ISOMORPHISM, ISOSTENISM AND CO VALENCE 255 



many other cases of mixed crystals could be found if the same effort were 

 expended in looking for them among other types of compounds. 



Let us not consider in more detail the cases of isomorphism given by 

 Barker : 



The theory of chemical structure given in the recent paper in the 

 Jour. Amcr. Cliem. Soc. is in full accord with these cases of isomorphism, 

 and affords an explanation of them. In the second method ^^ of regarding 

 complex compounds with a coordination number of 6 or more, it was ex- 

 plained how groups such as chloride ion, ammonia, water, etc., could be 

 held by electrostatic attraction to a positively charged central atom. We 

 have already seen how in typical octet compounds such as potassium sulfate, 

 etc., oxygen is unicovalent and may thus be replaced by fluorine without 

 involving a change in the crystalline structure. The compounds in Groups 

 A. B. G, and I are examples of this kind where fluorine, iodine or oxygen 

 replace one another. Group C gives an illustration of a positive central 

 ion (Sn"" or Fe **^) surrounded by 6 other groups (CI" or H2O) forming 

 a complex ion. The HoO group in the first compound replaces one of the 

 chlorine ions of the second. 



The compounds of Groups D and H afford interesting illustrations of 

 the replacement of negative by positive ions and vice versa in a manner 

 exactly analogous to that of Na20-MgF2, etc., of Table IL Thus in 

 BeNaF4 the two sodium ions replace the two chlorine ions of the MnCl2.- 

 4H2O, the beryllium ion replaces the manganese ion and the fluorine ions 

 replace the water comolecules. Similarly, in comparing Znl2.4NH3 with 

 K2SO4 we see that the iodide ions have replaced the potassium ions. This 

 is evident if we apply the octet theory in the ordinary way to the com- 

 pound Znl2.4NH3. The number of available electrons in the atoms of this 

 compound is c = 48. We place w = 7, assuming that the zinc, iodine and 

 nitrogen atoms all form octets. We then find from the octet equation 

 (2p = Sn — e) the value p = 4, from which we find the structure 

 l2'[Zn(NH3)4]'''', in which each nitrogen atom is quadricovalent and 

 shares a single pair of electrons with the octet of the zinc atom. The con- 

 stitution is thus exactly analogous to K2"'S04"", for in this case the quadri- 

 covalent sulfur atom shares a single pair of electrons with each of the oxy- 

 gen atoms. 



If we apply the octet theory in the same way to BeNa2F4 and MnCl2.- 

 4H2O we place for both compounds e = 32, m = 5 and find p =4. This 

 gives the structures 



[Mn(OH2)4]++Cl2- and [BeF*]— Na2+ 

 *' Jour. Amer. Chem. Soc. 41, 868 ("1919") beginning middle of p. 930. 



