Reactions of cyclobutenedione and cyclopentenetrione with zero-valent nickel triad complexes of isocyanide. X-Ray crystal structures of (3–4-η-3,4-diphenylcyclobut-3-ene-1,2-dione)- and (4–5-η-4,5-diphenylcyclopent-4-ene-1,2,3-trione)-tris(t-butyl isocyanide)nickel