Armand C. Oct 19, 2009:

XYZ? I very much suspect they meant exactly "thione compound", not "thionic compound".

The essential difference is that while a thionic (named using the "thio-" prefix to the function's name) compound is any organic molecule containing sulphur, a thione is specifically a thioketone (i.e., a ketone having an S atom instead of O double-bonded to C).

Virtually for any "regular" oxygen-containing function/substituent, a thio- (sulphur containing) equivalent exists. Thiones are only one of those classes, along with thiols (thioalcohols), thials (thioaldehydes), thiocyanates, thiocarboxilic acids and corresponding thioanhydrides, and so on, and so forth.

It could be helpful to know what that XYZ is, in order to see its structure and eliminate any doubt.