Acyclic donor ability

Figure 16.2 Donor abilities of acyclic carbenes in comparison with an NHC.

Over the past 20 years, there has been a growing interest in the use of acyclic (or open) ( 7 -pentadienyl)iron cations 1 (Scheme 1) as synthetic tools for G-C bond formation. Complexation of Fe(CO)3 to a diene distinguishes between two enantiotopic faces of the ligand and directs diastereoselective bond formation at unsaturated centers adjacent to the coordinated diene. In addition, the electron-donor ability of the carbonyliron group allows for the generation of cationic centers adjacent to the coordinated diene. ... 

Imines, either acyclic or macrocyclic but invariably multidentate, have a rich coordination chemistry that has been investigated at length. The 7r-accepting ability of imine donors results in the stabilization of lower oxidation states relative to their saturated amine analogs, and there exist many air-stable divalent imine complexes of Co, in contrast to amine relatives. The hexa-methyl-diene (52) has been the most intensively studied ligand of this class, particularly when complexed with Co. In addition, Co complexes of the dimethyl (53),295,296 tetramethyl (54),297 pentamethyl (55)298 and octamethyl (56)299 macrocyclic dienes are also known. In the presence of... 

Crown thioethers prompt attempts to impose poly(thioether) coordination upon metal ions, even those for which few thioether complexes were previously known. As a class, simple thioethers (e.g. MejS) are not particularly good ligands for both electronic and steric reasons. Thioethers exhibit weaker a-donor and 7t-acceptor ability than, for example, phosphines. Consequently they also show less binding affinity. In addition, the appreciable bulk of their terminal alkyl groups further hinders complexation. This latter factor is partially circumvented in acyclic polythioethers, and such ligands display respectable chelate effects. 

---