METALS AND METALLOIDS 699 



including lanthanum, do not give rise to organo-metallic 

 derivatives. In this vertical series, therefore, as in the fourth, 

 the two families show very different capacities for forming 

 compounds containing hydrocarbon radicals. 



The boron compounds with alkyl radicals are produced 

 by the general method from zinc alkyls : 



2BCI3 + 3 Zn(C 2 H 5 ) 2 - 3 ZnI 2 + 2B(C 2 H 5 ) 3 . 



The corresponding boron trimethyl is made by similar means 

 from ethyl borate. 



These alkyl boron compounds are possessed of somewhat 

 remarkable properties, one might even say inconvenient pro- 

 perties, regarded from the standpoint of the conventional theories 

 of valency. The existence of the great majority of boron com- 

 pounds can be readily explained on the assumption that the 

 element is uniformly tervalent, corresponding with the chloride 

 BCI3 and the oxide B 2 O s . But if this degree of combining 

 power represented all the chemical affinity possessed by boron, 

 then the boron trialkyls should be as inert as the paraffins, 

 for example, tertiary pentane (carbon tetramethyl, C(CH 3 ) 4 ), in 

 which, as we have already seen, the carbon is surrounded by 

 four methyl radicals situated at the apices of a regular tetra- 

 hedron containing the carbon atom at its centre. The boron 

 trialkyls behave, however, as highly unsaturated compounds ; 

 they combine additively with ammonia, and are readily absorbed 

 by the caustic alkalis. The compound [B(CH 3 ) 3 , NH 3 ] is 

 possessed of considerable stability, melting at 51 and boiling 

 at 110 . The compound with caustic potash has the composition 

 B(CH 3 ) 3 , KOH. 



If instead of regarding valency as being always entirely 

 integral we consider it as partly fractional and depending to 

 a large extent on the possibilities of arrangemert, we can obtain 

 an explanation for the existence of these additive compounds. 



The most symmetrical mode of arranging three methyl 

 groups round a central boron atom is at three points 120 apart 

 on a great circle of the boron sphere of influence. This would 

 also be the most symmetrical way of arranging the atoms in a 

 molecule of ammonia. This arrangement is, however, less 

 symmetric than the tetrahedral structure which could be pro- 

 duced by adding another associating unit to either of these 

 molecules. The boron and the nitrogen of the ammonia have 



