The Collaborative International Dictionary of English v.0.48:

Platinous \Plat"i*nous\, a. (Chem.)
   Of, pertaining to, or containing, platinum; -- used
   specifically to designate those compounds in which the
   element has a lower valence, as contrasted with the
   platinic compounds; as, platinous chloride (PtCl2).
   [1913 Webster]