 Most amazing field of inorganic chemistry is coordination chemistry.  Transition metal possess characteristic property of undergoing complex formation.  Coordination compounds are also formed by several non –transitional metals.  The branch of chemistry which deals with the study of coordination compounds or complex compound is known as Coordination chemistry.  Coordination compounds are widely distributed in minerals, plants, animals,they are play important biological functions.

# Chlorophyll is a coordination compound containing Mg. The red pigment hemoglobin present in blood, which is responsible for the regulation of respiration process in animal world, it is a coordination compound of iron. # A variety of metallurgical process, catalysts and analytical reagents make use of coordination compounds. # The systematic attempt of explaining the formation,reaction, structure of bonding of coordination compound was made by Alfred Werner.

Coordination compound contain a central metal ion surrounded by, number of oppositely charged ions or neutral molecules. Eg:- tetra ammine copper(ll) ion, [Cu(NH 3 ) 4 ] 2+  In this complex copper is the central metal ion while ammonia is the neutral molecule.  In this theory used two types of,linkages, such as primary and secondary by metal atom / ion in a coordination compound.  primary linkage is considered as ionic bond while secondary linkage is covalent bond.

Werner’s Theory of coordination compounds  Basic Postulates: 1)Most of the metallic elements exhibits two types of valence. Primary valence and secondary valence. 2)Every metal tend to satisfy both valences. 3)Every metal has fixed number of secondary valence. 4)The secondary valence is always directed toward fixed position in space.

Salient features  Principal valence is also known as,primary valence or ionisable valence, designated by solid line ( ____ ). In modern term it corresponds to the oxidation state of metal ion.  The secondary valence is called as, non-ionisable or subsidiary valence. It is designated by dotted line (……..).  There are two sphere of attraction around the metal ion in a complex, inner sphere and outer sphere.

 Groups present in the inner sphere are, firmly attached to the metal ion,and therefore cannot be separated easily.  The primary valences are those which a metal ion exercises towards the negative groups so that its normal charge is satisfied by the formation of simple salts. Primary valences of Co, Cu, Ag, are 3,2,1. to produces simple salts.

 Secondary valences are satisfied by –ve ions or neutral molecules.  Co has secondary valence is 6,Co forms a complexes like, [ Co (NH3) 6 ] 3+ [ Co (NH3) 5 Cl ]2 + [ Co (NH3) 4 Cl 2 ] +,[ Co (NH3) 3 Cl 3] 0  The primary valences are non rigid and non directional, while secondary valence, have directional properties.  Every element is characterized by fixed secondary valences. Fe(II), Co(III),Pt(IV).

 Werner combined the interpretation of both,geometrical and optical isomerism.  If the metal ion has, a secondary valence four, therefore geometry of the complex can be square planar or tetrahedral.  If the metal ion has secondary valence six, the geometry of the complex may be octahedral.

Werner’s theory in the light of modern electronic theory of valence The combing capacity of a metal in form of two sphere of attraction or two zones of attraction. The first or inner zone surrounding the metal is called as coordination sphere. Neutral molecules or negatively charged, ligands are linked by coordinate bonds to the central metal ion in this zone. The linking is rigid and tight, this sphere functions as a non ionisable zone.

The second or outer zone, surrounding the coordination zone is called as ionization sphere. In this zone the metal ion satisfies it’s primary valence,by linking with negatively charged ions which get ionized on dissolving the complex in water. Werner introduced the square brackets [ ] to enclose the coordination sphere of the complex compound which contains the central metal ion and the ligands. The portion outside the square bracket contains ions of ionization sphere.

[ Co (NH3) 6 ] Cl 3 -Hexa-amine cobalt (lll) -ChIoride In above example six ammonia molecules in coordination sphere and three chloride ions in ionization sphere. [Co(NH3) 5 Cl]Cl 2 - penta aminechloro cobalt(lll) - Chloride. Five ammonia molecules in coordination sphere and two chloride ions in ionization sphere The primary valence (oxidation state ) of Co is 3 while secondary valence is 6.

Ligands  The molecules or ions are coordinated to the metal atom or ions in a coordination compound is called as ligands or donor groups. Classification of Ligands 1) Monodentate ligands 2) Poly dentate ligands

 Mono or Unidentate Ligand: The ligand molecule / ion which has only one donor atom or,one point of attachment and can coordinate with metal ion,at only one site in a complex.eg. NH3 molecule has one donor atom i.e nitrogen.Hence it can be linked to the metal of complex through nitrogen atom.  Poly or Multidentate Ligands: Ligand molecule or ion which has more donor atoms or points of attachment and can be linked to same metal in a complex using more donating sites.

It is classified as bi, tri, ….. hexa dentate,it is depending upon the number of points attachments with the ligand.  Polydentate ligands involving ring structures, are called as metal chelates and ligands as chelate ligands, and the process of forming metal chlete is called as chelation.. Eg. Ethylene diamine.

Ambidentate ligands : If has two or more donor atoms, a complex is formed only one donor atom is attached to the metal is called as ambidentate ligands. Eg. NO2 - group has the donor atoms (N & O). Out of two only one donor atom are linked to metal as, M- ONO or M-NO2.

Type of ligandsNameAbbreviation Monodentate 1) Water 2)Ammonia aqua Amine Bidentate1) Ethylenediamine 2) Oxalate 3) Dimethyl glyoximeto En Ox dmg TridentateDiethylenetriamineDien TetradentateTriethylenetetraamineTrien HexadentateEthylene diamine tetra acetone edta The common mono and polydentate Ligands

 The number of ligands which are directly bonded to central metal atom or ion in a complex is called as coordination number.  The coordination number of metal ion is influenced by several factors such as: 1) Charge on the metal ion 2) Charge on the ligand 3) The relative sizes of metal ion and ligand 4) Forces of repulsion between ligand

 A complex ion is more or less stable charged aggregate formed when an ion, mostly of a metal is directly linked to a group of neutral molecules or ions.  The most common complex ions are the metal centered complex ions., neutral NH3 molecule linked to Cu (ll) ions.  Eg. In tetra amine copper (ll) ion,[Cu(NH3)4] 2+, neutral NH3 molecule linked to Cu (ll) ions.  In hexacyanoferrate (ll) ion, [Fe(CN)6], negative cynide ions attached to ferric ion.  In hexacyanoferrate (ll) ion, [Fe(CN)6] 2+, negative cynide ions attached to ferric ion.

Homoleptic and Hetroleptic complexes 1) Homoleptic complexes Complex in which a metal is attached to one kind of donor group are homoleptic complexes. Eg. In hexa amine cobalt ( lll) ion, [ Co (NH3)6 ] 3+ six ammonia groups are attached to cobalt.

2) Hetroleptic complexes Complexes in which a metal is bound to more kind of donor groups are hetroleptic. In tetra ammine dichlorocobalt (lll) ion, [ Co (NH3)4 CI2 ] + four ammonia and two chloride ions bound to cobalt. Co-ordination compounds /complexes,three types a)Cationic complexes b)Anionic complexes c) Neutral complexes

 Net charge carried by complex ion is called its charge number.  It is equal to algebraic sum of charges carried out by central metal ion and ligands attached to it.

The charge number of [Fe(CN) 6 ] -4 = charge on Fe x charge on CN - ion. = ( + 2 ) + 6 (- 1 ) = -4  Eg:- The charge number of frrocyanide ion or hexacyanoferrate (ll). [Fe(CN) 6 ] -4 is -4.

1) Double salts, are those molecular or addition compounds which exist in solid state but dissociate into their constituent ions when dissolved in water.  Thus double salt lose their identity in aqueous solution.  Eg. Mohr’s salt. It is a mixture of saturated sol. of ferrous sulphate and ammonium sulphate.

2) Coordination compounds, are those molecular or addition compounds which retain their identity in aqueous solutions and show properties entirely different from those of their constituent ions. Eg. Potassium ferrocynide.

Characteristics of complex ions The central metal ion in a complex is usually,a transition metal. 1) Exhibit multiple oxidation states. 2) small ionic size and high charge. 3) make available suitable orbital's of appropriate energy to accommodate the electron donated by the ligand. 4) Ones an ion forms a complex it losses its individual properties and acquires the properties which are the characteristic of the complex ion. 5) A complex ion retain its identity in solution may undergo dissociation.

 It is a total number of electrons around central metal ion present in a complex and calculated as sum of electrons on metal ion and number of electron donated by ligands  EAN= Z - X + Y Where Z=atomic number of metal ion X = no. of electrons lost during the formation of the metal ion from its atom Y = no. of electrons donated by the ligands

Example. In ferrocyanide ion [Fe(CN) 6 ] e 24e + 6 x 2 = = 36 e EAN = 26 – = 36 Here – 2, no of electrons lost during formation of Fe ++ Fe e > Fe CN - [Fe(CN) 6 ] 4-

MetalComplexZX no. of e lost. Y no. of e donated by ligands EAN NiNi(CO) 4 tetra carbonyl nickel Fe[Fe(CN) 6 ] 4- hexacyanoferrate (II) Co[Co(NH 3 ) 6 ] 3+ hexa ammine cobalt (III) ion Zn[Zn(NH 3 ) 4 ] 2+ tetra amine zincate (II) ion Pt [Pt(NH 3 ) 6 ] 4+ hexa amine platinum (IV) ion Effective atomic number (EAN)some metal ions.

MetalComplexZXYEAN Fe[Fe(CN) 6 ] 3- Hexacyanoferrat e (III) ion Cu [Cu(NH 3 ) 4 ] 2 + tetra amine copper (II) ion Pt[Pt(NH 3 ) 4 ] 2+ tetra amine platinum (IV) ion

 Warner was the first to discuss the bonding features in coordination compounds.  There are many theories which can explain the nature of bonding,as well as various properties of coordination compounds. They are: 1) Valence Bond Theory 2)Crystal field theory 3)Ligand field theory 4)Molecular Orbital Theory

 The valence bond theory was developed by Linus Pauling  This theory accounts for coordinate bond formation,due to overlap of vacant hybrid orbital of central metal atom with the filled orbitals of ligands each containing lone pair of electron.

 Central metal ion present in a complex provides a definite number of vacant orbitals,s, p and d for the formation of coordinate bonds with ligands.  The no. of vacant orbitals provided the central ion is the same as its coordination number.  Each ligand has at least one orbital containing a lone pair of electron.  Vacant orbitals undergo hybridization to form same no. of hybrid orbitals.

 The geometrical shape of the complex ion depends upon hybridization of metal orbital's.  When central metal atom of the complex compound contains one or more unpaired electrons it, exhibits paramagnetic property.  Coordinate bond is stronger, if the overlapping between orbitals is greater.

 If (n-1 )d orbitals are used for hybridization, complexes are called inner complexes. [Co(NH 3 ) 6 ] 3+ If nd orbital's are used for hybridization are called as outer complexes. [Co F 6 ] 3- when strong field ligands like NH 3 and CN - are involved in formation of complexes, they cause pairing of electrons present in metal ions. This process is called spin pairing.

 Crystal field theory (CFT) 1 ) It is the interaction between the metal ion and ligands is purely electrostatic. When ligands approach central metal atom / ion the five degenerate d-orbital of the central atom become split up into levels of different energy under the influence of electrostatic field of ligands.

Salient features of crystal field theory 1) In complex central metal atom / ion is surrounded by various atoms / groups of atoms called ligands. 2)Ligands are –vly or neutral molecules posses lone pair of electrons. 3)Both metal ion and ligands act as point charges. 4)Electron of metal atom occupy splited d – orbitals acc. To Hunds rule.

 Isomerism is very common phenomenon in coordination compounds.  Isos meaning equal and meros meaning part.  There are two types of isomerism in coordination compounds: A. Stereo isomerism B. Structural isomerism

 Stereo isomerism : In this type different atom or groups occupy different spatial positions around central metal atom. 1) Geometrical isomerism 2) optical isomerism B)Structural isomerism: It is classified as, 1) Ionization isomerism 2) Linkage isomerism 3) coordination isomerism 4) solvate isomerism

 Geometrical isomerism: It give rise to –cis and -trans isomers. Similar groups are arranged on the same side or opposite sides of central metal atom. -Cis-Trans

Optical isomerism : Optically compounds having same molecular formula,same structural formula but different action of plane polarized light are called as optical isomers and phenomenon is called as optical isomerism. D- form l- form

One form rotate the plane of polarized light in clockwise, while other in anticlockwise direction, is called as d- form and l-form. The phenomenon of optical isomerism is quite common in coordination compounds. Complex whose molecules are asymmetric exhibit optical isomerism. It will give rise to dextro (d) and leavo (l) form.

 Structural isomerism is further subdivided into: 1)Ionization isomerism. 2)Linkage isomerism. 3)Coordination isomerism. 4)Solvate isomerism.

 In Ionization isomerism there is an exchange of ions inside and outside the coordination. They have same formula but produce different ions in solution.it is also known as ion –ion exchange isomerism. CompoundsColorIons present [Co(NH3)SO4]BrRed violet[Co(NH3)SO4] - + Br + [Co(NH3)5Br]SO4Red[Co(NH3)5Br] - + SO4 + Ionization isomerism  In given isomers there is an exchange of ions i.e. Br - ions and SO4 2- ions inside and outside the coordination sphere.

Linkage isomerism  In linkage isomerism the same ligand is bonded to central metal atom or ion through different atoms. Linkage isomers have same molecular formula but differ in the linkage of the ligand to central metal atom.  In given eg. NO2 ligand can bind to the central metal through nitrogen or oxygen to give two isomers, CompoundsColor [Co(NH3)5NO2 ]Cl2Yellow [Co(NH3)5ONO]Cl2Red

Coordination isomerism  This type of isomerism arise from different complex ions having same molecular formula. There is inter change of ligands between cationic and anionic entities of different metal ions present in a complex. [Co(NH3) 6 ][Cr (CN) 6 ] and [Cr(NH3) 6 ][Co(CN) 6 ] In above example there is interchange of ligands i.e. NH3 are bound to cobalt and cyanide ligands bound to chromium. In its coordination isomers the NH3 ligands are bound to chromium and cyanide ligands to cobalt.

Solvate isomerism / Hydrate isomerism  In hydrate isomerism there is an exchange of H2O molecule inside or outside coordination sphere.  Hydrate isomers have the same molecular formula but differ in the number of molecules of H2O inside and out side the coordination sphere. CompoundColour Number of H2O molecule inside coordination sphere Number of H2O molecule outside coordination sphere [Cr(H2O)6]Cl3Violet60 [Cr(H2O)5Cl]Cl2.H2OBlue green51 [Cr(H2O)4Cl2]Cl.2H2OGreen42

 Compound containing carbonyl ligands only are known as homoleptic carbonyl.  Such type of compounds are formed by most of the transition metals.  For: pentacyarbonyl iron (O) is trigonal bypiramidal tetracarbonyl nickel (O) is tetrahedral while hexacarbonyl chromium (O) is octahedral.

Fe Co

Ni Co