Coordination sites organometallic chemistry

Although very few complexes capable of closely mimicking the structure of HDS active sites are known, in the following Chapters a wealth of results obtained from coordination and organometallic chemistry will be presented, some of which are very clearly in line with a number of the possibilities discussed above, thus adding to this very lively discussion. 

Organometallic chemistry of pyrrole is characterized by a delicate balance of the ti N)- and -coordination modes. Azacymantrene is an illustration of the considerable nucleophilicity of the heteroatom. However, azaferrocene can be alkylated at C2 and C3 sites. Ruthenium and osmium, rhodium, and iridium chemistry revealed the bridging function of pyrroles, including zwitterionic and pyrrolyne complex formation. The ti (CC) coordination of osmium(2- -) allows versatile derivatizations of the heteroring. 

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

I 3 Catalytic Properties of Single Site Catalysts Prepared via Suface Organometallic Chemistry Coordination [2+2]-cycioaddition [2+2]-cycioreversion De-coordination... 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

Vacancies were later called coordinately unsaturated sites (cus). This is more in line with terminology used in organometallic chemistry. In view of the present understanding of the nature of the active sites, SBMS or Co(Ni)-Mo-S, the following discussion describes mechanisms in terms of catalysis by organometallic complexes. The references available on this topic are too numerous to mention, and the mechanisms are very well understood. A particularly useful reference is the book by Candlin, Taylor, and Thompson (90), although there are many others that can be consulted. 

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