine)

(H2L) with general formula [Ln2(L3)] have also been

39 reported .

Lanthanide(II1)isopropoxide complexes of

N-(2-pyridy1)salicylidimine (HL) of type [Lr~(oPr)~Ll,

[Ln(OPr)L] and [LXI(L)~I with a coordination number of six

40 in all these cases have been reported . Sankhla et al. 41

reported the synthesis of lanthanide isopropoxide

complexes with the Schiff base N,N1-butylene-

bis(acetoacetani1ideimine) (H2L) to give three types of

complexes [Ln(OPr)L], [L~(HL)(L)] and [Ln2(L)3], where

Ln = Pr, Nd, Sm, Dy and Ho. Here the ligand is

quadridentate dibasic in nature. Agarwal and Tandon have

reported 2,4-pentanedi(anil)(A-R) complexes of the

rare earth nitrates having the composition 42

ILn(A-R)2N031(N0 3 ) 2 where Ln = Yb and R = C H - or 6 5

C H CH -. 6 5 2 The tris Schiff base complex have a

coordination number of eight and the bis complex six. The

Schiff base derived from acetyl acetone and benzidine,

which is bivalent quadridentate in nature, formed

complexes with lanthanide chloride of the type

43 [Ln(L)H O]C1 with a coordination number of five . 2

The rare earth nitrate complexes with the Schiff base

2,5-pyrrolediyl-bis-[N-(o-hydroxyphenylaldimine)] (SBH2)

having the general formula [Ln(SBH)nH20](N03)2 (where

n = 1 for Gd, Er, Dy and Pr and n = 2 for Sc, Y and La)

have been reported44. The Schiff base acts as uninegative

tridentate ligand in these complexes. Neodymium chloride

complexes of several Schiff base ligands obtained from

vanillin and propylenediamine, diethylene triamine,

2-naphthyl amine, o-phenitidine and p-phenitidine were

45 reported . Also the Schiff base of vanillin with

triethylene tetraamine gives complexes with lanthanide

chlorides of type [Ln(L)(C1)2(H20)12C1.H20, in which the

ligand is quadridentate and the coordination number of the

4 6 metal ion is eight . Similar type of complexes of

lanthanide chlorides with the Schiff base bis(vanil1in)-p-

phenylenediamine (H2L) having the general formula

47 [L~I(L)(H~O)~]C~ have also been reported . In these

complexes, the Schiff base acts as bidentate, coordinating

through the two phenolic groups, giving a coordination

number of five.

The rare earth nitrate complexes of the type [LnL]

with a heptadentate Schiff base, tris(3,5-dichlorosalicyl-

4 8 idene-2-iminoethy1)amine (H3L), was reported . The

complex formation of a quadridentate ligand biacetyl-bis-

(benzoylhydrazone) (H2L) with the rare earth ions of the

composition [Ln(L)(OH)(H20)] with a coordination number of

six 4 9 have also been reported. Lanthanide isopropoxides

with azines50 like salicylaldehyde azine, o-hydroxy-

acetophenone azine, 2-hydroxyl-1-naphthaldehyde azine

formed three different types of complexes, [Ln(OPr)L2Ir

[Ln2(L)31 and [Ln(L)(HL)] with coordination number 5, 4

and 5 respectively.

Sharma et al. reported51 the synthesis of the Schiff

base complexes of europium chloride and carried out its

Mossbauer studies. The ligands used were acetyl acetone

benzidine, vanillin benzidine, salicylaldehyde dianizidine

and vanillin-o-phenylenediamine and have suggested

coordination numbers 5, 6, 6 and 7 respectively in these

complexes. Another Schiff base diethyl(ethy1ene-

bisaminocrotonate) reacted with lanthanide nitrates and

chlorides yielding complexes of varying general formula

52 and coordination number in the same series .

Schiff base complexes of lanthanide~~~ with

sulphamethoxazole and salicylaldehyde and sulpha-

methoxazole and thiophene-2-aldehyde have the general

formula [M(LH)3.2H20]. In these complexes the Schiff base

acts as bidentate and two water molecules are coordinated

giving a coordination number of eight to the central ion.

Rare earth chloride complexes with o-hydroxy-

phenyl-4-benzamidethiosemicarbazide of type [LnL2]C1.H20

were reported54, where the ligand acts as tridentate with

coordination through sulphur also. The Schiff base

complexes of acetyl acetonethiosemicarbazide and benzoyl

acetonethiosemicarbazide with lanthanides were also

3 1 reported .

The rare earth nitrate and perchlorate complexes of

the Schiff base 1-benzoyl-2-(2'-hydroxy-1'-naphthyl-

idene)hydrazine of type [LnL2X] have been reported

recently55. The ligand acts as monovalent terdentate and - -

X ( = NO or C104 ) is coordinated to the lanthanide ion 3

monodentately. New complexes of lanthanide nitrates with

a Schiff base derived from 0-vanillin and p-toludine

having the general formula [LnL2(N03)2]N03 were

56 reported . The Schiff base and the nitrate groups are

bidentately coordinated to the central ion giving a

coordination number of eight. The Schiff base derived by

the condensation of or-pyridone with anthranilic acid and

p -alanine having N,N,O-donor atoms complexed with

lanthanide(II1) ions 57 having a coordination number of

six.

Recently T.R. Rao and G. Singh reported 58 the

synthesis and spectral studies of lanthanide metal

complexes of furfural isonicotinoylhydrazone of the type,

[Ln(L)2C13], where the Schiff base acts as neutral

bidentate ligand. Also a similar study was reported 58

using another Schiff base, viz., acetone

isonicotynoylhydrazone in which the complex is 1:2

electrolyte in nature with the general formula

[Lr~(L)~Cl]cl~. Here also the Schiff base is acting as

neutral bidentate ligand and the coordination number

remains the same, ie., seven. Lanthanide complexes of the

Schiff base N-thiophene-2-carboxamidosalicylaldimine (H L) 2

of type K[Ln(L) ] were prepared from alkaline solution of 2 5 9 Schiff base and lanthanide chloride . In these

complexes the Schiff base is binegative tridentate in

nature, suggesting a coordination number of six.

The rare earth perchlorate, nitrate and iodide

complexes of the Schiff base 4-N-(2'-hydroxy-l1-naphthyl-

idene)aminoantipyrine (HL) have been reported 60-62. The

perchlorate complexes 60 have the general formula

[Ln(L)2C104], where Ln = La, Pr, Nd or Sm with a

coordination number seven and [Ln(HL)4](C104)3, where

Ln = Gd, Tb, Dy, Ho or Y with a coordination number eight.

The nitrate complexes 61 of the same ligand have the

general composition [Ln(HL)2(N0 ) ] where Ln = La, Pr, Nd, 3 3

Sm, Gd, Dy, Ho and Y with coordination number seven.

Here the nitrate groups act as unidentate and HL molecules

as bidentate. This Schiff base forms two types of iodide

62 complexes , viz., [LnL21), where Ln = La, and Pr and

[L~I(HL)~I]I~, where Ln = Nd, Sm, Gd, Tb, Dy, Ho and Y.

The ligand is monovalent tridentate in La & Pr complexes

and neutral bidentate in other complexes.

The complexes of iodides, perchlorates and nitrates

of lanthanides with the ligand 4-N-(2'-hydroxybenzil-

idene)aminoantipyrine (HL) have also been reported 63,64

The perchlorate complexes are represented by

[Ln(HL)n](C104)3 where n = 4 for La, Pr or Nd and n = 5

for Ln = Sm, Gd, Tb, Dy, Ho or Y. But the iodide

complexes 6 3 is having the general formula [LII(HL)~III~,

where Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho or Y. The

nitrate complexes 64 have the general formula

[Ln(HL) (NO ) ] where Ln = La, Pr, Nd, Sm, Gd, Tb, Dy, Ho 2 3 3

or Y. Both the Schiff base and the nitrate groups are

bidentate giving a coordination number of ten in these

complexes.

CHAPTER II

REAGENTS AND PHYSICOCHEMICAL METHODS