26. General group trends in Binary Acid Strength. Variation of Binary Acid Strength Across Periods. Structure and Relative Strengths of Oxoacids.

27. Describe types of isomers of complex compounds. Cis-and trans-isomers. Trans effect ligands.

In chemistry, a coordination complex consists of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents.

What are isomers? Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole

The arrangement of the ligands is fixed for a given complex, but in some cases it is mutable by a reaction that forms another stable isomer. There exist many kinds of isomerism in coordination complexes, just as in many other compounds.

Stereoisomerism occurs with the same bonds in different orientations relative to one another. Stereoisomerism can be further classified into:

• Geometrical isomerism:

Geometrical isomerism is observed in heteroleptic complexes (complexes with more than one type of ligands) due to different possible geometric arrangements of the ligands. This behavior is mainly observed in coordination compounds having coordination numbers equal to 4 and 6.

• Optical isomerism:

Optical isomerism occurs when a molecule is not superimposable with its mirror image. It is so called because the two isomers are each optically active, that is, they rotate the plane of polarized light in opposite directions.

• Structural isomerism:

Structural isomerism occurs when the bonds are themselves different. There are four types of structural isomerism: ionisation isomerism, solvate or hydrate isomerism, linkage isomerism and coordination isomerism.

Ionisation isomerism – the isomers give different ions in solution although they have the same composition. This type of isomerism occurs when the counter ion of the complex is also a potential ligand. For example pentaamminebromocobalt(III) sulfate [Co(NH3)5Br]SO4 is red violet and in solution gives a precipitate with barium chloride

Solvate or hydrate isomerism – the isomers have the same composition but differ with respect to the number of solvent ligand molecules as well as the counter ion in the crystal lattice. For example [Cr(H2O)6]Cl3 is violet colored, [CrCl(H2O)5]Cl2·H2O is blue-green, and [CrCl2(H2O)4]Cl·2H2O is dark green.

Linkage isomerism occurs with ambidentate ligands that can bind in more than one place. For example, NO2 is an ambidentate ligand: It can bind to a metal at either the N atom or an O atom.

Coordination isomerism – this occurs when both positive and negative ions of a salt are complex ions and the two isomers differ in the distribution of ligands between the cation and the anion. For example [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].

Cis-and trans-isomers:

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry.

Cis means that functional groups are on the same side of the carbon chain,and trans means that functional groups are on the opposite side of the carbon chain. It is used to describe molecules that are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are rotated into a different orientation in three-dimensional space. It is not to be confused with E–Z isomerism, which is an absolute stereochemical description.

Trans effect ligands

It is observed that during the substitution reactions of square planar metal complexes, some ligands preferentially direct the substitution trans to themselves. i.e., the choice of leaving group is determined by the nature of ligand trans to it.

The Trans effect can be defined as the effect of a ligand over rate of substitution of another ligand positioned trans to it in the square planar complexes.

The trans effect ligands

'T' is the trans directing group and 'Nu' is the nucleophilic ligand which preferentially substitutes the ligand 'X' which is trans to ligand 'T'.

* The Trans effect was first recognized by Ilya Ilich Chernyaev, a russian chemist in square planar complexes of Platinum(II).

* However the Trans effect is also observed in octahedral complexes.

* The true Trans effect is a kinetic effect.