An X-ray study of isomorphism in simple organo-metallic series. — Part I. The tetraphenyls

Abstract

The present paper deals with a preliminary X-ray study of the isomorphous series of organo-metallic compounds formed by combining a fourth-group clement with four phenyl radicals, the resulting compounds being the tetraphenyls of carbon, silicon, germanium, tin and lead [C (C 6 H 5 ) 4 , etc.]. The simpler isomorphous organo-metallic compounds seemed of interest for the following reasons : (i) If the compounds of any one series, when examined by the methods of X-ray crystallography, are found to be truly isomorphous— that is, to belong to the same space group and to have unit cells of similar size— then the only variable in passing from one member to the next is the grouping element. The effect, therefore, of this part of the molecule on the intensity of the reflected X-rays can be studied, (ii) By choosing substances having one or more metal atoms in the molecule, comparatively large differences in scattering power of various parts of the molecule can be obtained. For example, the numbers of electrons in the various parts of a molecule of lead tetraphenyl Pb(C 6 H 5 ) 4 are as follows: whole molecule (246), lead atom (82), carbon atom (6), hydrogen atom (1) and phenyl radical (41). (iii) By choosing the simpler compounds ( e. g ., of the type M R n ) in which all the valencies of the grouping element (M) are occupied by groups (R) all alike, and not too complex, a molecule of high symmetry might be expected. A note must be made of the meaning here of simpler compounds . It would at first appear that the range of choice was very wide, but if the work is restricted to organic compounds which are stable solids in air at ordinary temperatures, many of the simpler series are eliminated, as some or all of their members are gases, liquids or unstable solids. Apart, however, from the experimental difficulties in working with these very simple compounds, there is another objection bearing on (i). It was there stated that if all the members of a series belonged to the same space group, then only one variable was altered in passing from one to another member of the series. This statement is not strictly true, for the difference in the size of the grouping atom causes a change in the dimensions of the unit cell, and may also change the relative arrangement of the parts of the molecule. This change may well be quite large if very simple radicals are attached to the grouping atom. If, however, fairly large radicals are attached, then the fractional change in size of the molecule accompanying the change made in the scattering power of one of its parts can be made fairly small, and a fair approximation to the varying of only one variable can be reached.