Ligand properties steric crowding

Ligands can be used to modify the electronic properties, steric crowding, and even chirality around the metal in some situations. 

The Mannich adducts are readily transformed to optically active a-amino-y-lac-tones via a one-pot diastereoselective reduction and lactonization sequence and the tosyl group exchanged for a Boc group via a two-step procedure. The cop-per(II) ion is crucial for the success of this reaction [21]. It has the properties necessary both to generate the enol species in situ and, in combination with the C2-symmetric ligand, coordinate it as well as the imine in a bidentate fashion. The reaction proceeds via a cyclohexane-like transition state with the R substituent of the enol in the less sterically crowded equatorial position, which is required to obtain the observed diastereoselectivity (Fig. 5). 

Treatment of thulium diiodide with substituted phospholide and arsolide salts afforded stable bis(phospholyl)- and bis(arsolyl)thulium(ll) complexes (Scheme 201) as green solids, that were characterized by multinuclear NMR and X-ray crystal structures. The latter clearly revealed the beneficial effects of the steric and electronic properties of crowded phospholyl and arsolyl ligands for the stabilization of divalent thulium.727 Several other homoleptic samarium(ll) and thulium(n) phospholyl sandwich complexes containing the 2,5-di-/-butyl-3,4-dimethylphospholide (=dtp) or 2,5-bis(trimethylsilyl)-3,4-dimethylphospholide (=dsp) ligand have been synthesized and structurally characterized. X-ray studies revealed that [Sm(dtp)2]2 and [Sm(dsp)2]2 are both dimeric in the solid state due to... 

The rhodium complexes are more resistant towards oxidative addition than their iridium counterparts and this is believed to be linked to steric crowding, especially when employing bulky tertiary phosphine ligands. Work by Wilkinson explored several aspects of the steric and electronic properties of the rhodium(i) analogues, but definite crystal structural confirmation of the reaction... 

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