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
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 dxy,
dyz, and dxz.
When the coordinated ethylene molecule lies perpendicular to the molecular plane , the back bonding may take place either through dyz or dxz filled orbitals , but it may take place only through dxyfilled orbital.
When the coordinated ethylene molecule lies perpendicular to the molecular plane , the back bonding may take place either through dyz or dxz 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.
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.
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 dxy metal orbital and in the second case it may take place either through dyz or dxz 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.
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 ?
wow...everything is explained in great detail with accuracy...it helped a lot...thank you
ReplyDelete