vanadous

from The Century Dictionary.

• Of or pertaining to vanadium: as, vanadous oxid: specifically noting compounds in which vanadium has a lower valence than in the vanadic compounds.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Of or pertaining to vanadium; obtained from vanadium; -- said of an acid containing one equivalent of vanadium and two of oxygen; specifically, designating those compounds in which vanadium has a lower valence as contrasted with the vanadic compounds

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Of, pertaining to or containing vanadium, especially in an oxidation number of 4.