Catalysts bonding character

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

Another type of metal-carbon bond, the metal carbene bond (with carbene of an electrophilic or nucleophilic character), appears to be the active bond in transition metal-based catalysts for the ring-opening metathesis polymerisation of cycloolefins. Such a bond, which is co-originated with metal by the sp2-hybridized carbon atom, possesses a a, n double bond character (Mt = C) [34,35], The enchainment of the coordinating cycloolefin at the active site... 

There is no concrete evidence that tin or lead alkyls can initiate coordinated anionic polymerization, and this is in accord with their covalent bond character and low electrophilicity. Polymerizations which have been initiated with these catalysts are most probably free radical in nature. 

Hydrogenation of benzothiophene (BT) to 2,3-dihydrobenzothiophene (DHBT) takes place much more readily than reduction of thiophene, since the C(2)=C(3) bond in this molecule has more of a localized double bond character, instead of the pseudoaromatic nature of an isolated thiophene ring or of a heterocycle in a fused-ring system like dibenzothiophene and its derivatives. As shown by the data in Table 3.2, several Ru, Os, Rh, and Ir complexes are known to be very efficient catalysts for the reduction of BT to DHBT under moderate reaction conditions [74-86]. 

Studies of catalysts used in conventional chemical reactions suggest that it is not entirely defensible to correlate the properties of a catalyst, which are basically concerned with its surface, with properties such as work function and, in the case of transition metals, with d-bond character. In chemisorption and in catalysis, the chemical properties of the individual surface species are more important than the collective properties of metal catalysts such as can be deduced from the band model. The most significant result is the correlation found between a low heat of chemi-mrption for the adsorbate and a high catalytic activity for the adsorl nt. 

Nitrogen is incorporated in a hexagonal ring having three double bonds. The compounds in this family are those which can give a basic character to petroleum products and are thus a poison to acid catalysts. 

Fig. XVIII-17. Correlation of catalytic activity toward ethylene dehydrogenation and percent d character of the metallic bond in the metal catalyst. (From Ref. 166.)...

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

The tertiary metal phosphates are of the general formula MPO where M is B, Al, Ga, Fe, Mn, etc. The metal—oxygen bonds of these materials have considerable covalent character. The anhydrous salts are continuous three-dimensional networks analogous to the various polymorphic forms of siHca. Of limited commercial interest are the alurninum, boron, and iron phosphates. Boron phosphate [13308-51 -5] BPO, is produced by heating the reaction product of boric acid and phosphoric acid or by a dding H BO to H PO at room temperature, foUowed by crystallization from a solution containing >48% P205- Boron phosphate has limited use as a catalyst support, in ceramics, and in refractories. 

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