Solvate Isomerism

Solvate isomerism, also known as hydrate isomerism, refers to isomers that have the same composition but differ in whether a solvent molecule is directly linked to the metal ion or is only present on free solvent molecules in the crystal. This is a slightly unique case of the above ligand exchange using neutral solvate molecules. Using Alfred Werner’s coordination theory, it appears that numerous water molecules are directly connected to the core chromium ion (through coordinate covalent bonds). In fact, there are multiple compounds that employ brackets to represent complicated bonding and dots to represent “water molecules that are not bonded to the core metal but are part of the lattice.” 

Isomerism

The presence of molecules with the same number of atoms of the same type (and consequently the same formula) but differing chemical and physical properties is referred to as isomerism. Timing and energy are also factors in isomerism. Molecules are mobile entities that change shape by a variety of rotating motions, all of which need energy. As a result, certain molecules may be identical on one timeline or under one set of energy conditions but different, or isomeric, on another.  

• Constitutional isomers like n-butane and isobutane are molecules with different connections.
• The connection is the same in stereoisomers, but the pieces are oriented differently in space.
• In chemistry, a structural isomer (also known as a constitutional isomer in IUPAC terminology) is a compound with the same number of atoms of each element but conceptually separate bonds between them.

Stereoisomers contain the identical formula and chemical linkages as one another, but their spatial arrangement is different. Stereoisomers are of two types: 

• Geometrical Isomers: There are two types of geometrical isomers: cis and trans isomers.This isomerism is common in complexes with coordination number 4 and 6.  Geometrical isomerism does not exist in tetrahedral complexes. One of the example of geometrical isomer is stilbene (C14H12) having 2 isomers -> cis and trans.
• Optical Isomers: The chiral complexes are those that have chiral structures. The chiral complexes exhibit optical isomers. When mirror images can’t be placed on top of each other, this problem develops. Enantiomers are mirror images of one another. The two types are known as dextro (d) and laevo (l). Optical isomerism can be seen in tetrahedral complexes with the formula [M(AB)2] and octahedral complexes (cis form).  

Explanation

The chromium chloride “CrCl3.6H2O,” which may include 4, 5, (or) 6 coordinated water molecules, is one of the best-known examples of Hydrate Isomerism or Solvate Isomerism type of isomerism. 

• [Cr(H2O)4Cl2]Cl.2H2O – Bright green

Tetraaquadichlorochromium(III) chloride dihydrate

• [Cr(H2O)5Cl]Cl2.H2O – grey-green

Pentaaquachlorochromium(III) chloride monohydrate

• [Cr(H2O)6]Cl3 – Violet

Hexaaquachromium(III) chloride

These isomers have highly distinct chemical characteristics, and when tested for Cl– ions using AgNO3, 1,2, and 3 Cl– ions would be found in solution, accordingly. They are distinguished by the fact that one ion is directly connected to the essential metal while the other is not. When a coordinating group is substituted by a solvent molecule, a comparable isomerism occurs (Solvate Isomerism).

Water of Crystallisation

Many chemicals absorb water molecules into their crystalline frameworks after crystallisation from water. Water contained in the crystalline framework of a metal complex or a salt that is not directly linked to the metal cation is referred to as “waters of crystallisation.” The entire weight of water in a substance at a certain temperature is known as water of crystallisation, and it is usually present in a definite (stoichiometric) ratio. A hydrate is a chemical with associated water of crystallisation. Because of the presence of hydrogen bonds that determine polymeric structures, the structure of hydrates can be highly complex. Consider the aquo complex NiCl2 .6H2O, which is made up of separated trans-[NiCl2(H2O)4] molecules that are only weakly connected to nearby water molecules. Only four of the formula’s six water molecules are bonded to the nickel (II) cation, with the remaining two becoming crystallisation waters when the crystal structure develops. 

Because it is tiny and polar, water is a particularly common solvent in crystals. Water is notable because it is reactive, although other solvents like benzene are thought to be chemically harmless.

Conclusion

‘Hydrate isomerism’ is the name given to the isomerism in which water is used as a solvent. Ionisation isomerism is a similar concept. It has a varied quantity of water molecules inside and outside the coordination sphere. The solvate isomer [Cr(H2O)5Cl]Cl2 .H2O (grey-green) and the aqua complex [Cr(H2O)6]Cl3 (violet). are two instances . Using Alfred Werner’s coordination theory, it appears that numerous water molecules are directly connected to the core chromium ion (through coordinate covalent bonds).