Actor ligand

Selected values oi / and g Ligand f (actor Metal ion g (actor6... 

Extracellular ligand binding portion of the human tumor necrosis f actor receptor linked to the Fc portion of... 

One role of spectator ligands is to block certain sites, say of an octahedron, to leave a specific set of sites available for the actor ligands so the desired chemistry can occur. These spectator ligands are commonly polydentate with the donor atoms arranged in specific patterns. A small sample of such ligands is shown in Fig. 1.10. The tridentate ligands can bind to an octahedron either... 

Tertiary phosphines, PR3, are important because they constitute one of the few series of ligands in which electronic and steric properties can be altered in a systematic and predictable way over a very wide range by varying R. They also stabilize an exceptionally wide variety of ligands of interest to the organometal-lic chemist as their phosphine complexes (R3P) M-L. Phosphines are more commonly spectator than actor ligands. 

The allyl group, commonly an actor ligand, binds in one of two ways. In the monohapto form, 5.12, it is a simple le X-type ligand like Me, and in the trihapto fotm, 5.13, it acts as a 3e LX enyl ligand. It is often useful to think of 5,13 in terms of the resonance forms 5.14a and S.14b. Intermediate cases between 5.12 and 5.13 (t -allyls) are also known. ... 

Arenes usually bind to transition metals in the 6e, j -form 5.41, but t) (5.42) and Tj (5.43) stmcturcs are also known. In the i form the ring is usually strongly folded, while an arene tends to be flat. The C—C distances are u.sually essentially equal, but slightly longer than in the free arene. Arenes are much more reactive than Cp groups, and they are also more easily lost from the metal so arenes are normally actor, rather than spectator. Ligands. 

We now look in detail at compounds with multiple bonds between metal and ligand. We are chiefly concerned with multiple bonds to carbon, as in metal carbene complexes L M=CR2, which have a trigonal planar carbon and at least formally contain an M=C double bond, and ntetal carbyne complexes, L MsCR, which are linear and contain an M=C triple bond, but we also look at complexes with multiple bonds to O and N. These are more often actor rather than spectator ligands. 

The M=C bonds in the majority of the terminal caibenes described up to now are reactive. Such carbenes are actor ligands because the M=C bonds are Inro-ken in typical reactions. A -heterocyclic carbenes, " OT NHCs, are an excqr-tion—their M=C bonds are so stable that they often act as spectator Hgantk. They are gaining increasing attention in that role because, like phosphines, dtey are electronically and sterically tunable. Also like phosphines, they promote a wide series of catalytic reactions. For the moment we lack the same level of detailed understanding of their steric and electronic properties that is provided for phosphines by the Tolman map (Fig. 4.4). 

Carbenes such as CH2 can either form single M-C bonds to each of two metals (3.21) or act as a terminal ligand, M=CH2. The latter type has a particularly important chemistry, being the key actors in alkene metathesis, a catalytic reaction of growing importance (Chapter 12). Similarly, CH can bridge three (3.22) or two metals (3.23) or form a... 

CO can act as an unreactive spectator or a reactive actor ligand. The reactions of Eq. 4.8-Eq. 4.12 all depend on the polarization of the CO on binding and thus also on the cohgands and net charge change. For example, types 1 and 3 are promoted by the electrophihcity of the CO carbon and type 2 by nucleophiUcity at CO oxygen. 

The allyl group is commonly a reactive actor ligand in catalysis by undergoing nucleophilic attack. It either binds in the monohapto form as a le X hgand (5.11) or in the trihapto form (5.12) as a 3e LX enyl ligand with resonance forms 5.13a and 5.13b. 

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