Steric effects encouraging

When some other reaction parameter, Z, such as the log of a rate constant, is plotted on to this steric and electronic map on an axis normal to the plane of the paper the comparative contributions of 6 and v should become apparent. A purely steric effect will slope north or south (the reader is encouraged to view Figures 26-28 of ref. 187 to appreciate this fully). Weimann and co-workers211 used Tolman s methodology to show the % steric effect in the oligomerization of butadiene catalyzed by nickel phosphine complexes. 

However, those for the left amide were very bad, presumably because of the profound negative effect of the portion of the left amide being closer to the side chain, which lacks a detailed description in this model (e.g., steric and inductive effects). Although the results were fairly encouraging, it is clear that other influences, such as steric effects (e.g., blocking of the amide hydrogen by bulky side chains) and inductive effects (e.g., polarization of aromatic rings), need to be included for better overall predictions. 

This overview of MM calculations on metal complexes demonstrated the versatility and increasing utility of the MM approach in understanding steric effects in coordination chemistry. The quantitative correlation of MM results with experimentally determined reactivity parameters should encourage the further application of MM as a tool for metal-selective ligand design. As additional force field parameters for metal complexes are developed and refined, the use of MM methods for metal complexes will continue to gain in popularity. 

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

A steric inhibitory effect can, in turn, be inhibited, as in the cation of 4,5-dimethylquinazoline, which, unlike that of 4-methylquinazoline, is highly hydrated, the water being added across the 3,4-double bond. Apparently. overcrowding in the C-4 and C-5 area, which is relieved by hydration, encourages this process.44... 

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