Carboxylate complexes bonding modes

As unidentate ligands, carboxylates are expected to (i) lose the equivalence of the two carbon- oxygen bonds found in the anion and (ii) have one metal-oxygen distance considerably shorter than the next shortest M—O contact. Lithium acetate dihydrate exemplifies this14 with C—O distances of 133 and 122 pm and Li—O distances of 227 and 257 pm. Most examples of unidentate carboxylate complexes have this classical configuration of M(0—C) and C=0 respectively so certainly the presence of features (i) and (ii) unambiguously determine this mode of coordination. 

Fig. 22 a Schematic illustration of a predominantly one-handed helix induction in 66 upon complexation with chiral carboxylic acids, b Expected hydrogen-bonding modes... 

Several studies have addressed how car-boxylated Ru complexes interact with Ti02 surface [303, 378, 380-382, 385], In most vibrational studies, an asymmetric C—O stretch consistent with carboxylate binding modes (bidentate or bridging) has been reported [378, 380, 385]. Evidence for ester linkages and H-bonded network have also been reported [303, 376, 382]. Qu and Meyer [378] demonstrated that high surface proton concentrations favor carboxylic acid type linkage where low proton concentrations favor carboxylate type binding modes for complexes. Possible modes of coordination of the... 

The enhanced nucleophilicity of coordinated CO2 in complex [Fe(C02)(depe)2] allowed reaction with organotin chlorides. In the resulting carboxylate complexes 31, the CO2 unit links the two metals via a fi-ij C) if(P,0 ) bonding mode. Although the X-ray data of 31b exclude the contribution of a carbenoid form in the solid state, chemical shifts of IX-CO2 in the NMR spectra indicate a borderline case between oxycarbonyl and dioxycarbene character... 

The fact that organosamarium allyl complexes of the type Cp 2Sm(CH2CH=CHR) can arise from the treatment of Cp 2Sm or [Cp 2Sm(/r-H)]2 with a variety of olefin and diene substrates makes samarium chemistry more intriguing. The reaction modes are illustrated in Scheme 18. These allylsamarium complexes 55 react with C02 to afford the carboxylate products 56, which participate in monometallic/bimetallic interconversions (Equation (10)). Carbon disulfide and 0=C=S also insert into carbon-samarium bonds, which form only monometallic species.29... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

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