CO-ORDINATION COMPOUND
CO-ORDINATION COMPOUND
• Coordination Compounds Introduction: The complexes show a wide variety of physical and chemical properties which are quite different from normal salts. These difference arise due to the difference in their structures. Molecular or addition compounds: When solution containing two or more salts in stoichiometric (i.e., simple molecular) proportions are allowed to evaporate, we get crystals of compounds known as molecular or addition compounds.
KCl MgCl2 6H 2 O KCl.MgCl2 .6H 2 O (carnallite)
K 2SO 4 Al 2 (SO 4 )3 24H 2O K 2SO 4 .Al2 (SO 4 )3 .24H 2O (potash alum)
Fe(CN)2 4KCN Fe(CN)2 .4KCN (potassium ferrocyanide)
These are of two types depending on their behaviour in aqueous solution. I) Double salts or Lattice compounds: The addition compounds having the following characteristic are called double salts or lattice compounds. a) They exist as such in crystalline state. b) When dissolved in water, these dissociate into ions in the same way in which the individual components of the double salts do.
FeSO4 .(NH 4 ) 2 SO 4 .6H 2O Fe2 (aq) 2NH 4 (aq) 2SO 42– (aq) Mohr 's salt
K 2 SO 4 .Al 2 (SO 4 ) 3 .24H 2 O 2K (aq) 2Al 3 (aq ) 4SO 24 – (aq) Potash alum
.II) Coordination (or complex) compounds: It has been observed that when solutions of Fe( CN) 2 and KCN are mixed together and evaporated, potassium ferrocyanide,
Fe(CN)2 .4KCN is obtained which in aqueous
solution does not give test for the Fe 2 and CN – ions, but gives the test for Fe(CN)6
K ion and ferrocyanide ion,
4
Fe(CN) 2 4KCN Fe(CN) 2 .4KCN 4K Fe(CN) 4– 6 Thus we see that in the molecular compound like
Fe(CN)2 4KCN, the individual compounds lose their identity.
Such molecular compounds are called coordination (or complex) compounds. A complex compound may contain a simple cation and a complex anion or a complex cation and a simple anion or a complex cation and complex anion, e.g. K 2 [Pt IV Cl 6 ], [Pt IV (NH 3 ) 4 Br2 ]Br2 and
[CO III (NH 3 ) 6 ][Cr III (C 2 O 4 )3 ] are all complex compounds. The term complex compound is used synonymously with the term coordination compound. In the above complex compounds, the ions
[ Pt IV Cl 4 ]2 – ,[ Pt IV ( NH 3 ) 4 Br2 ]2 ,[Co III ( NH 3 ) 6 ]3 and [Cr III (C2O 4 ) 3 ]3 – are called complex ions. Thus a complex ion is an electrically charged radical which is formed by the union of a metal cation with one or more neutral molecules or anions. Neutral complexes such as Ni CO 4 , Cr CO 6 , Co NH 3 3 Cl3 are also known.
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CO-ORDINATION COMPOUND
Different parts of Co-ordination compound :
•
COORDINATION SPHERE Aggregate of metal and ligands attached to it is called coordination sphere. It remains as a single unit in the solutions. Metal atom acts as Lewis acid and ligand acts as Lewis base in these complexes. e.g.
K4
[Fe(CN)6]
Ionic Co-oridnation Sphere sphere
•
COORDINATION NUMBER The number of lone pairs accepted by a given central atom from ligands. It may be different from number of ligands attached. In [Cu(NH3)4]+2 the co-ordination no. of Cu is 4. In [Co(en)3]3+, the co-ordination no. of cobalt is 6 and en is a bidentate ligand.
•
LIGANDS: 1.
The neutral molecules, anions or cations which are directly linked with central metal atom or ion in a complex ion are called ligands.
2.
The ligands are attached to central metal ion or atom through co-ordinate bond or dative bond.
3.
Denticity:The number of atoms (Sites) through which any ligand can attach to a metal atom is called its denticity.
Type of Ligand Monodentate Multidentate Ambidentate Bridging Ligands
Definition Ligands having only one site of attachement Ligands having many sites (atoms) through which these can attach to central metal atom Ligands which can attach a metal atom through more than one way /atom. Ligands which can be attached simultaneously to more than one metal atoms. (denoted by in normenclature)
4. Classification of Ligands:
1)
Neutral unidentate
:
2)
Univalent unidentate
: F , Cl , Br , I , OH , CN
3)
Neutral bidentate
: en, bipy, phen
4) 5) 6)
Univalent bidentate : acac, DMG, Glycine Bivalent bidentate : Oxalate, Sulphate, Carbonate Multidentate or Flexidentate : Dien, Tren, EDTA
7)
Ambidentate
: NCS , NO2 , CN
8)
Bridging
: Cl , OH , NH 2 , CO, NCS
5. SOME COMMON POLYDENTATE LIGANDS NAME ABBREVIATION Ethylenediamine
[181]
en
STRUCTURE
H2 N NH2
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CO-ORDINATION COMPOUND
N
N 2,2-bipyridyl
bipy
N
1,10-phenanthroline(phen
phen
Acetylacetanato
Acac
Oxalate
Ox
N
OOC COO
O Glycinato
NH2
gly
O
H3C Dimethylglyoximate
DMG
N C
O
C
OH N
H3C H2 N Diethylenetriamine
NH2
dien
NH CH2
Triethylenetetramine
tren
CH2 NH
CH2 NH CH2
H2N CH2
CH2
OOCH2C Ethylendiamine tetraacetate
E.D.T.A.
OOCH2C
NH2
CH2COO N CH2 CH2 N CH2COO (Hexadentate)
6. Chelation Polydentate ligands whose structure permit the attachment of their two or more donor atoms (or sites) to the same metal ion simultaneously and thus produce one or more rings are called chelate or chelating ligands (from the Greek for claw) or chelating groups. However, it should be noted that every multidentate ligand is not necessarily a chelating ligand — the coordinating atoms of the ligand may be so arranged that they cannot be coordinated to the same metal atom to produce a ring structure. Thus NH 2 — CH 2 — CH 2 — NH 2 is a chelating ligand, while
[182]
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CO-ORDINATION COMPOUND
—(CH 2 )2 — N — CH 2 — CH 2 — NH 2 is not, although both are diamines. The formation of such rings is termed chelation —(CH 2 )2 — and the resulting ring structures have been called chelate rings or chelates. The chelate rings are most stable, because of reducd strain, when they have 5 or 6 membered ring including the metal ion. The enhanced stability of complexes containing chelated ligands (i.e., multidentate ligands) is known as the chelate effect. 2
2
Cd NH 3 4 H 2 O ...............(1) Cd H 2O 4 4 NH 3 4 H 0 56.3 k J mol 1 , S 0 67.3J mol 1 K 1 , G 0 37.2 kJ / mol 2 2 Cd en 4 H 2O .....................(2) Cd H 2O 4 2en 2
H 0 56.8 kJ mol 1 , S 0 14.1 J mol 1 K 1 , G 0 60.7 kJ / mol
•
Note that H 0 is same due to formation of Cd - N bond. There is difference in entropy. It is called Entropy effect. In second case, 2en replace 4H 2 O molecules. So increase in entropy of the system. IUPAC Nomenclature Of Coordination Compounds 1. 2. 3.
Naming of salt: If the complex is a salt, the cation is named first followed by the name of the anion. For the complex entity, the name of the ligand(s) is put before the name of the metal atom. However, the reverse order is followed in writing the formula of the compound. Naming of the negative ligands: The names of all anionic ligands end in ‘o’ .
F : Fluoro, Cl : Chloro, O 2 : Oxo, CN : Cyano Cationic and neutral ligands have no special ending. There are a few exceptions like ‘aqua’ for H2O, ammine for NH3, carbonyl for CO, and nitrosyl for NO groups. 4.
Indication of the number of ligands: The number of ligands is indicated by adding prefixes di–, tri–, tetra–, penta–, hexa–, etc. for two, three, four, five, six, etc entities of the ligand. For example,
[Co(NH 3 )6 ]Cl3 will be called hexaamminecobalt(III) chloride. If the ligands are big complicated groups, instead of di–, tri–, tetra–, penta– prefixes we use bis–, tris– , tetrakis–, pentakis– etc. For instance 5.
Cu(CH 3COCHCOCH 3 ) 2 is called bis(acetylacetonato)
copper.(II) Order of Naming ligands: The ligands are quoted in alphabetical order. Numerical prefixes indicating the number of ligands are not considered in determining that order. For example, a compound like
[CoCl(NO 2 )(en) 2 ]Cl will be called chlorobis (ethylenediamine) nitrocobalt (III) chloride. 6. 7.
Oxidation state: The oxidation state of the metal ion in a complex is indicated by Roman (I), (II), (III) etc. or an Arabic (O) and placed in parenthesis immediately after the name of the metal. Naming of complex: The name of the complex anion ends in ‘ate’ and the Latin name of the metal atom is used.
8. 9.
K 2 [PtCl6 ] : Potassium hexachloroplatinate(IV), K[Ag(CN) 2 ] :Potassium
dicyanoargentate(I). A little space is given between the name of the cation and the anion. No space or hyphen is used anywhere else. Once the complex entity is completely identified according to the above rules, no mention of the number of cations or anions used for charge balancing is required. For example,
[Co(NH 3 )6 ]Cl3 is called
hexaamminecobalt(III) chloride and not hexaamminecobalt(III) trichloride. Similarly,
K 2 [PtCl6 ] is named
potassium hexachloroplatinate(IV) and not dipotassium hexachloroplatinate(IV).
[183]
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CO-ORDINATION COMPOUND
10.
Ligands having more than one donor atom: If a ligand has more than one donor atom, the actual atom involved in the bond formation with the metal ion is indicated by putting italicized symbol of the 3–
atom after the name of the ligand. For example, [ Ag (S 2O 3 ) 2 ] is called dithiosulphato S-argentate(I) ion.
11.
SCN
thiocyanato
CN - : Cyano
NO2
NCS
isothiocyanato
NC : Isocyano
ONO
nitro nitrito
Naming of Bridging ligands: A bridging group is indicated by putting the Greek letter ‘ ’immediately before its name and separated by hyphens from other ligands. For
example, H 2 O 4 Fe OH OH Fe H 2 O 4
4
is called -dihydroxobis [tetraaquairon(II)] ion and the
formula could be written as [(H 2 O) 4 Fe( OH) 2 Fe(H 2 O) 4 ]4 also. 12.
Structural information may be given in the names and formulae by prefixes such as cis–, trans– etc.
[Pt(NH 3 ) 2 Cl 2 ] can be written as cis-dichlorodiammineplatinum(II) or trans-dichlorodiammineplatinum(II), respectively. Note: When writing the formula of a complex, the central atom is listed first. The coordinated groups (i.e., ligands) are listed in the order: formally anionic ligands, neutral ligands followed by cationic ligands. Within each group, the ligands are listed alphabetically according to the first symbol Illustration 3: Write down IPUAC name of K2 [Fe(CN)3 Cl2(NH3)2] Solution: The positive part is named first followed by the negative part. In the negative part the names are written in alphabetical order followed by metal. So the name is Potassium diamminedichlorotricyano-N-ferrate (III). Illustration 4: Using IUPAC rules, write the formula for the following:Hexaamminecobalt(III) sulphate Solution: [Co(NH3)6]2(SO4)3
• •
Isomerism Compounds that have the same chemical formula but different structural arrangement are called isomers. Because of the complicated formula of many coordination compounds,the variety of bond types and the no of shapes possible, many different types of isomerism occur. Structural Isomersim i)
ii)
Polymerization Isomerism: This is not true isomerism because it occurs between compounds having the same empirical formula, but different molecular weights. For example, , [Pt(NH3)2Cl2], [Pt(NH3)4][PtCl4], [Pt(NH3)4] [Pt(NH3)Cl3]2. Ionization Isomerism: This type of isomerism is due to the exchange of groups between the complex ion and the ions outside it. [Co(NH3)5Br]SO4 is red – violet. An aqueous solution gives a white precipitate of BaSO4 with BaCl2 solution, thus confirming the presence of free SO 24 ions. In contrast [Co(NH3)5SO4]Br
iii)
[184]
is red. A solution of this complex does not give a positive sulphate test with BaCl2. It gives a pale yellow coloured precipitate of AgBr with AgNO3, thus confirming the presence of free Br– ions. Hydrate Isomerism: Three isomers of CrCl3.6H2O are known. From conductivity measurements and quantitative precipitation of the ionized chlorine, they have been given the following formulae: Complex Colour No.of AgNO3 Conc. Molar Cation cond. Exchange H SO Cl 2 4 [Cr(H2O)6]Cl3 violet 3 100% No wt.loss 430 3HCl [Cr(H2O)5Cl]Cl2.H2O
green
2
66.66%
1 H 2 O loss 220
2HCl
[Cr(H2O)4Cl2].Cl.2H2O
dark green
1
33.33%
2H 2 O loss 80
1HCl
a)
Conc. H 2 SO4 removes lattice water and not the coordinated water molecules.
b)
Molar conductivity is in Ohm1 cm2 mol 1
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CO-ORDINATION COMPOUND
c)
When the known amount of the complex is sent through cation exchange resin RSO3 H , HCl is
iv)
liberated. The acid can be estimated by titration with standard NaOH solution. Eg:- The first complex gives 3 moles of HCl per mole of the complex. Linkage Isomerism: Certain ligands contain more than one atom which could donate an electron pair. In the NO 2 ion, either N or O atoms could act as the electron pair donor. Thus there is the possibility of isomerism. Two different complexes [Co(NH3)5(NO2)]Cl2 and [Co(NH3)5(ONO)]Cl2have been prepared, each containing the NO 2 group in the complex ion.
v)
Coordination Isomerism: If the complex is a salt having both cation and anion as complex ions then the ligands can exchange position between the cation and the anion. This will result in the formation of coordination isomers. For example
Cr NH 3 6 Co CN 6 and Co NH 3 6 Cr CN 6 Pt NH 3 4 CuCl4 and Cu NH 3 4 PtCl4
[Co(en)3 ][Cr(C 2 O 4 )3 ] and [Co(en) 2 (C 2 O 4 )][Cr(en)(C 2 O4 )2 ] [Cr(en)2 (C2 O4 )][Co(en)(C2O4 )2 ] and [Cr(en)3 ][Co(C2 O4 )3 ] vi)
Coordination Position Isomerism: If in a multinuclear complex the distribution of ligands around the metal centre changes, it results in a different isomer. Such an isomerism is called coordination position isomerism. Some typical examples are : [(NH 3 ) 4 Co
[(R 3 P) 2 Pt viii)
NH 2 O2
Co(NH 3 ) 2 Cl2 ] Cl2 and [Cl(NH 3 )3 Co
NH 2 O2
Co(NH3 )3 Cl]Cl2
Cl Cl PtCl2 ] and [Cl(R 3P) Pt Pt(R 3P)Cl] , ( R3 P Tri alkyl phosphine) Cl Cl
Electronic Isomerism: The complex
[Co(NH3 )5 NO]Cl 2 exists in two forms. One is black
paramagnetic while the other is pink and diamagnetic. The black isomer is a Co(II) complex containing
• i)
neutral NO group whereas the pink one is a Co(III) complex with NO – . This kind of isomerism is known as electronic isomerism. Stereo Isomerisms: Stereoisomers have the same bonds but the arrangement of atoms in space is different. Stereoisomerism can be divided into two kinds: geometrical and optical. Geometrical isomerism or cis-trans isomerism: It occurs when ligands can assume different positions around rigid bonds with the metal ion. (1)
The compound [ Pt(NH 3 ) 2 Cl2 ]has a square planar structure. The two possible arrangements are. Cl
H3 N
Pt
Pt
Cl
H3 N
Cl
H3 N
Cl
NH3
They are differentiated by dipole moment. Cis has larger dipole moment than trans one.For square planar complexes
Ma 4 , Ma 3 b or Mab 3 where a and b are monodentate ligands, the geometrical
isomerism is not possible. The square planar complexes, Ma 2 b 2
Ma 2 bc, Mabcd and
M(AA)2 , M(AB) 2 where AA and AB represent symmetrical and unsymmetrical chelating ligands give geometrical isomers.
[185]
(2)
Geometrical Isomers - structures:
(a)
Ma 2 b 2
Pt(NH 3 ) 2 Cl2 Delhi: 48, Hasanpur, I.P. Extn., Patparganj, Pin-110092; Ph: 011-43094712, Mobile: 09811744029 Kashipur : Aryna Nagar, Near Himalyan Public School, Kashipur (Uttarakhand) Ph. : 05947-276252 Mobile : 9837617412 E-mail:
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CO-ORDINATION COMPOUND
H3 N
H3 N
Cl
Cl
Pt
Pt
H3 N
Cl
Cl
NH 3
Ma 2 bc
(b)
H3 N
Cl Pt
Pt(NH3 )2 Cl Br
Pt
H3 N (c)
NH3
Pt(NH 3 )(C 5 H 5 N)(Cl)(Br)
H3N
NC 5 H 5
H3N
NC 5 H 5
Pt
Cl
Cl
H3N
Pt Cl
(d)
Br
Br
Mabcd
Br
Cl
H3 N
Pt Br
Br
NC 5 H 5
Bridged binuclear planar complexes like
[Pt (PEt 3 )Cl 2 ]2 may exist in three isomeric forms: Et 3 P Cl
Cl Pt Cl trans–
ii)
(1)
Cl Pt PEt 3
Et 3 P Cl
Cl Pt
Pt Cl
PEt 3 Et 3 P Cl
cis–
Six coordinated octahedral complexs of the type
Et 3 P
Cl Pt
Cl Pt
Cl
Cl
unsymmetrical
Ma 4b 2 , Ma 3b 3 , Ma 3b 2c, Ma 3bcd, Ma 2b 2cd
Ma 2bcde, Mabcdef would all give geometrical isomers. Systems with one or two bidentate ligands (2)
(3)
[186]
and rest monodentate would also give geometrical isomers. A number of isomers are possible whether they can be isolated or separated is a different question which depends on so many factors. As we increase the number of different ligands, the possible number of isomers increases.
Ma 4 b 2 type of complex would give only two isomers cis–and trans–.
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CO-ORDINATION COMPOUND
(4)
Ma 3 b3 gives two isomers facial (fac–) and meridional (mer–) isomers. In the former (fac–) three ligands of one type form one triangular face of the octahedron and the other three on the opposite face. In the latter (mer–) one set of these ligands are arranged around an edge of the octahedron whereas the other set occupies the opposite edge as shown in figure.
a
a
a
b
b
b
M
M
a
b
a
b
a
b
Facial and meridional isomers of (5) iii)
Ma 3 b3 complex
Mabcdef is expected to give 15 isomers. 6 C2 . a to f are unidentate ligands.
Optical isomerism: (1) Two isomers which have almost identical physical and chemical properties like mp, bp, density, colour etc., but differ in the way they rotate the plane-polarised light are called optical isomers. (2) Optically active compounds exist in pairs and are known as stereoisomers or enantiomers. These isomers aer non-superimposable mirror images of each other. (3) Any molecule which contains either a centre of symmetry or a plane of symmetry will not show optical isomerism. (4) Optical isomerism is rarely observed in square planar complexes. Tetrahedral complexes of the type [M ( AB) 2 ] [AB = bidentate ligand] do give optical isomers as shown in figure especially where M
A A = Be, B etc.
B (5)
A M
M
A \
B
B
B
(a) Optical isomerism is very common with octahedral complexes. (b) But
Ma5b, Ma4b 2 , Ma 3b 3 , M ( AA) 3 , M ( AA) 2 ab, M ( AA)a 2b 2 , M ( AA)(BB)a 2 ,do not
show optical isomerism since they have symmetry element. (c) A few typical examples which show optical activity are [187]
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CO-ORDINATION COMPOUND 3
2
3
3
Co en 3 , Ir C2O4 3 , Cr C2O4 3 , Al C2 O4 3 (C2 O4 Oxalate) 3
CH 2 CH 2
H2N
CH 2
NH 2
H2N
H 2C
NH 2 Co
CH 2
NH 2
CH 2
and Cis Co en 2 Cl 2 are optically active. Trans isomer is not optically active sin ce it has symm etry element . Not unsymmetrical
N
N en
en
O2N
NO 2
N
N
N Co
Co
N
O2N
NO 2
en
en
N
N
Illustration 5:
H 2C
NH 2
CH 2
CH 2
C is Co en 2 N O 2
H 2C
H2N
NH 2
H2N H2N
(d)
H2N
H2N
Co
H 2C
3
CH 2
How do you distinguish between the following pairs of isomers?
i)
[Cr ( NH 3 )5 Br ]Cl and [Cr ( NH 3 )5 Cl ]Br
ii)
Co H 2O 4 Cl2 Cl.2 H 2O A and Co H 2O 6 Cl3 B
Solution:
AgNO3 reagent. One gives curdy precipitate of AgCl
i)
The isomers can be distinguished by using
ii)
soluble in ammonia while the other will form light yellow precipitate of AgBr partially soluble in ammonia. A B
AgNO 3
1 mol of AgCl
Molar Conductivity 1
2
60
3 mol of AgCl 420
1
(Ohm cm mol ) Wt.Loss on Conc. H 2 SO4 Cation exchange resin
•
i)
[188]
Loss of 2H 2 O 1 mol of HCl
No wt.loss 3 mol of HCl
RSO3 H Werner’s Theory Of Coordination Compounds(1893): Postulates Most elements exhibt two types of valenceis: (a) primary valency and (b) secondary valency. a) Primary valency: This corresponds to oxidation state of the metal ion. This is also called principal, ionisable or ionic valency. It is satisfied by negative ions and its attachment with the central metal ion is shown by dotted lines. b) Secondary or auxiliary valency: It is also termed as coordination number (usually abbreviated as CN) of the central metal ion. It is non-ionic or non-ionisable (i.e. coordinate covalent bond type). This is satisfied by either negative ions or neutral molecules. The ligands which satisfy the coordination number are directly attached to the metal atom or ion and shown by thick lines. While writing down the formulae these are placed in the coordination sphere along with the metal ion. These are directed towards fixed position in space about the central metal ion, e.g. six ligands are arranged at the six corners of a regular octahedron with the metal ion at its centre. This postulate predicted the existence of different types of isomerism in coordination complexes and after 19 years Werner actually succeeded in resolving various Delhi: 48, Hasanpur, I.P. Extn., Patparganj, Pin-110092; Ph: 011-43094712, Mobile: 09811744029 Kashipur : Aryna Nagar, Near Himalyan Public School, Kashipur (Uttarakhand) Ph. : 05947-276252 Mobile : 9837617412 E-mail:
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CO-ORDINATION COMPOUND
coordination examples into optically active isomers. H 3N Cl H 3N
NH 3
Cl
Co
NH 3
NH 3 Cl
Cl
NH Co
NH
Cl
H 3N
3
Cl
H 3N
NH 3
NH 3 Cl
C o C l 3 .5 N H 3 H 2 O o r [( C o III ( N H 3 ) 5 ( H 2 O )] 3 ( C l – ) 3
3
NH
Cl NH 3
Co
H 2O
C oC l 3 . 6 N H 3 o r [( C o III ( N H 3 ) 6 ] 3 ( C l – ) 3
H 3N
NH 3
H 3N
Cl NH 3
NH 3
Cl
Co
H 3N
3
Cl
Cl
NH 3
C o C l 3 .5 N H 3 o r [( C o II I ( N H 3 ) 5 C l ] 2 ( C l – ) 2
H3N
C o C l 3 .4 N H 3 H 2 O o r [( C o I II ( N H 3 ) 4 C l 2 ] C l –
Cl NH3 Co
Cl
Cl NH3
CoCl 3 .3NH3 or[CoIII (NH3 )3 Cl 3 ]0 ii)
Every element tends to satisfy both its primary and secondary valencies. In order to meet this requirement a negative ion may often show a dual behaviour, i.e. it may satisfy both primary and secondary valencies since in every case the fulfillment of coordination number of the central metal ion appears essential . iii) Basis of Werners theory: Different isomers, color of the complexes, precipitation, electrical conductivity, Ion exchange studies, dipole moment, dehydration temperature. Characteristic of Co(III) ammines Ammines (i.e. Complexes
•
–
No. of Cl ions Precipitated as AgCl by AgNO3
Molar Total No. of conductivity ions given range by complex -1 2 -1 (Ohm cm mol )
Charge type on ions in soln.
Ionic Formulation
CoCl3.6NH3
3(100%)
430
4
(3+,-1)
[CoIII(NH3)6]3+(Cl–)3
CoCl3.5NH3. H2O
3 (100%)
430
4
(3+,1-)
[CoIII(NH3)5(H2O)]3+(Cl–)
CoCl3.5NH3
2(66.66%)
250
3
(2+,-1)
[CoIII(NH3)5Cl]2++(Cl–)
CoCl3.4NH3
1(33.33%)
100
2
(1+,1-)
[CoIII(NH3)4Cl2]+Cl–
CoCl3. 3NH3
0 ( -- )
0
-
-
[CoIII(NH3)3Cl3]10 (non-electrolyte)
Effective Atomic Number: (EAN) 1) 2)
Sidgwick, with his effective atomic number rule, suggested that electron pairs from ligands were added until the central metal was surrounded by the same number of electrons as the next noble gas.
K 4 [Fe(CN)6 ] (potassium ferrocyanide): Fe2+ ; 24e [Fe(CN)6 ]4– = 24 (6 2) 36 (Kr - The nearest noble gas). The EAN rule correctly predicts the number of ligands in many complexes.
[189]
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CO-ORDINATION COMPOUND
3) Effective atomic numbers of some metals in complexes Atom
Atomic No.
Complex
Oxidation state
Ligand Electrons
EAN
Cr
24
[Cr(CO)6]
0
12
36
Fe
26
[Fe(CN)6]4-
2
12
36
Fe
26
[Fe(CO)5]
0
10
36
Co
27
[Co(NH3)6]3+
3
12
36
[Kr]
Ni
28
[Ni(CO)4]
0
8
36
Cu
29
[Cu(CN)4]3-
1
8
36
Pd
46
[Pd(NH3)6]4+
4
12
54
[Xe]
Pt
78
[Pt(Cl6)]2-
4
12
86
[Rn]
Fe
26
[Fe(CN)6]3-
3
12
35
Ni
28
[Ni(NH3)6]2+
2
12
38
Pd
46
[PdCl4]2-
2
8
52
2
8
84
Pt
2+
78
[Pt(NH3)4]
4) There are, however, a significant number of exceptions where the EAN is not found to have a noble gas configuration. a) Co CO 4 : 27 4 2 35 , In such cases the complex has odd electron. Co CO 4 is therefore unstable and dimerises to form Co2 CO 8 . b) Mn CO 5 : 25 5 2 35
Mn CO 5 is unstable and Mn2 CO 10 is stable. c) Co CO4 and Mn CO 5
36e
Kr are stable as they obey EAN rule.
d) V CO 6 : 23 12 35e , V CO 6 is stable. V CO 6 does not dimerise as it has complete coordination
sphere (coordination no.6) 5) Co NH 3 6
2
: 25 6 2 37 e
3
Co NH 3 6 : 24 6 2 36 e Co NH 3 6
3
is more stable than Co NH 3 6 .
2
Co NH 3 6
2
can be easily oxidised by air O2 to Co NH 3 6 .
3
6) NO is considered as 3e donor.
3CO 2 NO
From Cr CO 6 , one can get Cr CO 3 NO 2 and Cr NO 4 . No other complex can form.
Cr NO 4 : 24 4 3 36 Kr .
• 1.
Valence Bond Theory (Pauling) Pauling used hybridization theory to derive the geometry of the complex. The basis of this theory is the magnetic property of the complexes. He classified ligands into ‘Strong field’ (Low spin complexes) and ‘Weak field’ (High spin complexes). The complexes can be diamagnetic or paramagnetic. If paramagnetic, the magnetic moment
[190]
n n 2 BM
, n = no.of unpaired electrons.
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CO-ORDINATION COMPOUND
2.
Strong field and weak field ligands: (a)
Fe CN 6
4
Fe : 3d 6 4 s 2 , Fe 2 : 3d 6
d 2 sp 3 hybridization octahedral. Diamagnetic ( Experimentally shown) CN is a SFL, Low spin complex Inner orbital complex (uses inner 3d orbital) (b)
3
Fe CN 6 : Fe3 : 3d 5
3
Fe CN 6 :
d 2 sp 3 hybridization octahedral paramagnetic, low spin
1 2
3 1.73 BM
CN is a strong field ligand. Inner orbital complex. K 4 Fe CN 6 is diamagnetic and K 3 Fe CN 6 is paramagnetic. Both of them can be differentiated. (c)
FeF6
3
, Fe3 ; 3d 5
sp3 d 2 octahedral paramagnetic, HIgh spin complex. 5 5 2 35 5.87 BM outer orbital complex (uses outer 4d orbitals)
5 5 2 35 5.87 BM
Stereochemistry (a)
Ni CO 4 , Ni O ;3d 8 4s 2
(CO is a strong field ligand. The complex was found to be diamagnetic experimentally.) [191]
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CO-ORDINATION COMPOUND
sp3 Hybridization Tetrahedral (b)
NICl4
2
is found to be paramagnetic with a magnetic moment of 2.84 BM.
sp 3 , tetrahedral. (c)
Ni CN 4
2
is found to be diamagnetic.
2
Ni CN 4 : dsp 2 sq.planar (d)
PtCl4
2
is found to be diamagnetic.
Pt 2 :5d 8
dsp 2 , square planar Note that NiCl4 2 is paramagnetic but PtCl4 2 is diamagnetic. It is not that Cl is acting as weak or strong field ligand. Its field strength does not change. But the 5d orbitals in Pt are more open, outer and exposed,so it feels the Cl field strong and electrons pair up. 5 (e) [FeCl4]–. The electronic configuration of Fe3+ ion is 3d
•
Since Cl– ion is a weak field ligand it is unable to pair the unpaired electrons and hence, the Cl– ion uses 4s and 4p orbitals to form a tetrahedral complex of sp3 hybridisation. Illustration-6: Calculate the magnetic moment of the “Brown ring” complex. “Brownn ring” complex is
Fe H 2O NO 2 , x O 1 2, x 1 5
Fe : 3d 6 4 s 2 , Fe 1 : 3d 6 4 s1 Fe 1 :
Fe H 2O 5 NO
2
sp 3 d 2 , octahedral ‘outer orbital complex. Paramagnetic,
3 3 2 15 3.87 BM .
Illustration-7:. All octahedral Ni(II) complexes are paramagnetic and outer orbital complexes. Explain.
[192]
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CO-ORDINATION COMPOUND 2
Ni NH 3 6 , Ni 2 ; 3d 8 2
Ni NH 3 6 :
sp 3 d 2 , 2.84 BM On pairing up, we do not get
Ni H 2O 6
d 2 sp 3 . So no pairing up.
2
sp 3 d 2 , 2.84 BM Irrespective of whether the ligand is strong field or weak field ligand all the complexes are outer orbital and paramagnetic. Illustration 8: [Co(NH 3 ) 6 ]3 is diamagnetic and [CoF6 ]3– is strongly paramagnetic. Explain Solution:
Co3 diamagnetic due to paired electrons
[Co(NH 3 ) 6 ]3
d 2sp3 [CoF6 ]3–
sp3d 2 Paramagnetic due to four unpaired electrons
Co 3 has 3d 6 configuration with four unpaired electrons in ground state. In presence of
NH3 (strong
ligand)
all
the
unpaired
electrons
in
Co3 get
paired
and
thus
[Co(NH 3 ) 6 ]3 has d 2sp3 hybridization (octahedral structure), thus it is diamagnetic (no electron unpaired).
F – is a weak ligand hence six lone pairs of six 3– 3 2 F – are filled in outer d-orbitals of Co3 which has now four electrons unpaired. Thus CoF6 has sp d hybridization
in Co 3 and is thus paramagnetic due to unpaired electrons.
•
FAILURE OF VB THEORY: (a)
Pauling VB theory could not explain satisfactorily the geometry of
2
Cu NH 3 4 . Cu 2 3d 9
2
Cu NH 3 4 : sp3 hybridization, Tetrahedral. But the actual geometry (experimentally) is found to be square planar. This was explained as 2
Cu NH 3 4 : (b)
[193]
It could not explain the color of the complexes. Delhi: 48, Hasanpur, I.P. Extn., Patparganj, Pin-110092; Ph: 011-43094712, Mobile: 09811744029 Kashipur : Aryna Nagar, Near Himalyan Public School, Kashipur (Uttarakhand) Ph. : 05947-276252 Mobile : 9837617412 E-mail:
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CO-ORDINATION COMPOUND
(c)
It could not explain why certain ligands act as strong field ligands and others as weak field ligand.
Table: Hybridization and Geometry of the complexes:
Coordination Hybridization No. 2 Sp
Geometry Linear
Examples
Ag NH3 2 Ag CN 2
1
1
1
Cu NH 3 2 , Cu CN 2
Au CN 2 , Au NH3 2 3 (rare) 4
Sp 2 Sp3
Trigonal planar Tetrahedral
HgI3
1
2
1
BeCl4 , AlCl4 , FeCl4 ,
CoX 4 X Cl, Br , I , NCS 2 ZnX 4 X Cl , Br, I , NCS 2 CdX 4 X Cl, Br, I , NCS 2 HgX 4 X Cl, Br, I , NCS 2 2 MnCl4 , MnBr4 , Ni CO 4 , 2
2
2
NICl4 , SnCl4 , GeCl4 4
dsp 2
Square planar
2
2
Ni CN 4 , Ni DMG 2 DMG Dimethyl glyoxime ,
PtX 4 ( X Cl, Br, I , NCS , CN ) 2 PdX 4 X Cl , Br, I , NCS , CN 2 Pt NH 3 2 Cl2 , Pt en 2 Fe CO 5 , Zn tu 3 SO4 2
5 (rare)
sp3d
5 (rare) 6
3
dsp
d 2 sp3
Trigonal bipyramid Square pyramid Octahedral
Ctu Itiourea SO4 Ni CN 5
2
is abidentate ligand)
3
4
Fe CN 6 , Fe CN 6 3
Co NH3 6 Co en 3 3
3
3
Cr NH3 6 , Cr CN 6 6
Sp3d 2
Octahedral
3
3
Fe H 2O 6 , FeF6 , MN H 2O 6 3
2
2
2
Ni NH 3 6 Ni H 2O 6 , Ni en 3 7,8,9,12 (rare)
•
ZrF7
3
2
3
, Nd H 2O 9 , Lu H 2O 12
Ligand field theory Shapes of d-Orbitals: Since d orbitals are often used in coordination complexes it is important to study their shapes and distribution in space. The five d orbitals are not identical and the orbitals may be divided into two sets. The three t 2g orbitals have identical shape and point between the axes, x, y and z. The two eg orbitals have different shapes and point along the axes. Alternative names for t 2g and eg are
[194]
d and d respectively.
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CO-ORDINATION COMPOUND
x
x t 2g orbitals (d )
-
+
+
-
+
+
z
y
+
-
z +
d xz
d xy
•
y
d yz
(1) Ligand Field Splitting in Octahedral Complex: Let us consider the case of six ligands forming an octahedral complex. For convenience, we may regard the ligands as being symmetrically positioned along the axes of a Cartesian co-ordinate system with the metal ion at the origin. To simplify the situation, we can consider an octahedral complex as a cube, having the metal ion at the centre of the body and the 6 ligands at the face centres
and if we take the metal ion as the origin of a Cartesian co-ordinate, the ligands will be along the axes. It is obvious that not all of the orbitals will be affected to the same extent when the ligands approach the metal ion.
The orbitals lying along the axes d z2 and d x 2 y 2 will be more strongly repelled than the orbitals with lobes directed between the axesdxy, dxz, dyz). The d-orbitals are thus split into two sets with the
d z 2 and d x2 – y 2 at a
higher energy than the other three.
d
z2
, d x 2 y2
0
dxy, dyz, dxz
•
Outer orbital and Inner orbital complexes a)
In a weak ligand field such as [CoF6]3–, the approach of the ligand causes only a small split in the
energy level. O < P(pairing energy)
[195]
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CO-ORDINATION COMPOUND
4 4 2 24 4.87 B.M
Since the ligand is a weak field ligand, its repulsions with the electrons in d z 2 and d x 2 y2 orbitals are very less (or) in other words we can say that the electrons in d z 2 and d x 2 y2 cannot move away from the approaching ligands since they have insufficient energy to pair up with the electrons in dxy, dyz and dxz orbitals. Thus there are no vacant orbitals in the 3d shell and the ligands occupy the first six vacant orbitals (one 4s, three 4p and two 4d). Since outer d orbitals are used, this is an outer orbital complex. The orbitals are hybridised and are written sp3d2 to denote this. Since none of the electrons has been forced to pair off, this is a high spin complex and will be strongly paramagnetic because it contains 4 unpaired 3d electrons. (b)
Under the influence of a strong ligand field as in the complex [Co(NH3)6]3+, the approach of the ligand causes a greater split in the energy level. 0 > P
Since, the split is very high, we can say that the energy difference between the two sets of orbitals is much greater and this energy difference is sufficient to allow the electrons in d z 2 and d x 2 y2 orbitals to move into the half filled dxz, dxy and dyz orbitals, even though this pairing requires energy. We can also view this like, the ligand repel the electrons in higher energy level to an extent such that they get paired up against Hund’s rule The d z 2 and d x 2 y2 orbitals become vacant. The six ligands each donate a lone pair to the first six vacant orbitals, which are: two 3d, one 4s and three 4p. Inner d-orbitals are used and so this is an inner orbital complex. The orbital are hybridised and written d2sp3 to denote the use of inner orbitals. Since, the orginal unpaired electrons have been forced to pair off, there is a low spin complex and is in fact diamagnetic. The inner and outer orbital complexes may be distinguished by magnetic measurements. Since the outer orbital complexes use high energy levels, they tend to be more reactive. The inner orbitals are sometimes called inert orbitals. (c) Ions
[196]
Distribution of d-electrons in t 2g and eg sets in strong(er) and weak (er) octahedral ligand fields. Strong (er) field (low spin or spin paired 0 > P) Complexes) (
Weak (er) field (high -spin or spin free 0
