The Collaborative International Dictionary of English v.0.48:

Complemental \Com`ple*men"tal\, a.
   1. Supplying, or tending to supply, a deficiency; fully
      completing. "Complemental ceremony." --Prynne.
      [1913 Webster]

   2. Complimentary; courteous. [Obs.] --Shak.
      [1913 Webster]

   Complemental air (Physiol.), the air (averaging 100 cubic
      inches) which can be drawn into the lungs in addition to
      the tidal air, by the deepest possible inspiration.

   Complemental males (Zool.), peculiar small males living
      parasitically on the ordinary hermaphrodite individuals of
      certain barnacles.
      [1913 Webster]

WordNet (r) 3.0 (2006):

complemental
    adj 1: acting as or providing a complement (something that
           completes the whole) [syn: complemental,
           complementary, completing]