Recently, van der Kerk and his co-workers1'2 have suggested that (la) contributes strongly to the structure of methyl derivatives (R = Me) but that its importance decreases rapidly on ascending the homologous series. They consider the dipolar form (la) to be responsible for the antifungal and plant-growth promoting properties of certain derivatives of NN-dimethyldithiocarbamic acid. Since the biological activities of the NN-dialkyldithiocarbamate derivatives fall rapidly on ascending the homologous series, they conclude that the contribution of the form (la) to the structures also falls rapidly. At the time of their later publication2, we had examined the infra-red spectra of a number of complex NN-dimethyl- and NN-diethyldithiocarbamates and interpreted the so-called 'thioureide ion' band3 in terms of the canonical form (la). This band had previously been identified in many thioureides but not satisfactorily interpreted3'4. Van der Kerk's publication prompted us, however, to extend our investigation to higher homologues and to organic derivatives. We now report briefly our conclusions. In general, we find that the forms (la), (16) and (Ic) contribute about equally to the structures of all NN-dialkyldithiocarbamic acid derivatives, and that there is no significant decrease in the importance of the form (la) on ascending the homologous series. The canonical form of type (la) probably contributes about equally strongly to the structures of the N-monoalkyldithiocarbamates, but only in a comparatively minor way to the structures of the xanthates.Our evidence for this is based on an infra-red spectral investigation of some thirty derivatives of NN-dialkyldithiocarbamic acid, and on the dipole moments of some unsymmetrical derivatives. The types of compounds examined included salts, for example, Na(Me2NCS2), complex salts, for example, Co(Et2NCS2)3, an ester, Me2NCS2Me, and a sulphide (Me2NCS2)2. All have a strong absorption band, the 'thioureide ion' band, in the 1,542-1,480 cm.-1 region of the spectrum, usually around 1,500 cm.-1. It is too high in frequency to be due to a vibration, the form of which is largely determined by stretching of a single C-N bond, or of a double C=S bond3 ; its frequency is also too high for it to be assigned to a deformation mode of the methyl groups4, for this would not occur at frequencies greater than 1,480 cm.-1. The only possible assignment appears to be to a polar C-N double bond, and this implies that the canonical form (la) makes an important contribution to the ground-states of all NN-dialkyldithio- carbamates.If this conclusion is correct, unsymmetrical derivatives of dialkyldithiocarbamic acids should have large electric dipole moments, because the form I (a) is highly dipolar. Most complex dithiocarbamates presumably have centro-symmetrical structures, but those of type As(J?2NCS2)3 must be unsymmetrical, because arsenic (III) has a lone pair of electrons ; the esters i?2NCS2_R must also be unsymmetrical. In fact, the dipole moments of these two types of derivatives are high, of the order 4-5-5-0 and 3-0 Debye units respectively5. To account for these moments in the esters the C-N bond must have a bond order of 1 -25-1 -35, that is, the forms (la), (16) and (Ic) contribute about equally to the structure. The unsymmetrical N-alkyldithiocarbamates have similar dipole moments to the corresponding NN-dialkyldithiocarbamates5, and the spectra of the few monoalkyl derivatives which we have examined all have an absorption band in the 1,500 cm.-1 region. Apparently their electronic structures are similar to those of the NN-dialkyldithiocarbamates. The unsymmetrical xanthates have comparatively low moments of less than 2 Debye units in the cases of As(i?OCS2)3, and of the esters, ROCS2R 5. These are of the same order as those of the symmetrical complex dithiocarbamates5, xanthates5 and acetyl-acetonates6. We have therefore examined the spectrum of Ni(EtOCS2)2, and find that it is transparent in the double-bond region7. There is a broad band at 1,265 cm.-1 which is in the right place for an unsaturated ether8. We conclude, therefore, that_ + the canonical form -82C=OR contributes comparatively little to the structure of the xanthates.In view of the suggestion of van der Kerk and his co-workers, we examined a homologous series of copper NN-dialkyldithiocarbamates. Our observations are recorded in Table 1. It is evident that the C=N frequency is almost independent of the size of the alkyl groups, and such change as we observe in the solid state is mainly due to crystal forces. It appears that the observations of van der Kerk and his co-workers cannot be explained as due to the much greater contribution of form (la) to the structures of the lower homologues. Perhaps increasing lipoid- or decreasing water-solubility may provide an explanation of the decreasing biological activity as the series is ascended. Table 1. C-N STRETCHING FREQUENCIES (cm."1) OF CUPRIC NN-DlALKYLDITHIOCARBAMATES, CU(#2NCS2)2, IN CHLOROFORM SOLUTION (2 cm.-1) AND IN THE CRYSTALLINE STATE R Me Et Prn Bun PrSolution Solid insol. 1,5241,505 1,508 Prn1,502 1,501 Bun1,505 1,497 1,490 1,493The greater contribution of the canonical form of type (la) to the structure of the dithiocarbamates than to that of the xanthates undoubtedly arises from the greater mesomeric electron-releasing tendency of the -NJR2 group as compared with the -OR group. In the dithiocarbamates, this greater electron drift into the sulphur atoms will increase their electron donor capacity, and decrease the electron affinity of their d-orbitals as compared with the sulphur atoms in the corresponding xanthates. Thus the dithiocarbamate ion should form stronger complexes, with less tendency to dative 7r-bond formation from the metal to the sulphur atoms, than the xanthate ion. This may well provide some explanation of the rather striking differences between the chemistry of the complex dithiocarbamates and complex xanthates9. Full details of this work will be published elsewhere.