Ligand properties, structure, reactivity effects

Recent progress in the chemistry of structurally well-defined lanthanide ketyl and ketone dianion complexes is reviewed, with particular emphasis on the ligand effects on the reactivity of these complexes. It has been demonstrated that the stability and reactivity of the ketyl radical and ketone dianion species strongly depend on the steric and electronic properties of the ancillary ligands, the structure of their parent ketones, as well as the nature of the metals to which they are bound. Fine-tuning these factors can control the reactivity of these species. Generation and reactions of dianionic thioketone and imine species are also briefly described. 

The concept of a structure-reactivity relationship implies that changes in structure should be quantitatively reflected in some measurable reactivity parameters associated with the molecule. For metal complexes, the capacity of the ligand structure to influence chemical properties is measurable in terms of reactivity parameters such as stability constants, rates of ligand dissociation, and reduction potentials. The influence of structure on chemical reactivity can often be rationalized in terms of steric and electronic components. MM calculations can be used to quantify the steric components present in the system. For metal complexes with series of ligands in which electronic effects are relatively constant (same number and type of donor atoms), reactivity differences can be attributed primarily to steric effects. In such cases, MM calculations have been used to obtain correlations between structure and reactivity. 

In the gas phase, ions may be isolated, and properties such as stability, metal-ligand bond energy, or reactivity determined, full structural characterization is not yet possible. There are no complications due to solvent or crystal packing forces and so the intrinsic properties of the ions may be investigated. The effects of solvation may be probed by studying ions such as [M(solvent) ]+. The spectroscopic investigation of ions has been limited to the photoelectron spectroscopy of anions but other methods such as infrared (IR) photodissociation spectroscopy are now available. 

---