Carbene complexes geometries

Carbene Complex Geometries. Molecular orbital studies of the various conformations of several transition metal-carbene complexes have been undertaken by the groups of Fenske and Hoffmann (8,13). Of the two... 

Figure 23 X-ray structures of the copper carbene complexes 53 and 54 and geometry of the molecules of 55. 54 reproduced with permission from ACS publications. 

Nucleophilic carbene 4 (R = Me, R = H) has been shown to disrupt the polymeric structure of beryllium dichloride to form an ionic carbene complex, 10 (15). The crystal structure of 10 revealed that the cation possesses a distorted tetrahedral coordination geometry at the beryllium center. The average Be-Cl bond distance (2.083(7) A) is... 

The 1 1 reaction of 76 or 77 (R = Me, R = H) with bromine resulted in oxidation and the formation of a carbene complex of SBr2 (79) or SeBr2 (80), respectively (55, 58-62). The X-ray crystal structure of 79 has been reported and showed that this sulfurane possesses an almost collinear axial three-center geometry with elongated S-Br... 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

Alkylation via approach of the electrophile from the least hindered side of an a-alkoxy vinyl -rhenium complex of antiperiplanar (alkoxy oxygen anti to the carbon monoxide oxygen) geometry has been proposed. The NMR spectroscopic data are consistent with the transient presence of rhenium-carbene complexes, such as 2 and 597. 

The ease with which the geometry of the metal carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction124. 

When we go from palladium to nickel, we stay within group 10 and retain the square planar geometry common to most d -complexes. We do not need to concern ourselves with the synthesis and structure of nickel(II) pincer carbene complexes. They are analogous to the palladium(II) homologues. However, in their applications, the Kumada-Corriu reaction [291,292] takes a prominent place besides the Heck and Suzuki reactions encountered also with palladium. 

Note Since the pincer geometry calls for meridonal coordination, tetrahedral nickel(II) pincer carbene complexes are not expected to exist. 

Figure 3.153 Coordination geometries in rhodium pincer carbene complexes.

Danopoulos and coworkers [473] then developed a ruthenium(II) pincer carbene complex exactly on the lines described above starting from the Grubbs catalyst. Reaction with the free carbene ligand results in displacement of the phosphane ligands and coordination of the central pyridine unit changes the coordination geometry from square pyramidal to octahedral (see Figure 3.157). 

The titanium(IV) complex just seen is not the only titanium carbene complex generated with this hydroxy functionalised NHC ligand. Arnold and coworkres reacted the potassium salt also with [Ti(thf)3Cl3], a titanium(III) precursor for which they established anew, cost efficient synthesis [39]. The product is a nonsymmetrical titanium(in) complex with octahedral geometry (see Figure 4.8). 

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