Steric effects and electron delocalization
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص219-220
2025-09-01
349
Steric effects and electron delocalization
Key point: The stability of chelate complexes of d metals involving diimine ligands is a result of the chelate effect in conjunction with the ability of the ligands to act as π acceptors as well as donors. Steric effects have an important influence on formation constants. They are particularly important in chelate formation because ring completion may be difficult geometrically. Chelate rings with five members are generally very stable because their bond angles are near ideal in the sense of there being no ring strain. Six-membered rings are reasonably stable and may be favoured if their formation results in electron delocalization. Three-, four-, and seven-membered (and larger) chelate rings are found only rarely because they normally result in distortions of bond angles and unfavourable steric interactions. Complexes containing chelating ligands with delocalized electronic structures may be stabilized by electronic effects in addition to the entropy advantages of chelation. For example, diimine ligands (78), such as bipyridine (79) and phenanthroline (80), are constrained to form five-membered rings with the metal atom. The great stability of their complexes with d metals is probably a result of their ability to act as π acceptors as well as donors and to form π bonds by overlap of the full metal d orbitals and the empty ring π* orbitals (Sec tion 20.2). This bond formation is favoured by electron population in the metal t2g orbitals, which allows the metal atom to act as a π donor and transfer electron density to the ligand rings. An example is the complex [Ru(bpy)3]2 (81). In some cases the chelate ring that forms can have appreciable aromatic character, which stabilizes the chelate ring even more. Box 7.3 describes how complicated chelating and macrocyclic ligands might be synthesized.
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