241 



-A-nim. -try i> that of the axial group D 3 , and therefore tin- 

 arrangement must be different from its mirror-iin 



!! ic \\v have an excellent example of a molecule compos< 

 cvfii identical units, which possesses a rather highly symmetrical 

 configuration, and which, notwithstanding this, may be obtained 

 in t\vo enantiomorphously related modifications, because its sym- 

 mrtry belongs to the groups which have only symmetry-properties 

 of the first order. The enormous rotatory power of these compounds 

 i^ tlnTrloiv merely due to the non-superposable arrangement as such, 



Fig. 164. 



The Symmetry of the Ions: [Me(X")a]. 



and to the special nature of the central metallic atom, not however 

 to any chemical contrast between the groups round the pluri- 

 valent atom *). 



That the special nature of the central metal-atom also takes a 

 predominant part in this, can be seen on comparing the magnitude 

 of the molecular rotation of some analogously composed salts of 

 the metals Cr and Co: 



(Eine) 



Br, (M)z, = ca. 1100; (M) c =ca. 545 

 Br, (M) D = ca. 2525; (M) c = ca. 906, 



1) Cf. W. J. Pope, Presid. Address to the Chem. Section. Brit. Assoc. Rep. 

 (1914): "It must however be insisted, that the observed optical activity is the 

 result of the enantiomorphism of the molecular configuration; the asymnu-trv 

 of a particular atom is not to be regarded as the cause of the optical activity, but 

 merely as a convenient geometrical sign of molecular enantiomorphism." 



16 



