C19H17NsNiO (NIFORM 2): Mr=390"l, monoclinic, P21/a, a = 11.2599 (4), b = 13.972 (8), c= 11.8702 (4) A, fl= 109.17 (1) °, V= 1763.9 A 3, z=4, D x=l.47gcm -3, p(CuKa)=16.99cm -1, 2(C uKa) = 1.5418 A, room temperature, F(000) = 808, R =0.031 for 2725 reflexions. C25H21NsNiO (NIFORM 3): Mr=466"2, monoclinic, P2t/c, a= 8.411(1), b=24.938(4), c=11.235(2)A, fl= 111.77(1) °, V=2189.2A 3,Z=4,D x=l.41gcm -3, p(Cu Kct) = 14.62 cm -~, 2(Cu Ka) = 1.5418 A, room temperature, F(000) = 968, R = 0.032 for 2993 reflexions. Both structures have planar, approximately square coordination of Ni by three N and one O. In NIFORM 2 formazan is coordinated to Ni via one terminal and one non-terminal N, giving rise to a five-membered Ni-formazan ring. In NIFORM 3 formazan is coordinated to Ni via both terminal N atoms resulting in a six-membered Ni-formazan ring.