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1. INTRODUCTION
Co-ordination compounds play a vital role. The importance can be realised that life would not have been possible without the existence of chlorophyll (Mg - complex) in plants and haemoglobin (Fe- complex) in the blood of human beings. The study of these compounds will enlarge our understanding of chemical bonding, physical properties such as magnetic properties of co-ordination compounds
2. MOLECULAR OR ADDITION COMPOUNDS
When solution containing two or more simple stable compounds in molecular proportions are allowed to evaporate, crystals of new substances called molecular or addition compounds are obtained. Example KCl + MgCl 2 + 6H 2 O (Carnallite )
KCl.MgCl 2. 6 H 2 O
CuSO 4 + 4 NH 3 (Tetrammin sulphate)ecopper (II)
[Cu(NH 3 ) 4 ]SO 4
Fe(CN) 2 + 4KCN (Potassiumferrocyanide )
K 4 [Fe(CN) 6 ]
2.1 Types of Molecular compounds
2.1.1 Double Salt A double salt is a substance obtained by the combination of two different salts which crystallize together as a single substance but ionise as two distinct salts when dissolved in water. These salts lose their identity in solution i.e. when dissolved in water they give test of all the ions present in the salt. eg. Potash alum, Mohr’s salt FeSO 4. (NH 4 ) 2 SO 4 .6H 2 O Fe2+^ (aq) + 6H 2 O + 2NH 4 +^ (aq) (Mohr’s salt) + 2 SO 4 2–^ (aq) K 2 SO 4. Al 2 (SO 4 ) 3. 24 H 2 O 2K+^ (aq) + 2Al3+^ (aq) +
(Potash alum) 4SO 4 3–^ (aq) + 24H 2 O
2.2 Coordination Compounds A coordination compound is a molecular compound that results from the combination of two or more simple molecular compounds and retains its identity in the solid as well as in dissolved state
Example
[Cu (NH 3 ) 4 ]SO 4 [Cu (NH 3 ) 4 ]2+^ + (^) SO^24
K 4 [Fe(CN) 6 ] 4K+^ + [Fe (CN) 6 ]4–
3. COORDINATION COMPOUNDS
A Co-ordination compound consists of a ligand, central atom, complex ion, a cation or an anion. The complex ion is generally written in a square box and the ion (cation or anion) is written outside complex ion. eg : [Co (NH 3 ) 6 ] Cl 3 [Complex ion] anion eg : K 4 [Fe (CN) 6 ] cation [Complex ion] General formula : Ax [MLn]/[MLn]By where : M is the central metal atom/ion L is the ligand A is the cation B is the anion Some Important Terms pertaining to Coordination Compounds
3.1 Coordination entity
It is the central metal atom or ion which is bonded to a definite number of ions or molecules which is fixed. For example, in [Co(NH 3 ) 6 ]Cl 3 , a coordination entity, six ammonia molecules are surrounded by three chloride ions.
3.2 Central atom/ion
It is the central cation that is surrounded and coordinately bonded to one or more neutral molecules or negatively charged ions in a definite geometrical arrangement. For example, in the complex [Co(NH 3 ) 6 ]Cl 3 , Co3+^ represents the central metal ion which is positively charged and is coordinately bonded to six neutral NH 3 molecules within the coordination sphere. The central metal/ion is also referred to as Lewis acid.
COORDINATION COMPOUNDS
3.3 Ligands
The ions or molecules bound to the central atom/ion in the coordination entity are called ligands. These may be simple ions such as Cl–, small molecules such as H 2 O or NH 3 , larger molecules such as H 2 NCH 2 CH 2 NH 2.
3.4 Co-ordination Number (C.N)
The number of atoms of the ligands that directly bound to the central metal atom or ion by co-ordinate bonds is known as the co-ordination number of the metal atom or ion. It is also equal to the secondary valency.
Complex Co-ordination numbers
K 4 [Fe (CN) 6 ] 6
[Ag (CN) 2 ]–^2
[Pt (NH 3 ) 2 Cl 2 ] 4
[Ca (EDTA)]2–^6
3.5 Coordination sphere
The central metal atom or ion and the ligands that are directly attached to it are enclosed in a square bracket. This had been called coordination sphere or first sphere of attraction. It behaves as a single unit because the ligands present in the coordination sphere are held tightly by the metal ion.
3.6 Co-ordination Polyhedron
A coordination polyhedron is the spatial arrangement of the ligand atoms that are directly attached to the central atom/ion. For example, [Co(NH 3 ) 6 ]3+^ is octahedral, [Ni(CO) 4 ] is tetrahedral and [PtCl 4 ]^2 is square planar.
3.7 Oxidation Number of Central Metal Atom
It is defined as the charge that the central metal ion would carry if all the ligands are removed along with electron pairs. It is calculate as follows :
Example K 4 [Fe (CN) 6 ] K 4 [Fe (CN) 6 ] 4 K+^ + [Fe (CN) 6 ]4– Charge on complex ion = – 4 Let charge on Fe = x, Now charge on cyanide ion (CN–) = – x + 6 × (–1) = – 4 x = + 2 Hence oxidation no of Fe = + 2 (or II)
3.8 Homoleptic and Hetroleptic Complexes
Complexes in which central atom is coordinated with only one kind of ligands are called homoleptic complexes, eg. [Co(NH 3 ) 6 ]3+. Complexes in which central atom is coordinated with more than one kind of ligands are called hetroleptic complexes, eg. [Co (NH 3 ) 4 Cl 2 ]+.
4. NOMENCLATURE OF COORDINATION
COMPOUNDS
4.1 Nomenclature
Following rules are adopted for naming a complex ion; (a) Cations are named before anions (b) Oxidation state (O.S.) of the central metal ion is denoted by Roman numeral.
Compound Cation O.S. anion
CuCl Copper (I) chloride CuCl 2 Copper (II) chloride FeCl 2 Iron (II) chloride FeCl 3 Iron (III) chloride
(c) The names of ligands are given first followed by the name of the central metal ion. (d) The names of ligands that are anions and ending with
‘ide’ are changed to ‘o’ ‘ite’ are changed to ‘ito’ ‘ate’ are changed to ‘ato’ (e) Many ligands that are molecules carry the unmodified name
(f) Positive groups end in – ium
hydrazinium.
EXAMPLE : What are the secondary valency of [Co (NH 3 ) 6 ] Cl 3 & K 4 [Fe (CN) 6 ]?
Sol. In [Co (NH 3 ) 6 ] Cl 3 the secondary valency is 6.
K 4 [Fe (CN) 6 ] : six ligands are coordinated to Fe. Hence secondary valency is 6. The primary valency is satisfied by ions attached to the complex ions. It is shown by dotted lines. Primary valency is also known as ionisable valency. The secondary valency is satisfied by the ligands, they are non ionisable and are shown by a solid line [Co (NH 3 ) 6 ] Cl 3 can be represented as
An anion present in co-ordination and ionization sphere is shown by Every element tends to satisfy both its primary and secondary valencies. A negative ion when present in the coordination sphere shows a dual behaviour. It may satisfy both primary and secondary valencies.
The ligand which satisfy the secondary valencies are directed toward fixed positions in space. The geometry of the complex ion depends on the coordination number. If the metal has coodination number 6, the complex is octahedral, i.e. six positions around the metal are occupied by six donor atoms of the ligands octahedrally. On the other hand, if the coordination number is 4, the geometry of the complex may
be tetrahedral or square planar. This postulate predicted the existence of different types of isomerism in coordination compounds.
EXAMPLES :
Octahedral Square planar Tetrahedral
(C.N = 6) (C.N = 4) (C.N. = 4)
[Cr(CH 3 ) 6 ]3+^ [Ni(CN) 4 ]2–^ [Ni(CO) 4 ]
[Co(NH 3 ) 6 ]3+; [Cr(H 2 O) 6 ]3+^ [Ni(NH 3 ) 4 ]2+^ [CuX 4 ]2–;[ZnCl 4 ]2–
[Fe(CN) 6 ]2–; [Fe(F 6 )]3–^ [Cu(NH 3 ) 4 ]2+^ [NiX 4 ]2–
[Pt(NH 3 ) 6 ]4+; [PtCl 6 ]2–^ X = Cl–, Br–, I–
Familiar C.N.’s of some common metal ions.
Univalent C.N. Divalent C.N.
Ag+^2 V2+^6 Au+^ 2, 4 Fe2+^6 Ti+^2 Co2+^ 4, 6 Cu+^ 2, 4 Ni2+^ 4, Cu2+^ 4, 6 Zn2+^4 Pd2+^4 Pt2+^4 Ag2+^4
Trivalent C.N. Tetravalent C.N.
Sc3+^6 Pt4+^6 Cr3+^6 Pd4+^6 Fe3+^6 Co^3 + 6 Os^3 +^6 Ir3+^6 Au3+^4
6. EFFECTIVE ATOMIC NUMBER (EAN)
Sidgwick proposed effective atomic number abbreviated as EAN, which is defined as the resultant number of electrons with the metal atom or ion after gaining electrons from the donor atoms of the ligands. The effective atomic number (EAN) generally coincides with the atomic number of next
7. VALENCE BOND THEORY
The bonding in coordination compounds can be explained by Valence Bond Theory (VBT) since majority of the complexes formed by the transition metals have their d- orbitals incomplete. Valence bond takes into account the hybridisation of orbitals since penultimate d-orbitals are near in energy to s and p-orbitals of the outer most shell, various kinds of hybridization is possible. VBT makes the following assumption
(i) A number of empty orbitals are available on the central metal ion which can accomodate electrons donated by the ligands. The number of empty d-orbitals is equal to the coordination number of the metal ion for the particular complex.
(ii) The metal orbitals and ligand orbitals overlap to form strong bonds. Greater the extent of overlapping, more is the stability of the complex. Different orbitals (s, p or d) hydridize to give a set of equivalent hybridized orbital which take part in bonding with the ligands.
(iii) Each ligand donates a pair of electrons to the central metal ion/atom.
(iv) The non-bonding metal electrons present in the inner orbitals do not take part in chemical bonding.
(v) If the complex contains unpaired electrons, the complex is paramagnetic. If it does not contain unpaired electron, the complex is diamagnetic in nature.
(vi) Under the influence of strong ligand (CN, CO) the electrons can be forced to pair up against the Hund’s rule of multiplicity.
Complex Metal (Oxid. state) At. No. of metal Coordination number Effective atomic number
K 4 [Fe (CN) 6 ] + 2 26 6 (26 – 2) + (6 × 2) = 36 [Kr]
[Cu (NH 3 ) 4 ] SO 4 + 2 29 4 (29 – 2) + ( 4 × 2) = 35
[Co (NH 3 ) 6 ] Cl 3 + 3 27 6 (27 – 3) + (6 × 2) = 36 [Kr] Ni (CO) 4 0 28 4 (28 – 0) + (4 × 2) = 36 [Kr] K 2 [Ni(CN) 4 ] + 2 28 4 (28 – 2) + (4 × 2) = 34.
COMMON TYPES OF HYBRIDISATION
Coordination Hybridi- Shape Geometry Number zation
2 sp Linear X — A — X
4 sp^3 Tetrahedron
4 dsp^2 Square planar
5 sp^3 d Trigonal
or dsp^3 bipyramid
6 d^2 sp^3 Octahedral
or sp^3 d^2
inert gas in some cases. EAN is calculated by the following relation : EAN = Atomic number of the metal – number of electrons lost in ion formation + number of electrons gained from the donor atoms of the ligands. (2 × CN) The EAN values of various metals in their respective complexes are tabulated below :
EFFECTS OF CRYSTAL FIELD SPLITTING
CFSE and electronic arrangements in octahedral complexes Number Arrangement in weak ligand field Arrangement in strong ligand field of d
electrons t2g eg CFSE Spin only t2g eg CFSE Spin only o magnetic moment o magnetic moment s (D) s (D)
d^1 1.73 1.
d^2 2.83 2.
d^3 3.87 3.
d^4 4.90 2.
d^5 5.92 1.
d 6 4.90 0.
d^7 3.87 1.
d^8 2.83 2.
d^9 1.73 1.
d^10 0.00 0.
8.2 Tetrahedral Complexes A regular tetrahedron is related to a cube. One atom is at the centre of the cube, and four of the eight corners of the cube are occupied by ligands as shown.
The directions x, y and z point to the centres of the faces of the cube. The e orbitals point along x, y and z axes (that is to the centres of the faces). The t 2 orbitals point between x, y and z axes (that is towards the centres of the edges of the cube). The direction of approach of the ligands does not coincide exactly with either the e or the t 2 orbitals.
Thus the t 2 orbitals are nearer to the direction of the ligands than the e orbitals. The approach of the ligands raises the energy of both sets of orbitals. The energy of the t 2 orbitals is raised most because they are closest to the ligands. The crystal field splitting is the opposite way round to that in octahedral complexes The t 2 orbitals are 0.4t above weighted average energy of the two groups (the Bari centre) and the e orbitals are 0.6t below the average. The magnitude of the crystal field splitting t in tetrahedral complexes is considerably less than in octahedral fields. There are two reasons for this :
- There are only four ligands instead of six, so the ligand field is only two third the size ; hence the ligand field splitting is also two third the size.
- The direction of the orbitals does not coincide with the direction of the ligands. This reduces the crystal field splitting by roughly a further two third.
9. ORGANOMETALLIC COMPOUNDS
Compounds that contain at least one carbon–metal bond are called organometallic compounds.
Grignard reagent, RMgX is a familiar example of organometallic compounds where R is an alkyl group. Diethyl zinc [Zn(C 2 H 5 ) 2 ], lead tetraethyl [Pb(C 2 H 5 ) 4 ], ferrocene [Fe(C 5 H 5 ) 2 ], dibenzene chromium [Cr(C 6 H 6 ) 2 ], metal carbonyls are other examples of organometallic compounds.
Organometallic compounds may be classified in three classes :
- Sigma ( ) bonded complexes.
- Pi ( ) bonded complexes,
- Complexes containing both – and –bonding characteristics.
9.1 Sigma bonded complexes
In these complexes, the metal atom and carbon atom of the ligand are joined together with a sigma bond, i.e., the ligand contributes one electron and is, therefore, called one electron donor. Examples are :
(i) Grignard reagent, R–Mg–X where R is an alkyl or aryl group and X is halogen.
(ii) Zinc compounds of the formula R 2 Zn such as (C 2 H 5 ) 2 Zn. This was first isolated by Frankland in 1849. Other similar compounds are (CH 3 ) 4 Sn, (C 2 H 5 ) 4 Pb, Al 2 (CH 3 ) 6 , Al 2 (C 2 H 5 ) 6 and Pb(CH 3 ) 4 , etc.
9.2 –bonded organometallic compounds
These are the compounds of metals with alkenes, alkynes, benzene and other ring compounds. In these complexes, the metal and ligand form a bond that involves the electrons of the ligand. Three common examples are Zeise’s salt, ferrocene and dibenzene chromium. These are shown here :
The number of carbon atoms bound to the metal in these compounds is indicated by the Greek letter ‘ ’ (eta) with a number. The prefixes 2 , 5 and 6 indicate that 2, 5 and 6 carbon atoms are bound to the metal in the compound.
9.3 – and –bonded organometallic compounds
Metal carbonyls, compounds formed between metal and carbon monoxide belong to this class. These compounds posses both – and – bonding. The oxidation state of metal atoms in these compounds is zero. Carbonyls may be monomeric, bridged or polynuclear.
In a metal carbonyl, the metal–carbon bond possesses both the – and –character. A –bond between metal and carbon atom is formed when a vacant hybrid orbitals of the metal atom overlaps with an orbital on C atom of carbon monoxide containing a lone pair of electrons.
Formation of –bond is caused when a filled orbital of the metal atom overlaps with a vacant antibonding * orbital of C atom of carbon monoxide. This overlap is also called back donation of electrons by metal atom to carbon. It has been shown below :
The –overlap is perpendicular to the nodal plane of –bond. In olefinic complexes, the bonding –orbital electrons are donated to the empty orbital of the metal atom and at the same time back bonding occurs from filled orbital of the metal atom to the antibonding –orbital of the olefin.
Thus the tetrahedral crystal field splitting t is roughly 2/3 × 2/3 = 4/9 of the octahedral crystal field splitting o.
geometrical isomers and the phenomenon is called geometrical isomerism. 10.2.1.1 Geometrical Isomerism in square planar complexes A square planar complexe having similar ligands at adjacent positions (90º a part) is called cis - isomer while a square planar complex having two similar ligands at opposite positions (180º a part) is called trans-isomer.
- Ma 2 b 2
Draw the geometrical isomers of [PtCl 2 (NH 3 ) 2 ]
Sol.
- Ma 2 bc
Draw the geometrical isomers of [PtCl 2 (NH 3 )py]
Sol.
- Mabcd
Draw the geometrical isomers of [PtClBrpy (NH 3 )]
Sol.
4. M (AB) 2
Draw the geometrical isomers of [Pt(gly) 2 ]
Sol.
Example - 1
Example - 2
Example - 3
Example - 4
10.2.1.2 Geomertical Isomerism in octahedral complexes
- Ma 4 b 2
Draw the geometrical isomers of [CrCl 2 (NH 3 ) 4 ]+
Sol.
- Ma 3 b 3
Draw the geometrical isomers of [RhCl 3 (py) 3 ]
Sol.
- Mabcdef : They form 15 isomers
- M (AB) 3
Draw the geometrical isomers of [Cr(gly) 3 ]
Sol.
5. M(AA) 2 b 2
Draw the geometrical isomers of [CoCl 2 (en) 2 ]+
Sol.
Cis and trans-isomers of [CoIII^ (en) 2 Cl 2 ]+^ ion. (a) Cis-isomer (b) trans-isomer
Example - 5
Example - 6
Example - 7
Example - 8
2. M (AA) 3
Draw the optical isomers of [Co(en) 3 ]3+
Sol.
The two optical isomeric forms of the complex [Co(en) 3 ]3+
- M (AB) 3
Draw the optical isomers of [Cr(gly) 3 ]
Sol.
- cis M (AA) 2 b 2
Draw the optical isomers of RhCl 2 (en) 2 ]+
Sol.
Optical active isomers of cis [RhCl 2 (en) 2 ]+
- cis Ma 2 b 2 c 2
Draw the optical isomers of [PtCl 2 Br 2 (NH 3 ) 2 ]
Sol.
- cis M(AA)b 2 c 2
Draw the optical isomers of [CoCl 2 (en) (NH 3 ) 2 ]+
Sol.
- cis M(AA) 2 bc
Draw the optical isomers of [CoCl (en) 2 Br]2+
Example - 13
Example - 14
Example - 15
Example - 16
Example - 17
Example - 18
11. STABILITY OF COORDINATION COMPOUNDS
The stability of a complex in solution refers to the degree of association between the two species involved in the state of equilibrium. If we have a reaction of the type :
M + 4L ML 4 then the larger the stability constant, the higher the proportion of ML 4 that exists in solution. Free metal ions rarely exist in the solution so that M will usually be surrounded by solvent molecules which will compete with the ligand molecules, L, and be successively replaced by them. For simplicity, we generally ignore these solvent molecules and write four stability constants as follows :
M + L ML K 1 = [ML]/[M][L]
ML + L ML 2 K 2 = [ML 2 ]/[ML][L]
ML 2 + L (^) ML 3 K 3 = [ML 3 ]/[ML 2 ][L]
ML 3 + L (^) ML 4 K 4 = [ML 4 ]/[ML 3 ][L]
where K 1 , K 2 , etc., are referred to as stepwise stability constants. Alternatively, we can write the overall stability constant thus :
M + 4L ML 4 4 = [ML 4 ]/[M][L]^4
The stepwise and overall stability constant are therefore related as follows :
4 = K 1 × K 2 × K 3 × K 4 or more generally, n = K 1 × K 2 × K 3 × K 4 ................. Kn The instability constant or the dissociation constant of coordination compounds is defined as the reciprocal of the formation constant.
12. IMPORTANCE AND APPLICATIONS OF
COORDINATION COMPOUNDS
- Analytical chemistry : The analytical applications of coordination chemistry are in (a) Qualitative and quantitative analysis : Metals ions form colored coordination compounds on reaction with a number of ligands. These reactions are used for detection of the metal ions. The colored complexes formed can be used for the estimation of metals by classical or instrumental methods such as gravimetry or colorimetry. Some examples are given as follows : The presence of iron ions (Fe3+) can be detected by the addition potassium ferrocyanide solution, which results in formation of Prussian blue complex. Fe2+^ + K 3 Fe(CN) 6 KFe[Fe(CN) 6 ] + 2K+
(b) Volumetric analysis : Hardness of water can be estimated by titration with EDTA. The metal ions causing hardness, that is Ca2+^ and Mg2+, form stable complexes with EDTA.
- Metal extraction and purification : Extraction of metals, such as silver and gold, is carried out by forming their water soluble cyanide complexes with the ore. Pure gold can then be obtained from the solution by addition of zinc. Similarly, metals can be purified by formation and then decomposition of their coordination compounds. For example, impure nickel obtained after extraction may be converted into pure nickel by first converting it to nickel carbonyl and then decomposing.
- Catalysis : Coordination compounds are used as catalysts in important commercial processes. For example, (a) The Zeigler-Natta catalyst (TiCl 4 and trialkyl aluminium) is used as a catalyst in the formation of polyethene. (b) The Wilkinson catalyst - RhCl(PPh 3 ) 3 is used in the hydrogenation of alkenes. (c) In the Monsanto acetic acid process, various rhodium complexes, such as [Rh(CO) 2 I 2 ], [Rh(Cl)(CO)(PPh 3 ) 2 ] or [Rh(Cl)(CO) 2 ] 2 , are used as catalyst in the presence of CH 3 I, I 2 or HI as activator.
- Electroplating : Coordination compounds of gold, silver and copper are used as components in the baths used for electroplating articles of other metals with these metals. For example, in silver plating, K[Ag(CN) 2 ] is used as an electrolyte; in gold plating, K[Au(CN) 2 ] is used as an electrolyte; and in copper plating, K 3 [Cu(CN) 4 ] is used as an electrolyte.
- Biological importance : Some important biological compounds are coordination complexes. For example, chlorophyll is a complex of Mg2+. This green pigment plays a vital role in photosynthesis in plants. Similarly, haemoglobin, the red pigment present in blood, is a coordination complex of Fe2+^ and vitamin B 12 , an essential nutrient, is a complex compound of Co3+.
- Medicinal uses : Complexing or chelating agents are used in treating metal poisoning, wherein, the coordination complex is formed between toxic metal in excess metal and the complexing agent. For example, EDTA is used in lead poisoning. EDTA, when injected intravenously into the bloodstream, traps lead forming a compound that is flushed out of the body with the urine. Other heavy metal poisonings that can be treated similarly with chelation therapy are mercury, arsenic, aluminium, chromium, cobalt, manganese, nickel, selenium, zinc, tin and thallium. Similarly, chelating ligands D-penicillamine and desferrioxime B are used for removal of excess copper and iron, respectively. New potent drugs are being created using various derivatives of metallocene. A platinum complex [PtCl 2 (NH) 32 ] called cis- platin is used in treatment of cancer.
(c) Tridentate Ligand
The ligands having three donor atoms are called tridentate ligands. Example :
(d) Tetradentate ligand
These ligand possess four donor atoms
Example :
(e) Pentadentate ligands
They have five donor atoms
Example :
(f) Hexadentate Ligands They have six donor atoms.
Example :
14.2.2 Chelating ligands : A bidentate or a polydentate ligand is known as a chelating ligand if on co-ordination it results in the formation of a cyclic ring structure. The complex thus formed are called chelates.
The chelates containing 5 or 6 membered rings are more stable. Ligands with larger groups form more unstable rings than with smaller groups due to steric hinderance.
14.2.3 Ambidentate ligands : The ligands which have two donor atoms but in forming complexes only one donor atom is attached to the metal atom at a given time. Such ligands are called ambidentate ligands.
Example :
15. COORDINATION NUMBER (C.N)
The number of atoms of the ligands that directly bound to the central metal atom or ion by co-ordinate bonds is known as the co-ordination number of the metal atom or ion. It is also equal to the secondary valency.
Complex Co-ordination numbers
K [Fe (CN) 6 ] 6 [Ag (CN) 2 ]–^2 [Pt (NH 3 ) 2 Cl 2 ] [Ca (EDTA)]2–^6
16. STABILITY OF COORDINATION COMPOUNDS
IN SOLUTIONS
16.1 In general, higher the charge density on the central ion, the greater is the stability of its complexes, i.e., the higher value
of
charge (^) , radius of the ion the greater is the stability of its complexes. Electronegativity of the central ion influences the stability. The higher the electronegativity of the central ion, the greater is the stability of its complexes. 16.2. The higher the oxidation state of the metal, the more stable is the complex. The charge density of Co3+^ ion is more than Co2+^ ion and thus, [Co (NH 3 ) 6 ]3+^ is more stable than [Co (NH 3 ) 6 ]2+. Similarly, [Fe (CN) 6 ]3–^ is more stable than [Fe (CN) 6 ]4–. The cyano and ammine complexes are far more stable than those formed by halide ions. This is due to the fact that NH 3 and CN–^ are strong Lewis bases.
The complexes of bivalent cations (M2+) of 3d-series shown the following order of stability :
Cation Mn2+^ Fe2+^ Co2+^ Ni2+^ Cu2+ Ionic size 0.91 0.83 0.82 0.78 0.69 decreases
Stability of increases
the complex (Irving William order)
Chelating ligands form more stable complexes as compared to monodentate ligands. Greater is the chelation, more is the stability of complex.
