Aluminium chloride is removed from cyclopenta (azathienium) tetrachloroaluminate by tetrahydrofuran t o give pure [S,N,]CI and this reacts with aqueous tetrafluoroboric acid t o give [S,N,] [ BF,]. Cyclic voltammograms of [S,N,] CI in liquid SO, show reduction of S , N : at € ; $ = 0.34 V vs. a saturated calomel electrode; the process is reversible. Cyclic voltammograms of [S,N,] [BF,] in MeCN were more complex and a mechanism for this electroreduction is proposed. Electrodeposition of (SN)x o n platinum-foil and on (SN)x-film cathodes was achieved by potentiostatic electrolysis. [S,N,] CI crystallizes in the monoclinic space group C2/c, with a = 7.98(1), b = 14.37(1), c = 7.30(1) A, p = 97.15(5)", and Z = 4. 1 366 Observed reflections were refined t o R = 0.054. The mean bond angles are NSN 1 1 1.3 and SNS 140.2". Three of the five unique S-N distances are 158.2 pm but t w o [S(2)-N(2) 156.5, S(3)-N(3) 159.8 pm] are affected by short S . -. CI contacts [3.277(1) and 3.1 48(1) A at S(2) and 3.243(1) and 3.279(1) A at S(3)]. Strain in the azulene-shaped cation arising from cation-anion interactions can be detected by comparing bond distances and angles with the strain-free values deduced from either bond distance-bond angle correlation functions or M N DO calculations.The monocyclic cation SSNS+ is the 14-2 member of the Huckel set of cyclothiazenes. Most reported salts contain large counter ions [e.g. S3N306,' SnCl,(OPC13)-,2 A1C14-,3 GaC14-,4 Ga,Cl,-' 1 and, with colourless anions, the salts are yellow. The chloride, however, is red indicating either significant cation-anion interaction and/or a different cation conformation. Two isomers of SSNS+ have been reported from X-ray structure determinations (with an azulene 1*2*4 or 'heart' shape ,v4).The apparent heart shape may well arise from cation size di~order,~ but since MNDO calculations show that the two shapes probably have similar energy, genuine isomerism is still possible. We have therefore determined the structure of SSNS+CI-and find significant cation-anion interaction associated with the reentrant portion of an azulene-type SSNS+ ring. We also report a new preparation of [S,N,]CI from [S,N,][AICl,] and tetrahydrofuran (thf) and describe a solvate, We have investigated further the electrochemical reduction of SSNS+ (S,N,+Cl--SO,, S,N,+BF~-LiClO,-MeCN) to the conducting polymer (SN),, on both a macro-and microelectrode scale. In a broader context, the purpose of this investigation was (i) to elucidate the reaction mechanism at the electrode surfaces under various conditions, (ii) to work out a preparation method for pure polycrystalline (SN), (in quantities of tenths of a gram per batch) suitable as a source for vacuum deposition of electrically conductive films, and (iii) to explore the possibility of a direct electrochemical coating with compact layers of (SN), (lo2-lo4 A thick) on conducting or semi-conducting substrates.[SSN,]Cl*SO,.t Cyclopenta(azathienium) chloride.