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Werner’s coordination theory:
It explains the nature of bonding in complexes. Metals show two different kinds of valencies:
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– Primary valency: Nondirectional and ionizable. It is equal to the oxidation state of the central metal ion.
– Secondary valency: Directional and no ionizable. It is equal to the coordination number of the metal. It is commonly satisfied by neutral and negatively charged or some times by positively charged ligands.
The ionization of the coordination compound is written as:
Addition compounds: These are the compounds formed by the combination of two or more simple compounds are called addition compounds. They are of two types:
Double salt: A compound formed by the combination of two or more simple compounds, which is stable in solid-state only is called double salt. In solution, it breaks into component ions. e.g.,
K2SO4·Al2(SO4)3·24H2O Potash alum
FeSO4·(NH4)2SO4·6H2O Mohr’s salt
Complex compound: A compound formed by the combination of two or more simple compounds that retain its identity in solid and solution states both are called a complex compound. e.g.,
K4[Fe(CN)6] Potassium ferrocyanide
[Cu(NH3)4]SO4 Cuprammine sulfate
The central metal atom or ion and ligand took together is called a coordination entity. It may be positive, negative or neutral.
e.g., [Cu(NH3)4]2+, [Fe(CN)6]4–, [Ni(CO)4]
The atom or ion with which a definite number of ligands are attached in a definite geometry is called central atom/ion. Any atom/ion which has high positive charge density or vacant orbitals of suitable energy may be central atom or ion. e.g., transition metals, lanthanoids. It is Lewis acid (electron acceptor).
Molecules or ions which are bound to the central atom/ion in the co-ordination entity are called ligands. A molecule or ion which has high negative charge or dipole or lone pair of electrons may be ligands. It is Lewis base (electron donor).
Homoleptic and heteroleptic complexes:
Homoleptic complexes: Complexes in which a metal is bound to only one kind of ligand are called homoleptic complexes.
e.g., [Co(NH3)6]3+, [Ti(H2O)6]3+, [Cu(CN)4]3–
Heteroleptic complexes: Complexes in which the central atom is bound to different types of ligands are called heteroleptic complexes.
e.g., [Co(NH3)4Cl2], K2[Fe(CN)5NO], [Fe(H2O)5NO]SO4
Classification of ligands:
Nomenclature of coordination compounds:
Rules for writing the formula of coordination compounds:
– The formula of the cation whether simple or complex must be written first followed by anion.
– The coordination sphere is written in square brackets.
– Within the coordination sphere, the sequence of symbols is, first the metal atom followed by anionic ligand then neutral ligand finally cationic ligand. Ligands of the same type are arranged alphabetically.
– Polyatomic ligands are enclosed in parentheses.
– The number of cations or anions to be written in the formula is calculated on the basis that the total positive charge must be equal to the total negative charge, as the complex as a whole is electrically neutral.
Rules for naming coordination compounds:
– The cation is named first then the anion.
– In the naming coordination sphere, ligands are named first in alphabetical order followed by metal atom and then oxidation state of metal by a Roman numeral in parentheses.
– The name of coordination compounds is started with a small letter and the complex part is written as one word.
The naming of ligands:
– The name of anionic ligands ends in –o. e.g., Cl– : Chloride
– Neutral ligands (with a few exceptions) retain their names e.g., NH3: Ammine
– The name of cationic ligands ends in – ium. e.g., NO2 + : Nitronium
– Certain ligands are represented by abbreviations in parentheses instead of their complex structural formulae. e.g., ethylenediamine(en).
– Ambidentate ligands are named by using different names of ligands or by placing the symbol of the donor atom. e.g.,
—SCN– (_iocyanato-S or _iocyanato),
—NCS– (_iocyanato-N or Isothiocyanato),
—ONO– (Nitrito-O or Nitrito),
—NO2 – (Nitrito-N or Nitro)
– The prefixes di-, tri-, tetra-, penta– and hexaare used to indicate the number of each ligand. If the ligand name includes such a prefix, the ligand name should be placed in parentheses and preceded by bis-(2), tris-(3), tetrakis-(4), pentakis-(5) and hexakis-(6).
– When the coordination sphere is anionic, the name of central metal ends in –ate.
Bonding in coordination compounds:
Valence bond theory: It was developed by Pauling.
– A suitable number of vacant orbitals must be present in the central metal atom or ion for the formation of coordinate bonds with the ligands.
– Central metal ion can use an appropriate number of s, p or d-orbitals for hybridization depending upon the total number of ligands.
– The outer orbital (high spin) or inner orbital (low spin) complexes are formed depending upon whether outer d-orbitals or inner d-orbitals are used.
– Low spin complexes are generally diamagnetic and high spin complexes are paramagnetic.
– Paramagnetism ∝ No. of unpaired electrons.
– Magnetic moment = n(n + 2) B.M. where n = number of unpaired electrons.
Crystal Field Theory:
It assumes the ligands to be point charges and there is the electrostatic force of attraction between ligands and metal atom or ion. When ligands approach the central metal ion, then the five degenerate orbitals do not possess equal energy anymore and results in splitting, which depends upon the nature of ligand field strength.
– Greater the ease with which the ligand can approach the metal ion, the greater will be the crystal field splitting caused by it.
– Crystal field splitting in octahedral coordination complexes can be shown as:
– If Do < P (where ‘P’ is the energy required for the forced pairing of electrons) then the electrons will remain unpaired and a high spin complex is formed.
– If Do > P, then pairing of electrons takes place and a low spin complex is formed.
– Crystal field splitting in tetrahedral complexes can be shown as:
– The difference in energy between e and t2 level is less in tetrahedral complexes
– Spectrochemical series: Arrangement of ligands in the order of increasing field strength.
Applications of coordination compounds:
Coordination compounds are of great importance in the biological system. e.g., chlorophyll, hemoglobin, vitamin B12, etc. are coordinate compounds of Mg, Fe, and Co respectively.
Coordination compounds are used for qualitative and quantitative analysis, extraction of metals, electroplating, and photography and as dyes.
cis-platin is used in cancer treatment, EDTA is often used for the treatment of lead poisoning.
Coordination compounds are used as a catalyst.