Recent chemical investigations (Gibson 1938) have led to the view that gold in its auric compounds is always 4-covalent, and crystal structure determinations (Cox and Webster 1937; Burawoy and others 1937) have shown that the arrangement of the four valencies is a planar one as predicted by Pauling (1931). The compound diethylmonobromogold achieves these two conditions by dimerization with formation of the ring structure (I). Et Br Et ⭩ / Au Au (I) / ⭧ Et Br Et It is of particular interest to examine a corresponding cyanogold compound, since it is impossible in this case for the square containing two gold atoms to be formed. The co-ordinate link from the nitrogen atom to the next gold atom must be collinear with the Au—C≡N links, and the simplest structure in which this principle of the 4-covalency of auric gold is adhered to is by the arrangement of four molecules as in (II). Et Et I I Et—A vk-N = C —Au—Et I t C N I I I III (II) N C i | Et—Au—C=N-*Au—Et Et Et This structure was suggested by Gibson and co-workers (Burawoy, Gibson and Holt 1935) and is supported by molecular weight evidence. This compound is, however, unstable, and the corresponding methyl compound has not yet been prepared. The next homologue, di-
n
-propylmonocyanogold, is comparatively stable, and its crystal structure determination was therefore undertaken. Previous observations on this compound make a determination of its molecular structure desirable, since, although the molecular weight is in agreement with the tetrameric form, it has a small dipole moment, 1.47 D.
Quantitative determination of a nano-object's atom density without atomic resolution
