2,3-Bis(diphenylphosphino)-1,4-diazadienes RN=C(PPh2)-C(PPh2)=NR (1a, R = 4-tolyl; 1b, R = 4-tert-butylphenyl; 1c, R = mesityl) were used as novel ligands for transition metals. The metal complexes [(1c)Mo(CO)(4)] (2a), {(1c)[Mo(CO)(4)](2)} (2b), [(1a)Cu(Cl)(PPh3)](3), and {(1b)[(NiBr2(THF))](2)} (4) were characterized by elemental analysis, MS, and P-31{H-1}, H-1, and C-13 NMR spectra (except the paramagnetic complex 4). Additionally, the molecular structure of the complexes in the solid state was determined by single-crystal X-ray diffraction. In 2a and 2b the chelating ligand coordinates via the N,P donor set, whereas in 3 the chelating ligand coordinates via the two P atoms. 4 contains a square-planar (P,P)NiBr2 Moiety on the one side of the bridging ligand 1b. On the opposite side the 1,2-diimine unit bonds to another Ni center having octahedral geometry. The bulkier ligand 1c reacts to form the mononuclear compound 5. X-ray diffraction analysis of single crystals shows that 5 contains a quinoxaline derivative with a cyclohexa-1,3-diene ring in the peripheral position. Furthermore, it contains a bis(diphenylphosphino)ethylene unit coordinating the NiBr2. This arrangement is the result of an intramolecular [4 + 2] cycloaddition between the 1,2-diimine unit (as diheterodiene) and the benzene ring of the 4-tolyl-N substituent (as dieneophile). The same type of ring-closing reaction followed by a tautomerization reaction to form the mononuclear compound 6 occurred by dissolution of the binuclear complex 4 in methanol. This reaction can be used as a simple method for the synthesis of novel 1,2-bis(diarylphosphanyl)ethylenes containing a quinoxaline backbone.