UNIT - IV Carbon pi-donor complexes

Rajah Serfoji Govt. College (Autonomous), Thanjavur – 613 005

            M.Sc., Chemistry – CBCS Pattern           SEMESTER – II

Inorganic Chemistry - II                                                                     Code: R2PCHEL2

UNIT  - IV                         Carbon  pi-donor  complexes 

HYDROGENATION OF ALKENES

The double bond of an alkene consists of a sigma (σ) bond and a pi (π) bond. Because the carbon-carbon π bond is relatively weak, it is quite reactive and can be easily broken and reagents can be added to carbon. Reagents are added through the formation of single bonds to carbon in an addition reaction.

Introduction

An example of an alkene addition reaction is a process called hydrogenation.In a hydrogenation reaction, two hydrogen atoms are added across the double bond of an alkene, resulting in a saturated alkane. Hydrogenation of a double bond is a thermodynamically favorable reaction because it forms a more stable (lower energy) product. In other words, the energy of the product is lower than the energy of the reactant; thus it is exothermic (heat is released). The heat released is called the heat of hydrogenation, which is an indicator of a molecule's stability Although the hydrogenation of an alkene is a thermodynamically favorable reaction, it will not proceed without the addition of a catalyst 

Common catalysts used are insoluble metals such as palladium in the form Pd-C, platinum in the form PtO2, and nickel in the form Ra-Ni. With the presence of a metal catalyst, the H-H bond in H2 cleaves, and each hydrogen attaches to the metal catalyst surface, forming metal-hydrogen bonds. The metal catalyst also absorbs the alkene onto its surface. A hydrogen atom is then transferred to the alkene, forming a new C-H bond.  A second hydrogen atom is transferred forming another C-H bond. At this point, two hydrogens have added to the carbons across the double bond.  Because of the physical arrangement of the alkene and the hydrogens on a flat metal catalyst surface, the two hydrogens must add to the same face of the double bond, displaying syn addition.

Common Applications

Hydrogenation reactions are extensively used to create commercial goods.Hydrogenation is used in the food industry to make a large variety of manufactured goods, like spreads and shortenings, from liquid oils. This process also increases the chemical stability of products and yields semi-solid products like margarine. Hydrogenation is also used in coal processing. Solid coal is converted to a liquid through the addition of hydrogen. Liquefying coal makes it available to be used as fuel.

Hydroformylation, also known as oxo synthesis or oxo process, is an important homogeneously catalysed industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon double bond.  Hydroformylation is also used in specialty chemicals relevant to the organic synthesis of fragrances and natural products. The development of hydroformylation, which originated within the German coal-based industry, is considered one of the premier achievements of 20th-century industrial chemistry

The process typically entails treatment of an alkene with high pressures (between 10 to 100 atm) of carbon monoxide and hydrogen at temperatures between 40 and 200 °C. Transition metal catalysts are required.

The original catalyst was HCo(CO)_4.

 A generic rhodium catalyst, where PAr₃ = triphenylphosphineor its sulfonated analogue Tppts. Seetris(triphenylphosphine)rhodium carbonyl hydride.

The overall mechanism resembles that for homogeneous hydrogenation with additional steps. The reaction begins with the generation of coordinatively unsaturated metal hydrido carbonyl complex such as HCo(CO)₃ and HRh(CO)(PPh₃)₃. Such species bind alkenes, and the resulting complex undergoes amigratory insertion reaction to form an alkyl complex.

A key consideration of hydroformylation is the "normal" vs. "iso" selectivity. For example, the hydroformylation ofpropylene can afford two isomeric products, butyraldehyde or isobutyraldehyde:

H₂ + CO + CH₃CH=CH₂ → CH₃CH₂CH₂CHO ("normal")

vs.

H₂ + CO + CH₃CH=CH₂ → (CH₃)₂CHCHO ("iso")

These isomers result from the differing ways of inserting the alkene into the M–H bond. Of course, both products are not equally desirable. Much research was dedicated to the quest for catalyst that favored the normal isomer.

When the hydrogen is transferred to the carbon bearing the most as tributyl phosphine), then this steric effect is greater. Hence, the mixed carbonyl/phosphine complexes offer a greater selectivity toward the straight chain products.

Olefin Complexes (I): Zeise's Salt

Zeise's salt, the first p-complex ever obtained was made in 1827 by the Danish Chemist Zeise who boiled a mixture of KCl and PtCl₄ in ethanol. Today, the compound is obtained in somewhat higher yield by bubbling ethylene through a solution of K₂PtCl₄.Zeise's Salt was also the first organometallic compound ever published.

Olefin Complexes (II): The Chatt-Duncanson Model

 

The now generally accepted description of the bonding situation in olefin complexes was given by Chatt and Duncanson.

By measuring the IR-spectrum of Zeise's salt, Chatt and Duncanson recognized that the CC-bond of ethylene in Zeise's salt still possesses double bond character but to a lesser degree than free ethylene. The CC-stretching frequency was lower than that of free ethylene.The metal donates electrons to the antibonding p*-orbital of the olefin thus reducing the bond order and accordingly, the CC stretching frequency.

Olefin p-complexes can also be conveniently described with two mesomeric structures:

With increasing strength of the olefin-metal interaction, the metal-carbon bond distance will decrease and the CC-bond distances will increase. If this geometric distortion is strong enough, it is legitimate to describe the olefin-complex as metalla-cyclopropane. It is possible to estimate the strength of the metal-olefin interaction from structural data. More convenient is the analysis of the IR-spectrum.

 The strength of the metal-olefin interaction depends on the olefin's   substituents as well. Electron withdrawing substituents (-CN, F etc.) favor complexation and the cyclopropane structure.

An entirely different type of olefin complex was obtained by Norton in 1982:

Olefin Complexes (III): Synthesis

Olefins act as two electron donors and can replace other to electron donors in metal complexes. These processes are often thermodynamically unfavorable and require activation of the complex.

This is typically done my transforming an 18 VE complex into an 16 VE complex. With very few exceptions, 16 VE complexes react spontaneously with olefins to give h²-olefin complexes.

Especially reactive are cationic 16 VE complexes:

The replacement of CO ligands in carbonyl complexes usually requires photochemical activation, but dienes like 1,3-butadiene or norbornadiene can react thermally:

Olefin complexes are usually more reactive than carbonyl complexes and are valuable starting materials for organometallic research. In favorable cases, homoleptic olefin complexes can be stable. The complex Ni(COD)₂ is commercially available. The best synthesis uses NiCl₂ as starting material, triethylaluminum as reducing agent and butadiene as stabilizer.

Butadiene prevents the precipitation of metallic nickel by forming a labile olefin complex and becomes subsequently replaced with COD.The exchange of CO for olefins is usually thermodynamically unfavorable. Nevertheless, the reaction often occurs readily because any CO formed can rapidly escape from the reaction solution into the gas phase.

The reactivity of the olefin also plays a significant role. Particularly reactive is norbornadiene:

More often, the more basic nitriles are used to obtain the labile nitrile complexes first:

The use if propionitrile instead of the more readily available acetonitrile allows higher reaction temperatures and reduced reaction times (a few h for propionitrile, many days for acetonitrile.

Allyl Complexes

The first allyl-complex was obtained 1959 by Smidt and Hafner.The allyl group can coordinate to transition metal fragments in as h¹- or h ³-ligand:

The allyl ligand can be counted as cation (2e-donor), anion (4e-donor) or as radical (3e-donor).

Depending on the nature of the metal and other co-ligands, allyl-complexes can behave as electrophilic or nucleophilic allyl-synthons (sources of allyl⁺ or allyl^-).

The bonding situation in p-Allylcomplexes can be described by MO theory.