Ligands steric effect

Tetrakisligand nickel(0) complexes have tetrahedral stmctures. Electronic stmctures have been studied and conformational analysis performed. Quantitative equiUbria measurements of the ligands in these complexes imply a dominant role for ligand steric effects when the complexes are employed as catalysts (94). 

Professor Cotton s studies have considerably clarified our understanding of di- and tri-nuclear metal-metal bonded compounds. For higher nuclearity clusters, rationalisation of structures, bonding, reactivity etc., must be much more tenuous because of the increased number of variables (metal-metal, metal-ligand, steric effects) now present. However, I would briefly like to present a few trends which seem to be emerging in this area. 

Guezi, I.A. and Wendt, M. (2006) An improved method for the computation of ligand steric effects based on solid angles. Dalton Trans., 3991. 

Composition does not necessarily, of course, reflect coordinate number at the metal. This is especially so for the species for which n > 6 there is no evidence for [MnN7] or [MnN8] polyhedra except where special ligand steric effects promote higher coordinate numbers. 

Except where specific ligand steric effects add kinetic stability, the compounds of Mn1", including the low spin [Mn(CN)6]3-, are quite labile. 

In metal clusters that are more highly substituted with phosphines, ligand steric effects may be significant. The rates of phosphine dissociation from fr4(CO)gL4 show evidence of a steric effect ... 

T. L. Brown, Inorg. Chem., 31, 1286 (1992). A Molecular Mechanics Model of Ligand Effects. 3. A New Measure of Ligand Steric Effects. 

Olefins are also carbonylated, although at a slower rate than 1-olefins. Similarly, cis-2-heptene is five times more active than trans-2-heptene. Ligand steric effects play a prominent role with the phosphine modified Pd catalysts. For example, tris(p-meAoxy-phenyl)-phosphine gives acceptable rates, whereas the catalyst employing the builder, tris(o-methoxyphenyl)-phosphine, has only 5% of the activity. 

Ariafard A, Yates BF (2009) Subtle balance of ligand steric effects in Stille transmetalation. J Am Chem Soc 131 13981-13991... 

---