See also: complexion and complexión English Noun complex ion (plural complex ions) (chemistry) Any ion containing more than one atom. e.g. sulphate as opposed to sulphide. (chemistry) Any ion in which a central metal atom (normally a transition element) is surrounded by a group of ions or molecules (ligands). e.g. ferricyanide. See also coordination compound Wikiwand - on Seamless Wikipedia browsing. On steroids.Remove ads