On the structure of dithizone

The ¹³C NMR spectrum of dithizone, unlike that of its oxygen analogue, shows only five different types of carbon atom. The ¹³C chemical shift of the C?S group in dithizone is only 7 ppm to lower field of that of the C?O moiety in its analogous compound. The results can be rationalized by consideration of tautomeric equilibria which lead to C_2v time-averaged symmetry.