What is Zeise’s salt ? Discuss the structure and bonding  of Zeise’s  salt?

Platinum (Pt ), a 4d transition metal, which forms a large number of organo-metallic compounds. Zeise’s salt is one of them. 

The salts containing the ion  [ C₂H₄PtCl₃] ^-1 are known as Zeise’s salt after the name of discover such salts may be K[C₂H₄PtCl₃], Na[C₂H₄PtCl₃] , NH₄[C₂H₄PtCl₃] etc.

Preparation of Zeise’s salt 

When platinum tetra chloride is react with ethanol an adduct is formed, which react with KCl gives Zeise’s salt. The following two method for preparation of Zeise’s salt are shown below.

The nature of bonding in such olefinic compound like Zeise’s salt is still imperfectly understood, but it is clear that localized σ-bond between the metal atoms and carbon do not exist.

The is generally attributed to the interaction between π-electrons in the unsaturated molecule and the hybrid orbitals of metal atom. Several olefin complexes have been studied by X-ray diffraction method.

In complexes such as [C₂H₄PtCl₂]₂ and [(C₂H₄)₂RhCl]₂ the coordinated double bond is normal to the coordination plane and similar in the case for [ C₂H₄PtCl₃] ^-1 ion.

The bonding in Zeise’s salt can be explained by the molecular orbital (M.O) approach. The theory assumes that the metal to olefin bond consists of two parts.

(I)overlap of the π-electron density of the olefin with a σ-type acceptor orbital on the metal atom. 

 (II)A back bonding resulting from flow of electron density from filled metal dπ orbital into antibonding vacant orbitals of C-atom .

This view is similar to bonding in carbonyls, but some how different from that and this is known a µ-bonding.

Why the C –C olefinic bond length in Zeise’s salt is greater than the C –C  bond in free hydrocarbon ?

 In Zeise’s salt back bonding also takes place from suitable filled metal dπ orbital into the vacant anti bonding π*orbital on the carbon atom. In the free C₂H₄ molecule, the C –C bond order should be decreases.

As a consequence, the C –C bond length in  Zeise’s salt increases from free C₂H₄ molecule ( 1.34 Å) to coordinated C₂H₄ molecule (1.4 -1.47 Å).

Why the ethylene molecule is perpendicular to the PtCl₃ molecular plane in Zeise’s salt ?

In Zeise’s salt the metal ion , Pt(II) contains three π-type filled d-orbital which are d_xy, d_yz, and d_xz.

When the coordinated ethylene molecule lies perpendicular to the molecular plane , the back bonding may take place either through d_yz or d_xz filled orbitals , but it may take place only through dxyfilled orbital.

When C₂H₄  molecule exists in the plane of the molecule . As a consequence, C₂H₄ molecule lies lies perpendicular to the PtCl₃ molecular plane for better scope of back bonding. 

Why Cl ^– ion trans to the ethylene has the large Pt –Cl bond distance than the other two Pt –Cl bond distance in Zeise’s salt? 

The above  fact can be explained with the help of trans orienting power of ligands. 

The metal olefin bond has multiple bond character due to back bonding and due to this multiple bond character (π-bonding effect ) decreases the metal–carbon bond length.

As a result, the metal chlorine bond trans to the ethylene molecule weakens, that is, Pt –Cl bond length increases. 

But Pt –Cl bond character cis to C₂H₄ molecule is not affected due to this π-bonding.

Therefore , Pt–Cl distance trans to C2H4 is larger than the other two Pt –Cl bond distance .

Is  the rotation of C₂H₄ molecule in Zeise’s salt hampering the stability of the complex ?

From bonding in Zeise’s salt it can be said that ethylene molecule can either lie on the same plane to which the PtCl₃ unit belongs on lie perpendicular to the plane of the molecule.

In each case back bonding is possible . In first case back bonding takes place through filled d_xy metal orbital and in the second case it may take place either through d_yz or d_xz metal orbitals.

Since in the second case possibility of such back bonding is greater and hence the stability is more .

Experimental measurement has shown that stability difference is of the order of 5 – 6 kcal per mole , that is, the energy barrier of C₂H₄ molecule is only 5 – 6 kcal per mole.

So it is clear from the above explanation that free rotation of C₂H₄ molecule is possible in Zeise’s salt. 

What is Zeise’s salt ? Discuss the structure and bonding of Zeise’s  salt  ?

Why the C –C olefinic bond length in Zeise’s salt is greater than the C –C bond in free hydrocarbon ?

Why the ethylene molecule is perpendicular to the PtCl₃ molecular plane in Zeise’s  salt ?

Why Cl ^–  ion trans to the ethylene has the large Pt –Cl bond distance than the other two  Pt –Cl bond distance in Zeise’s salt ?

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