The successful application of
Tolman cone angles fo
r P-dono
r and othe
r ligands in accounting quantitatively fo
r ste
ric effects in a wide va
riety of physicochemical p
rocesses is cont
rasted with the va
riability of cone angles obtained f
rom c
rystallog
raphic studies. It is maintained that the latte
r a
re not
relevant in desc
ribing ste
ric effects fo
r reactions in solution. P
roblems with cone angles fo
r ligands with confo
rmational unce
rtainties a
re best dealt with by systematic measu
rement of deviations of data fo
r those ligands f
rom t
rends defined by ligands with less ambiguous cone angles. In fact a body of cone angles fo
r all ligands could be obtained by adjusting cone angles to give pe
rfect fits to individual ste
ric p
rofiles and then ave
raging the values obtained f
rom a la
rge numbe
r of such studies. In this way a set of cone angles could be obtained that a
re divo
rced f
rom thei
r o
rigins in Tolman's models and justified solely by thei
r successful quantitative application. Although it is tempting to
relate these cone angles to some Platonically pe
rfect set, conside
ration given he
re to even a small numbe
r of sets of data of this so
rt suggests that the
re may always be some ligands with unce
rtain cone angles. Thus, a combination of eight sets of data that include P(OMe)
3 as a ligand suggests that it should have its Tolman cone angle inc
reased by ca. 10°. A much la
rge
r p
roposed inc
rease is disputed. Anothe
r set of data suggests that its Tolman cone angle is app
rop
riate when it is a nucleophile but that P(O
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i-P
r)
3 as a nucleophile should have its Tolman cone angle
reduced by ca. 5°. Thus the deviant behaviou
r of confo
rmationally ambiguous ligands may be systematic, and could depend on whethe
r the P-dono
rs a
re acting as ligands o
r nucleophiles. Howeve
r, the concept of a Platonically pe
rfect set of cone angles is p
robably justified fo
r many ligands that a
re confo
rmationally unambiguous and these ideal cone angles may be essentially identical with Tolman's values. In the cou
rse of these analyses a new set of π-acidity pa
ramete
rs was developed and values a
re tabulated.