Seven 2-(N-cycloalkylamino-1,3-dithiocarbamoyl)-carboxylic acids and seven mesoionic 2-N-cycloalkylamino-5-alkyl-1,3-dithiolium-4-thiolates have been conveniently synthesized. They were characterized by elemental analysis, LR., U.V., mass and 'H NMR spectrometry, plus I3C N M R spectrometry in some cases. Important questions concerning the structure of the mesoionic compounds, relevant to mesoionic compounds in general, are addressed. In particular, we refer to the degree of separation of regions of positive and negative charge, bond orders, electron and charge delocalization and aromaticity. In this discussion we cite some of our X-ray diffraction and theoretical studies. We conclude that there are regions of positive and negative charge in which there is delocalization of electrons and charge with bond orders between 1 and 2. However, the shared regions are separated by what are essentially single bonds. Thus, they, and mesoionic compounds in general, should not be considered as formally aromatic. This is supported by estimates of Bird Aromaticity indices-experimentally based for two of the title mesoionic compounds-of the order of 50: substantially less than values for thiophene and pyrrole and much less than values for benzene and pyridine. Finally, based on this discussion, we introduce a new notation for the structures of mesoionic compounds which we believe more accurately represents both the bonding situation and the chemical patterns of reactivity of such compounds.

Abstract. C9H13NS3, Mr=231"39, triclinic, PT, a =6-783(1), b=7.617(2), c=11"725(1) A, a= 82"61 (1), /3 = 74.31 (1), y = 66"06 (2) °, V = 533"0 (2)/~3, Z = 2, Dx = 1.442 g cm-3, A(Mo Ka) = 0"7107 A,/z = 6"2 cm-1, F(000) = 244, T = 295 K, R = 0.034 for 1127 reflections with IFol > 3~r(IFol). This mesoionic compound has, in the pentatomic ring system, two short C--S bonds [1.717 (3) and 1.715 (4)A] which are comparable to the C--S-(thiolate) bond of 1.714 (4)A, and two longer C--S bonds [1.777 (3) and 1.745 (3) A].Introduction. The concept of mesoionic compounds has developed gradually, undergoing a number of changes over the years. A reasonably well accepted current description is that mesoionic compounds are a class of heteroaromatic betaines, characterized by possessing a pentatomic heterocyclic system with a sextet of zr electrons associated with the heterocycle and which also supports a formal positive charge. This is counter-balanced by a formal negative charge on the a atom of a side chain.

Two new platinum(II) complexes have been synthesized and their anti-tumour and anti-HIV activities have been evaluated. The new complexes are: (i) cis-tetrafluorophthalate-ammine-morpholine-platinum(II) or MMF3 and (ii) cis-tetrafluorophthalate- ammine-piperidine-platinum(II) or MPF4. They were characterized by elemental analysis, IR spectra and 1H and 13C NMR spectra. They were tested against five human ovarian carcinoma cell lines, viz., CH1, CH1cisR, A2780, A2780cisR and SKOV-3. They were less active than cis-platin and showed cross-resistance with cis-platin in the CH1cisR and A2780cisR acquired resistance lines. They were also tested for possible anti-HIV activity using the HIV-I IIIB virus and C8166 cells, but they were inactive compared with AZT.