NEET Chemistry Notes Coordination Compounds – Effective Atomic Number Rule

NEET Chemistry Notes Coordination Compounds – Effective Atomic Number Rule

Effective Atomic Number Rule

Effective Atomic Number Rule

Effective atomic number EAN of a metal in a complex= atomic number of nearest inert gas.
or
EAN = atomic number of metal ± valency + 2XCN
where, CN = coordination number

Werner’s Theory

According to this theory

•  In complex compounds, metal atom exhibit two
  types of valencies—primary valency and secondary valency. .
•  Primary valencies are satisfied by anions only while secondary valencies are satisfied by ligands. Primary valency depends upon oxidation number of central metal atom while secondary valency represents the coordination number of central metal atom.
• Primary valencies are ionisable and are nqn-directional while secondary valencies are non-ionisable and directional. Therefore, geometry of complex is decided by secondary valencies.
  e.g. . [Cr(H₂0)₆)]Cl₃ has primary valency =3 (OS of Cr) and secondary valency = 6 (CN of Cr)

Nomenclature of Coordination Compounds
Cationic Complex

• Tetramminechloronitrocobalt (III) nitrate
•  Prefixes mono, di, tri etc., are used to indicate the number of the individual ligands and ligands are named in alphabetical order.
• Central metal atom and its oxidation state is indicated by Roman numeral in parenthesis.
•  Name of ionisable anion.

Anionic Complex

• Potassiumhexacyanoferrate (III)
•  Name of ionisable metal.
• Name of ligand in alphabetical order.
•  Central metal atom + ate and oxidation state.

Neutral Complex

• Diamminechloronitrito-N-platinum (II)
•  Name of ligands in alphabetical order.
• Central metal atom and its oxidation state.

Isomerism in Coordination Compounds

It is a phenomenon, in which compounds have the same molecular formula but different physical and chemical properties on account of different structures. These compounds are called isomers.

Stereoisomerism

It occurs due to different arrangements of ligands around central metal atom.
It is of two types; geometrical isomerism and optical isomerism.

Geometrical Isomerism

• It arises in heteroleptic complexes due to different possible geometric arrangements of the ligands. Important examples of this behaviour are found in square planar and octahedral complexes.
• Square planar complex of formula [MX₂L2] (X and L are unidentate), the two X ligands may be arranged adjacent to each other in a cis-isomer or opposite to each other in a trans-isomer, e.g. . [Pt(NH₃)₂Cl₂],
•  Square planar complex of the type [MABXL] (where A, B, X, L are unidentate) shows three isomers, two cis and one trans. Such isomerism is not possible for tetrahedral geometry, e.g. (Pt(NH₃)(Br)(Cl)(Py)].
• Square planar complex of type M(XL)2, here, XL is unsymmetrical didentate ligand, e.g.[Pt(gly)₂]-
•  Octahedral complexes of formula [MX₂Li] in which the two ligands X may be oriented cis or trans to each other, e.g. [Co(NH₃)₄Cl₂]⁺..
•  Octahedral complexes of formula [MX₂AX₂] (where, X are unidentate ligands and A are didentate ligands) form cis and trans-isomers, e.g. [CoCl₂(en)₂]
• Octahedral coordination entities of the type[Ma₃b₃] like [Co(NH₃)₃(N0₂)₃. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer.
• When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer.

Optical Isomerism

It arises when mirror images cannot be superimposed on one another. These mirror images are called enantiomers. The two forms are called dextro [d) and laevo (7).
Optical isomerism is common in octahedral complexes having atleast one didentate ligand.
e.g.[Co(en)₃ ]³⁺, [PtCl₂ (en)₂ ]²⁺ etc.

Structural Isomerism

In structural isomerism, isomers have different bonding pattern.
Different types of structural isomerism is as follows:

• Linkage isomerism arises in a coordination compound containing ambidentate ligand
  .e.g. [Co(NH₃)₅(N0₂)]Cl₂ and [Co(NH₃)₅(ONO)]Cl₂
  Coordination isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex.
  e.g. [Co(NH₃)₆][Cr(CN)₆] and [Cr[NH₃)₆][Co(CN)₆]
  Ionisation isomerism when the ionisable anion
  exchange with anion ligand, e.g. [Co(NH₃)₅S0₄]Br and [Co(NH₃)₅Br]S0₄
• Solvate isomerism is also known as “hydrate isomerism”. In this case water is involved as a solvent.
  e.g.[Cr(H₂0)₆]Cl₃, [CrP₂0)₅Cl]Cl₂ -H₂0, [Cr(H₂0)₄Cl₂]Cl • 2H₂0

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