452 Dr. S. H. C. Briggs : The Elements 



chlorine atoms by taking the chlorine from the potassium 

 chloride : 



2KCl + PtCl 4 =K ? [PtCl 6 ]". 



Potassium chloride may therefore be regarded as a co- 

 ordination compound of potassium and chlorine in the same 

 way as potassium platinichloride is regarded as a co-ordination 

 compound of potassium chloride and platinic chloride. Or 

 again, we may compare the chlorine kernel with its octet of 

 electrons in potassium chloride with the molybdenum atom 

 and its octet of cyanide groups in the compound 



K 4 '[Mo(CN) 8 ]"". 



(2) Co-ordination compounds with a complex cation. — 

 A polar compound such as cupric chloride may combine with 

 another polar compound like ammonia to give a co-ordination 

 compound with a complex cation : — 



*CiT<-— CI 2 ' > + 6(V NH 3 * ) = ->(Cu6NH 3 )"C] 2 ' . 



We may therefore expect polar elements to combine in a 

 similar manner. But the strongly polar elements are the 

 metals, hence the co-ordination compounds in question 

 should be sought for among the alloys. 



These considerations lead to the following equation to 

 represent the formation of an amalgam KHg x : — 



>KV— E >+ x ( *Hg A E 26 >) = >(#Hg*E 26 K,)E --> 



The amalgams of the alkali metals according to this view 

 would therefore be co-ordination compounds with the 

 mercury atoms grouped round the kernel of the alkali metal. 

 And since the attractive force acting upon the mercury of 

 atoms is distributed equally over the surface of the sphere 

 of the kernel, the atoms of mercury should tend to group 

 themselves symmetrically in space around the kernel. 



The number of different compounds of the alkali metals 

 with mercury described in the literature is very large and 

 many are of doubtful individuality, whereas others are well 

 defined. There is a marked tendency to form spatially 

 symmetrical combinations in which the mercury atoms may 

 be supposed to be situated around the kernel at the vertices 

 of a regular tetrahedron, octahedron, cube or icosahedron, 

 corresponding to the formulae XHg 4 , XHg 6 , XHg 8 , and 

 XHg 12 respectively. The following compounds may be 

 noted: PbHg 6 , RbHg 12 , CsHg 2 , 0sHg 4j CsHg 6j CsHg fs , 

 MgHg 4 , MgHg 6 , CaHg 8 , SrHg 6 , SrHg^ and SrHg ls . 

 Potassium amalgams KHg r have been described in which x 



