ZEISE’S SALT – KPtCl3(C 2 H 4) Paper #3
November 18, 2014
ZEISE’S SALT – KPtCl3(C 2 H 4) Paper #3
November 18, 2014
ZEISE’S SALT – KPtCl3(C 2 H 4)
This is the first metal complex identified as an organometallic compound KPtCl3(C 2 H 4) obtained from reaction of ethylene with platinum (II) chloride by William Zeise in 1825. It was not until much later (1951–1952) that the correct structure of Zeise’s compound was reported in connection with the structure of a metallocene compound known as ferrocene. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand and features a platinum atom with a square planar geometry. Zeise’s salt is of historical importance in the area of organometallic chemistry as one of the first examples of an alkene complex and that is the major reason for selecting this title.
Inorganic chemistry is the study of the synthesis and behaviour of inorganic and organometallic compounds. This field covers all chemical compounds except the myriad organic compounds (carbon based compounds, usually containing C-H bonds), which are the subjects of organic chemistry.
Organometallic compounds are considered to contain the M-C-H group. The metal (M) in these species can either be a main group element or a transition metal. Operationally, the definition of an organometallic compound is more relaxed to include also highly lipophilic complexes such as metal carbonyls and even metal alkoxides.
In organometallic compounds, most p-electrons of transition metals conform to an empirical rule called the 18-electron rule. This rule assumes that the metal atom accepts from its ligands the number of electrons needed in order for it to attain the electronic configuration of the next noble gas. It assumes that the valence shells of the metal atom will contain 18 electrons. Thus, the sum of the number of d electrons plus the number of electrons supplied by the ligands will be 18. Ferrocene, for example, has 6 d electrons from Fe(II), plus 2 × 6 electrons from the two 5-membered rings, for a total of 18.
Zeise’s salt is a coordination compound, K+ ion and water molecule is present outside the coordination sphere. Both, the Cl-ion and ethylene are coordinated with Platinum ion, hence inside the coordination sphere. Molecular formula of the salt is given as K[PtCl3(C2H4)]·H2O
ZEISE’S SALT PREPARATION
W. C. Zeise, a professor at the University of Copenhagen was the first person to prepare zeise’s salt, he prepared this compound in 1820s while investigating the reaction of PtCl4 with boiling ethanol, and proposed that the resulting compound contained ethylene. in 1868 Birnbaum prepared the complex using ethylene. Zeise’s salt compound is now commercially available as a hydrate. Hydrates are inorganic salts “containing water molecules combined in a definite ratio as an integral part of the crystal that are either bound to a metal center or that have crystallized with the metal complex. Such hydrates are also said to contain water of crystallization or water of hydration. If the water is heavy water, where the hydrogen involved is the isotope deuterium, then the term deuterate may be used in place of hydrate. The hydrate is commonly prepared from K2[PtCl4] and ethylene in the presence of a catalytic amount of SnCl2. The water of hydration can be removed in vacuo.
ZEISE’S SALT PROPERTIES
|EISE’S SALT CHEMICAL PROPERTIES|
|mp||220 °C (dec.) (lit.)|
|density||2.88 g/mL at 25 °C (lit.)|
The first olefin complex of a platinum group metal to be discovered was Zeise’s salt, K[C,H,PtCl,]. These olefin complexes have, however,been the subject of intensive study during the past decade (2, 3), with the result that not only are large numbers of these compounds known but the structures of many are alsounderstood and applications for them are now being discovered.
In a homogeneous catalytic reaction where the net process is the conversion of anolefin to some other species, the original platinum metal salt or complex may be regarded as the catalyst and the olefm-metal complex as the catalyst-substrate combination.
There is much evidence to suggest that an olefin molecule co-ordinated to a metal atom has a reactivity which is different in kind from the normal reactivity of olefins. The act of co-ordination decreases the electron density between the olefinic carbon atoms, and hence renders the olefin liable to attack by nucleophilic reagents such as hydroxyl or acetate ions, or indeed any electron-rich species which is seeking an electron deficient site. It is not possible to judge at the present how far this hypothesis accounts for the reactions undergone by co-ordinated olefins, although it almost certainly explains the occurrence of oxidation and hydrogenation processes. study of olefin oxidation, it was observed that if the reaction was stopped before completion the olefin had isomerised and that an equilibrium mixture of isomers was present .This led to the discovery that palladous chloride and its olefin and nitrile complexes catalysed olefin.
ZEISE’S SALT STRUCTURE AND CHARACTERIZATION
Zeise’s salt K[PtC13. C2H4]. H20 forms monoclinic crystals, a = 10.750_+ 0″006, b = 8.405 _+ 0-003, c 4.836+0-002 A, fl= 97-73 + 0.06 °, space group P21, with two formula units in the unit cell.In Zeise’s salt and related compounds, the alkene rotates about the metal-alkene bond with a modest activation energy. Analysis of the barrier heights indicates that the π-bonding between most metals and the alkene is weaker than the σ-bonding. In Zeise’s anion, this rotational barrier cannot be assessed by NMR spectroscopy because all four protons are equivalent. Lower symmetry complexes of ethylene, e.g. CpRh(C2H4)2, are, however, suitable for analysis of the rotational barriers associated with the metal-ethylene bond. CC-bond of ethylene in Zeise’s salt still possesses double bond character but to a lesser degree than free ethylene. This is because the metal donates electrons to the antibonding p*-orbital of the olefin thus reducing the bond order and accordingly, the CC stretching frequency. With increasing strength of the olefin-metal interaction, the metal-carbon bond distance will decrease and the CC-bond distances will increase.
In zeise’s salt Pt(II) is the central metal ion which is a square planar complex and the complex is diamagnetic. Central Pt 2+ ion coordinated with 3 Cl-ions and 1 ethene molecule in a square planar geometry while the 4 coplanar H’s are not coplanar with the 2 C’s. Pt—Cl (1) bond length is longer than the Pt—Cl (2) and Pt—Cl (3) bond length. Which can be termed as the trans effect is and is kinetically controlled. The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence: F−, H2O, OH− < NH3< py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− < SO32− < PR3, AsR3, SR2, CH3− < H−, NO, CO, CN−,C2H4. This can be proved with X-Ray Crystallography, and neutron diffraction. The bond stretching frequency for:C=C is 1827 cm-1 (B.O.=2), C—C is 1650 cm-1(B.O.=1) and Ethene present in the Zeise’s salt 1738 cm1 (B.O.=1.54). the pi-acid alkene donates electron density into a metal d-orbital from π-symmetry bonding orbital between the carbon atoms (II). The metal donates electrons back from (a different) filled d-orbital into the empty π* anti bonding orbital (I).Both of these effects tend to reduce the carbon-carbon bond order, leading to an elongated C-C distance and a lowering its vibration frequency.
The coordination round the platinumatom is essentially square planar. The deviations from the coordination plane (i. e. the best plane throughthe platinum and three chlorine atoms) are Pt, + 0-002;CI(1), 0.000; C1(2),o-0.015; CI(3), +0-005; centre ofC-C bond, -0.15 A. The ethylene group is bound bya re-type bond to the platinum atom, with the C-Cbond approximately perpendicular to the coordinationplane• The deviations from ideal symmetry are small,though significant: the C-C bond is inclined at 86 ° to the coordination plane instead of 90 o, and its mid-point is 0.15 A below the plane; in addition, C(1) is displaced from the plane perpendicular to the coordination plane and passing through Pt and CI(1), by 0.12 A. The Pt-C distances are effectively equal, 2.15(2) A, and are rather longer than 2.08 A, the distance predicted for a single Pt-C a-bond. The C-C bond-length shows that coordination to the metal atom reduces the bond-order in ethylene from 2 in the uncoordinated molecule to about 1½ in this complex. Similar results were obtained in the substituted acetylene complex [PtClz(ButC = CBut) (ptoluidine)],where coordination reduces the C–C bond order by about ½, and the Pt-C bond [2.19(2) A] is also longer than expected for a single bond. It may be that olefins bond more strongly to platinum than to palladium.
Two broad conclusions emerge from this research of platinum metal salts as homogeneous catalysts. First, we sense the enormous scope for the development of novel chemical processes which is offered by the metalolefin system: there are without doubt many more applications awaiting discovery and exploitation. It is significant and important that homogeneous catalytic processes are often more selective and specific than the corresponding heterogeneous processes would be. Secondly, there is a pattern of behaviour which may have implications in wider fields. The types of complex formed depend in a way not yet clearly formulated on the structure of the metal atom.
The reactivity of metal-olefin complexes has resemblances to the reactivity of olefins adsorbed on metal surfaces. Thus it is probably no coincidence that palladium salts feature largely as homogeneous catalysts for reactions of olefms, while olefins are known to be more weakly adsorbed by palladium than by platinum.
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