Some important terms:
- Coordination Compounds − Complex compounds in which the transition metal atoms are bound to a number of anions or neutral molecules
- Coordination Entity– Constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. Example: [CoCl3(NH3)3] is a coordination entity
- Central Atom or Ion– The atom or ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it in a coordination entity
- Ligands– Ions or molecules bound to the central metal atom or ion in a coordination entity
- Coordination Number– Number of ligand-donor atoms bonded directly to the metal
- Coordination Sphere– The central atom or ion and the ligands attached to it are enclosed in square brackets, which are collectively known as the coordination spheres.
- Coordination Polyhedron– The spatial arrangement of the ligand atoms which are directly attached to the central atom or ion
- Oxidation Number of Central Atom– The charge an atom would carry if all the ligands are removed along with the electron pairs that are shared with the central atom
- Homoleptic complexes: Complexes in which the metal is bound to only one kind of donor group. Example: [Co(NH3)6]3+
- Heteroleptic complexes: Complexes in which the metal is bound to more than one kind of donor groups. For example: [Co(NH3)4Cl2]+
Theories related to coordination compounds:
- Werner’s theory: In coordination compounds, there are two types of linkages (valences) − primary and secondary. The primary valences are ionisable, and are satisfied by negative ions. The secondary valences are non-ionisable, and are satisfied by negative ions or neutral molecules. The secondary valence is equal to the coordination number of a metal, and remains fixed for a metal. Different coordination numbers have characteristic spatial arrangement of ions or groups bound by the secondary linkages.
- Valence bond theory: The metal atom or ion under the influence of ligands can use its (n−1)d, ns, np or ns, np, nd orbitals for hybridisation, to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar, and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.
- Crystal-field theory: An electrostatic model which considers the metal−ligand bond to be ionic, and arises purely from the electrostatic interaction between the metal ion and the ligands. Ligands are treated as point charges in the case of anions, or dipoles in the case of neutral molecules. The five d-orbitals in an isolated gaseous metal atom/ ion are degenerate (i.e., have the same energy). Due to the negative fields of the ligands (either anions or the negative ends of dipolar molecules), the degeneracy of the d-orbitals is lifted, resulting in the splitting of the d-orbitals.
Coordination compounds:
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Features
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Coordination compounds
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| Formula writing |
- Central atom is listed first.
- The ligands are then listed in the alphabetical order.
- Polydentate ligands are also listed in the alphabetical order.
- The formula of the entire coordination entity is enclosed in square brackets.
- Ligand abbreviations and formulas for polyatomic ligands are enclosed in parentheses.
- For the charged coordination entity, the charge is indicated outside the square brackets, as a right superscript, with the number before the sign.
- The charge of the cation(s) is balanced by the charge of the anion(s).
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| Nomenclature |
- The cation is named first in both positively and negatively charged coordination entities.
- The ligands are named in alphabetical order before the name of the central atom/ion.
- Names of the anionic ligands end in −o and those of neutral and cation ligands are the same.
- To indicate the number of the individual ligands, the prefixes mono−, di−, tri−, etc., are used. If these prefixes are present in the names of ligands, then the terms −bis, −tris, −tetrakis, etc., are used.
- Oxidation state of the metal is indicated by a Roman numeral in parentheses.
- If the complex ion is cation, then the metal is named as the element.
- If the complex ion is anion, then the metal is named with ‘−ate’ ending.
- The neutral complex molecule is named as the complex cation.
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| Isomerism |
- Geometrical– Due to different possible geometrical arrangement of ligands
- Optical– Due to chirality.
- Linkage– Only with ambidentate ligand
- Coordination– Interchange of ligands between cationic and anionic entities of different metal ions present in the complex
- Ionization– The counter ion in the complex salt is itself a potential ligand and can displace a ligand, which can then become the counter ion
- Solvate- Water is involved as a solvent
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| Magnetic properties |
- Complexes with unpaired electron(s) in the orbitals are paramagnetic.
- Complexes with no unpaired electron(s) in the orbitals (i.e., all the electrons are paired) are diamagnetic.
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| Colour |
The colour of the coordination compounds is attributed to d−d transition of electrons. In the absence of ligand, crystal-field splitting does not occur; hence, the substance is colourless. |
| Stability |
The stability of a complex in a solution refers to the degree of association between the two species involved in the state of equilibrium. Stability can be expressed quantitatively in terms of stability constant or formation constant. |
Questions that were asked previously:
Q. Name the following coordination entities and describe their structure: (2012 Set 3)
(i) [Fe(CN)6]4-
(ii) [Cr(NH3)4Cl2]+
(iii) [Ni(CN)4]2-
(Atomic numbers: Fe-26, Cr-24, Ni-28)
Q. Write the name, stereochemistry and magnetic behavior of the following: (2011 Set 1)
(Atomic number: Mn-25, Co-27, Ni-28)
(i) K4[Mn(CN)6]
(ii) [Co(NH3)5Cl]Cl2
(iii) K2[Ni(CN)4]
Q. Name the following coordination compounds according to the IUPAC system of nomenclature: (2010 Set 3)
(i) [Co(NH3)4(H2O)Cl]Cl2
(ii) [CrCl2(en)2]
Best of luck,
Team Meritnation!
Add Comment Total Comments (4)
1. jyoti | January 20th, 2015 at 4:36 pm
answer kha hea ???
2. Hirak | March 14th, 2013 at 2:31 pm
FANTASTIC…..
3. Manish | March 10th, 2013 at 8:49 pm
awesome,really helpful tricks.
4. himanshu | March 10th, 2013 at 5:02 pm
thanks a lot for this great help
🙂
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