Table 1 Amine component Co Cu Cyclohexylamine -f -f( )-Menthylamine + + (-f-)-neoMenthylamine(-j-)-isoMenthylamine 0 4- ()-weoisoMenthylamine 0 4- 4- = isolation of the complex bis (salicylideneamine) M.= no reaction: formation of metal hydroxide and unchanged Schiff's base. 0 = formation of oils or tar and unchanged Schiff's base.The inability of salicylidene-(+)-neomenthylamine to form complexes with either cobalt or copper points to a strong steric hindrance with respect to both the nitrogen atom and the phenolic hydroxyl in this case. ( )-Menthylamine and (+)-neomenthylamine differ only in the configuration of the amino-group at Ct), and the ready formation of complexes with ( )-menthylamine (with both copper' and cobalt) is explainable on the basis of this relationship. A consideration of the probable conformations of strainless rings for the amin.es3>4 suggests that the c^/cZohexane ring contributes considerably to the steric hindrance preventing complex formation with (-f-)-neomenthyl-amine, in which the amino-group occupies an axial position, whereas no such effect impedes complex formation with ( ) -menthylamine, which has an equatorial amino-group. With copper and salicylidene- (-f)-somenthyl-amine the expected complex was isolated as readrly as in the case of ( ) -menthylamine; but with cobalt the reaction produced only a reddish tar together with unchanged base, and no solid complex could be isolated from the mixture. It is not immediately obvious why cobalt does not form a derivative with the (+)-somenthylamine base while copper does, even on the assumption of a tetrahedral arrangement for the expected cobalt complex. This difference observed between the behaviour of salicylidene -( ) -menthylamine and that of salicylidene -(-f-) -isomenthylamine towards cobalt ions was not expected, as the difference between the two amines is believed to be only in the position of the methyl group (le in the case of menthylamine and Ip in fc'somenthylamine4) . Although no complex could be obtained with cobalt and salicylidene-()-neo^somenthylamine, but only a reddish tar and unchanged Schiff's base, the copper derivative, on the other hand, was readily isolated. The lesser hindrance towards complex formation of ()-neoisomenthylamine compared with that of (+)-neomenthylamine is in agreement with the greater reactivity of neoisomenthol over that of neomenthol towards acid chlorides6. If the inability of salicylidene-(+)-neomenthylamine to form complexes with both copper and cobalt ions is due chiefly to the spatial arrangement of the ^'sopropyl and amino-groups, it seems unlikely that neo^somenthylamine has, in these reactions at least, a conformation in which the amino-group occupies an axial position. Rather, our results tend to support the view that neo^somenthyl derivatives are examples of conforma-tional instability (Mills, J. A., private communication), and that it is possible for the amino-group in neo^somenthylamine to adopt an equatorial position3. Relative pKa values have been determined for the four menthylamine epimers by potentiometric titra-ti,on in 50 per cent aqueous ethanol against hydrochloric acid. As can be seen from Table 2, the order of decreasing basic strength is ^omenthylamine, menthylamine, neomenthylamine, and neoisomenthyl-amine.The apparent non-reactivity of several of the salicylidene -menthylamine bases with cobalt, and of the neomenthylamine derivative with copper and cobalt cannot be ascribed to differences in the basicities of the amines. They are all comparably strong bases, and the electron density on the nitrogen atom in each case should not differ greatly throughout the series. Steric effects related to the tendency to form planar (copper) or tetrahedral (cobalt) derivatives as outlined above seem to provide the only other explanation of the difference in reactivity between copper and cobalt with the various bases studied. Table 2. j>K VALUES OF FOUR EPIMERIC MENTHYLAMINES DETERMINED IN 50 PER CENT AQUEOUS ETHANOL AT 20(+)-isoMenthylamine 9 '93 ) -Menthylamine 9-73+)-w0oMenthylamine 9-69 )-neoi0Menthylamine 9 -47