A new (E,E)-dioxime (H4L) containing a 21-membered trioxadithiadiaza macrocycle was synthesized from the reaction of 2,3:17,18-dibenzo-4,16-dithia-7,10,13-trioxa-1,19-diazanonadecane (2), prepared from 3,6,9-trioxa-1,11-dibromoundecane (1) and 2-aminothiophenol, and cyanogendi-N-oxide. Only mononuclear Ni(ii) and Co(iii) complexes with a metal:ligand ratio of 1:2 were isolated. These hydrogen-bridged complexes were converted to their BF2+-bridged analogues by reaction with boron trifluoride etherate. The reaction of the BF2+-bridged complexes with Pd(ii) and Ni(ii) gave heterotrinuclear complexes. These compounds are significant in that they provide binding sites not only within but also outside the macrocycles. Structures for the dioxime and its complexes are proposed in accordance with the elemental analysis, H-1 and C-13 NMR, IR and mass spectral data, magnetic susceptibility measurements and semi-empirical quantum chemical calculations.