Stereo-isomerism

ISOMERISM IN COORDINATION COMPOUNDS

·      The compound which have the same molecular formula but different arrangement of their atoms are called isomers and the phenomenon is called isomerism 

·      Isomers have different physical and chemical properties because of different arrangement of their atoms.

·      Isomerism in coordination compounds may be divided into two

             (i)Stereo-isomerism

             (ii)Structural Isomerism

i). Stereo-isomerism-

·     
stereoisomers have the same chemical formula and chemical bonds but they have different special arrangement of atoms or ligands.

                          Or

When two compounds contain the same ligands coordinated to the central ion, but the arrangement of ligands in space is different. The two compounds are said to be stereoisomers and the type of isomerism is called stereoisomerism.

i)                   Geometrical or cis-trans Isomerism

ii)                 Optical or mirror- image Isomerism

           i). Geometrical isomerism –

·      Isomers having same molecular formula but different arrangement of ligands around central atom/ion, are called geometrical isomers and phenomena is called geometrical isomerism

This isomerism is due to ligands occupying different positions around the central metal atom or ion.

·      The ligands occupy positions either adjacent or opposite to one another. This type of isomerism is also known as cis- and trans- isomerism.

·      Geometrical isomers in which two similar ligands are arranged adjacent to each other are called cis-isomers

·      Geometrical isomers in which two similar ligands are arranged opposite to each other are called Trans-isomers

·      Square planar complexes (coordination number four) exhibit geometrical isomerism

·      complexes of the type Ma₄b₂ and Ma₃b₃ exhibit geometrical isomerism(coordination no-6)

 

                                                                                                      

 Facial and Meridional Isomers

·      A set of three, ligands (similar) may be arranged on an octahedron in all cis-fashion, giving facial or 'fac' isomer. In other words, three similar ligands on the same face of octahedron give rise to the facial isomer.

·      A set of three similar' ligands may be arranged on an octahedron with one pair trans (meridian of octahedron), giving rise to meridional or 'mer' isomer.

 Note:

 1. Geometrical isomerism is not observed in complexes of coordination number 2 and 3.

 2. Geometrical isomerism is not observed in complexes of coordination number 4 of tetrahedral geometry.

(ii). Optical isomerism:

·      A coordination compound which can rotate the plane of polarised light is said to be optically active.

·      When the coordination compounds have same formula but differ in their abilities to rotate directions of the plane of polarised light are said to exhibit optical isomerism and the molecules are optical isomers.

·      The isomer which rotates the plane of polarised light to right direction is termed dextro (d-form) while the isomer which rotates the plane of polarised light to left direction is termed laevo (l-form).

The two optically active isomers are collectively called enantiomers.

·      Optical isomers which are mirror image that can not superimposed on one another are called enantiomers 

If Mirror image are superimposable,are called non-enantiomers

·      The essential requirement for a substance to be optically active is that the substance should not have a plane of symmetry in its structure.

·      Optical isomerism is expected in tetrahedral complexes of the type [Mabcd] but No mirror image isomerism is possible with tetrahedral and square planner complex of the type – [Ma₄], [Ma₃b], [Mab₃] because all possible arrangement of ligands round the central metal ion, M are exactly equivalent.

Example-

 

Chemical bonding -HYDROGEN BONDING

 

HYDROGEN BONDING

·     ·       In 1920, Latimer and Rodebush introduced the idea of "hydrogen bond" to explain the nature of association in liquid state of substances like water, hydrogen fluoride, ammonia, formic acid, etc.

     ·       Hydrogen bond can be defined as the attractive force which binds hydrogen atom of one molecule with the electronegative atom (F, O or N) 

     ·       It exerts in polar molecule or molecule having polar group.

     ·       This bond is represented by dotted line (---------) while solid represents the covalent bond.

     ·       The magnitude of the hydrogen bonding depends on the physical state of the compound.

     ·        order of  increasing strength of hydrogen bond in various physical state as

        gas < Liquid< solid  

        Type of hydrogen bonding-

         a)       Intermolecular hydrogen bonding: This type of bonding results between the positive and negative ends of different molecules of the same or different substances. Example-

   a) H^δ+ — F^δ--------H^δ+ — F^δ+--------H^δ+ — F^δ-

   b) H^δ+ — F^δ--------H^δ+ — O —H^δ+-------H^δ+ — F^δ       

        b).    Intramolecular hydrogen bonding: This type of bonding results between hydrogen and an electronegative element both present in the same molecule.

Importance of Hydrogen bond-

   ·      It helpful to explain that H2O exits in liquid state while H2S exists in gaseous state since in H2O, oxygen is more electronegative element than Sulphur in H2S therefore oxygen has ability to form H-bond with other water molecules. so it exists.

   ·      Ortho-nitrophenol has low boiling point than para-nitrophenol since P-nitrophenol form intermolecular hydrogen bond which is stronger than intra molecular hydrogen bond which is formed by ortho-nitrophenol.

     ·      Solubility of compound in water can be explained on the basis of h-bonding. Example alcohol is soluble in water but oil not. Because alcohol forms H-bond with water but oil does not form.

IUPAC NOMENCLATURE OF COORDINATION COMPOUNDS

 IUPAC NOMENCLATURE OF COORDINATION COMPOUNDS-The present system of nomenclature recommended by the Inorganic Nomenclature Committee of the I.U.P.A.C.

The main rules of naming of complexes are -

 I). If a coordination compound is ionic, the name of cation is given first whether or not it is the complex ion followed by the name of the anion just like naming a simple salt. The names of cation and anion are separated by a space.

       Ex.

                   K₄ [Fe (CN)₆]

 the naming of this complex starts with potassium.

(II) Within a complex ion/coordination entity, the ligands are named first followed by the metal ion.

i.e., ligands are named in alphabetical order before the name of the central     atom/ion

         The ligands be be neutral, anionic or cationic.

Names of the ligands

(i)            The neutral ligands are named as the molecule

ligand	IUPAC name	ligand	IUPAC name
C5H5N	(pyridine)	CO	carbonyl
(C6H5)3P	Triphenyl phosphine	NO	nitrosyl
H2N CH2CH2NH2	ethylene diamine	H2O	Aqua
		NH3 ammonia	ammine

 

                       

    (ii)(a) Anionic ligands ending with 'ide' are named by replacing the 'ide' with  suffix 'O' or  replacing ~e by -O.

           Example

anion	Symbol	IUPAC name
Chloride	Cl-	Chlorido
Bromide	Br-	Bromido
Nitride	N3–	Nitrido
Cyanide	CN-	Cyano/ Cyanido
Amide	NH2–	Amido
Imide	NH2–	Imido
Phosphide	P3-	Phosphido
Sulphide	S2–	Sulphido
Oxide	O2-	Oxo
Hydroxide	OH–	Hydroxo
(b)Ligands whose names end in 'ite' or 'ate' by replacing the ending 'e' with 'o' as follows.
anion	symbol	IUPAC name of ligand
Carbonate	CO32–	Carbonato
Sulphite	SO32–	Sulphito
Oxalate	C2O42–	Oxalato (Ox)2–
Acetate	CH3COO—	Acetato
Sulphate	SO42–	Sulphato
Nitrite	ONO — (bonded through oxygen)	Nitrito-O-
	NO2 — (bonded through nitrogen)	Nitrito-N- [ or Nitro]
Thiosulphate	S2O3 –2	Thiosulphato

                          (EDTA^4-)- ethylenediaminetetraacetate

                          (EDTA^3-)- ethylenediaminetriacetate

    iii).Positive ligands naming ends in 'ium'

NH₂—NH₃ ⁺ (Hydrazinium),

NO₂ ⁺ (nitronium);

NO⁺ (nitrosonium)

(III) If ligands are present more than once, then their repetation is indicated by prefixes   like di, tri, tetra etc. However, when the name of the ligand includes a number. 

Ex. - dipyridyl, ethylene diamine, then bis, tris, tetrakis are used in place of di, tri, tetra,             etc.

 Example-

                    [Pt(NH₃)₆]Cl₄      Hexaamineplatinum(IV) chloride

                       [Cu(en)₂] S0₄   Bis (ethane-l,2-diamine) copper(II) sulphate

(IV)Order of naming ligands: When more than one type of ligands are present, they are   named in alphabetical order without separation by hyphen.

 Example-

                       [Co(NH₃)₄H₂OC1]Cl  Tetraaquamineaquochloridocobait(llI) chloride

                       [Cr(H₂0)₄Cl₂]⁺            Tetraaquodichloridochromium(III) ion

(V)The oxidation state of the central metal is shown by Roman numeral in small bracket     (parenthesis) followed by its name. Ex-

               [Cr(H20)₄Cl₂]⁺  Tetraaquodichloridochromium(III) ion

(VI) Complex positive ions and neutral coordination compounds have no special ending but complex negative ions always end in the suffix -ate. In most of the cases, the  suffix -ate is fixed to English names of the metals but in some cases -ate is fixed with Latin names of metals.  

Example-

        K₃[Fe (CN)₅NO]     

 Potassium pentacyanonitrosylferrate(II)