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Delta bonding and Catalysis

Metal-metal multiple bonding is an important feature of the chemistry of many transition
elements and is very important to the field of metal cluster chemistry. A typical property of
metals is that rather than forming straight chains or rings, metals tend to agglomerate so as to
form maximum number of bonds with minimum number of adjacent metal atoms, giving rise to
either metal-metal multiple bonds or metal cluster compounds. The existence of a quadruple
bond in inorganic systems was first recognized in 1964 when the compound [Re2Cl8] 2– was
Inorganic clusters may contain quadruple bonds due to the formation of an extra bond called δ
bond which is formed by the overlapping of the dx2-y 2 or dxy orbitals.
[Re2Cl8]2- is characterised by shorter M-M bond distances Re-Re 0.224nm and eclipsed
configuration of Cl atoms linked to Rhenium.
Since Re-Re bond distance is shorter, the Cl atoms are very close at a distance less than the sum
of their van der Waal’s radii. So they are expected to be in staggered configuration.
[Re2Cl8]2- contains two ~square planar ReCl4 units
Re [Xe] 4f14 5d5 6s2
Re3+ [Xe] 4f14 5d4 6s0
2In [Re2Cl8] the Z axis is taken as the line joining the two Re atoms.
Each Re atom is linked to four Cl atoms through dsp2 hybridised orbitals involving dx2-y2 orbital.
dz2 forms Re-Re σ bond. dxz and dyz form two π bonds.
The dxy orbitals of both Re atoms overlap confacially to form δ bond. Thus there is quadruple
bonding between Re-Re atoms in [Re2Cl8]2-.
This δ bond can be formed only if Cl atoms are eclipsed, thereby forcing Cl atoms to take
eclipsed configuration.
The degree of overlapping and hence the bond strength is in the order σ > π > δ.
Other compounds with δ bonding-
Catalysis A catalyst is a substance that increases the rate of a reaction but it is not itself
consumed. A catalyzed reaction is faster (or in some cases more specific) than an uncatalyzed
version of the same reaction because the catalyst provide a different reaction pathway with a
lower activation energy. Catalysis plays a vital role in the production of fuels, commodity
chemicals, fine chemicals and pharmaceuticals as well as providing the means for experimental
safeguards all over the world.
More than 60% of all chemical products and 90% of all chemical processes are based on
catalysis. A whole new technology appeared based on organometallic catalysis in olefin
[ Nobel prizes for chemistry have been awarded to Ziegler and Natta (1963), Fischer and
Willkinson (1973) for their discoveries in organometallic chemistry and homogeneous catalysis •
More recently, in 2005, Chauvin, Schrock, and Grubbs were awarded Nobel Prize for
developing organometallic catalysts for olefin metathesis. ]
Desired Properties of Catalysts
(a) Selectivity • A selectivity catalyst yields a high proportion of the desired product with
minimum amounts of side product. In industry there is considerable economic incentive to
develop selective catalysts.
(b) Lifetime • A small amount of catalyst must survive through a large number of cycles of it is
to be economically viable • A catalyst may be destroyed by side reactions to the main catalytic
cycle by the presence of small amounts of impurities in the starting material. • For example,
many alkene polymerization catalysts are destroyed by O2, so these polymerizations are carried
out in absence of air.
Catalysis can be of two types: Heterogeneous and Homogeneous
Homogeneous Catalysis: They are present in the same phase as the reagents
Heterogeneous Catalysts: They are present in a different phase from that of the reactants
Steps involved in catalysis
In oxidative addition, a metal M inserts into a covalent bond of a compound XY. The XY
bond is broken and two new bonds form: MX and MY. The metal .loses two valence
electrons and gains two new ligands, X and Y. Oxidative addition is a key step in many
catalytic cycles. Often, it is the slow (i.e. rate-determining) step, because a covalent bond
(usually in the substrate) is broken. This creates a metastable species that easily reacts further
in the cycle.
Reductive elimination involves the elimination or expulsion of a molecule from a
transition metal complex. In the process of this elimination, the metal centre is reduced by
two electrons.
An insertion or migration step involves the introduction of one unsaturated ligand into
another metal–ligand bond on the same complex.
Insertion and reductive elimination are common bond-forming steps, just as oxidative
addition is a common bond-breaking step.
Wilkinson’s catalyst - Hydrogenation of alkenes is an important industrial reaction that requires
high temperature and pressure. Wilkinson’s catalyst can bring about hydrogenation of a wide
variety of alkenes at pressure of hydrogen close to 1 atm or less • Wilkinson’s catalyst is highly
sensitive to the nature of phosphine ligand and alkene substrate.
+ H2
- PPh3
- PPh3
+ H2
Disadvantages 1. Rhodium is costly and the conversion of rhodium to the catalyst is also a costly
2. Wilkinson’s catalyst is a soluble catalyst, separation of the catalyst from the product is
Hydroformylation reaction is the addition of H2 and CO to an alkene to form an aldehyde.
Hydroformylation increases the carbon chain by one carbon and introduces O atom in the chain.
Hydroformylation was discovered by Otto Roelen in 1938 during an investigation of the origin
of oxygenated products occurring in cobalt catalyzed Fischer-Tropsch reactions. Roelen's
observation that ethylene, H2 and CO were converted into propanal, and at higher pressures,
diethyl ketone, marked the beginning of hydroformylation.
Roelen's original research into hydroformylation involved the use of cobalt salts that, under
H2/CO pressure, produced HCo(CO)4 as the active catalyst. In 1960 and 1961 Heck and Breslow
proposed what is now accepted as the general mechanism for hydroformylation
Roelen’s catalyst - Co2CO8
1) In the first step Co2CO8 combines with H2 at high pressure and forms 18 electron
tetracarbonyl hydrido cobalt complex.
Co2CO8 + H2 → 2 HCoCO4
HCoCO4 → HCoCO3 + CO
2) HCoCO3 gets co-ordinated to the alkene and forms 18 electron complex which
undergoes insertion reaction with the hydrido ligand.
HCoCO3 + CH2 CHR→ H (CO)3Co
→ (CO)3CoCH2 CH2R
3) At high pressure the alkyl complex undergoes migratory insertion and forms an acyl
(CO)3CoCH2 CH2R +CO→(CO)4CoCH2 CH2R→ (CO)3CoCH2 CH2R
4) In the next step the reaction with H2 finally releases the aldehyde and regenerates the
In 1965 Osborn, Young and Wilkinson reported that Rh(I)-PPh3 complexes were active and
highly regioselective hydroformylation catalysts for 1-alkenes, even at ambient conditions, it was
Wilkinson's work that really ignited serious interest in rhodium phosphine hydroformylation
catalysts. The initial catalyst system was derived from Wilkinson's catalyst, RhCl(PPh3)3, but it
was rapidly discovered that halides were inhibitors for hydroformylation.
Polymerisation of Alkenes: The Ziegler-Natta catalyst, is a powerful tool to polymerize αolefins with high linearity and stereoselectivity. A typical Ziegler-Natta catalyst system usually
contains two parts: a transition metal (Group IV metals, like Ti, Zr, Hf) compound and an
organoaluminum compound (co-catalyst). The common examples of Ziegler-Natta catalyst
systems include TiCl4 + Et3Al and TiCl3 + AlEt2Cl.
In 1953, German chemist Karl Ziegler discovered a catalytic system able to polymerize ethylene
into linear, high molecular weight polyethylene which conventional free radical polymerisation
techniques could not make. The system contained a transition metal halide with a main group
element alkyl compound. This was the first synthesis of high density polyethylene (HDPE),
Following the catalytic design, Italian chemist Giulio Natta found that polymerisation of αolefins resulted in stereoregular structures, either syndiotactic or isotactic, depending on the
catalyst used. Using a mixture of triethylaluminum and titanium tetrachloride, Natta reported the
first synthesis of linear, isotactic and syndiotactic polypropylene, which result in crystalline
polymers opposed to the atactic form which is amorphous. Because of these important
discoveries, Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry in 1963.
The activation of ZN catalyst system by coordination of AlEt 3 to Ti atom.