Acid-catalyzed reactions of the hydrazido(2-) complexes cis,mer-[WX(2)(NNH2)(PMe(2)Ph)(3)] (X=Cl, Br) with phthalaldehyde gave the (phthalimidin-2-yl)imido complexes cis,mer-[WX(2)(NNCH2C6H4CO)(PMe(2)Ph)(3)], via the condensation of the terminal NH2 group with one of the formyl groups and the following cyclization to form a phthalimidine ring. Crystal structure of the chloro complex 3a was unambiguously determined by X-ray analysis. Reaction of 3a with HBr liberated the (phthalimidin-2-yl)imido ligand as 2-aminophthalimidine in moderate yield, while treatment of 3a with KOH in THF selectively cleaved the N-N bond to give phthalimidine. A similar condensation of the hydrazido(2-) complex trans-[WF(NNH2)(dppe)(2)](+) (8a(+); dppe=Ph(2)PCH(2)CH(2)PPh(2)) with phthalaldehyde resulted in the formation of the diazoalkane complex trans-[WF(NN=CHC6H4CHO)(dppe)(2)](+). However, further treatment of the latter complex with AlCl3 afforded the corresponding (phthalimidin-2-yl)imido complex trans-[WF(NNCH2C6H4CO)(dppe)(2)](+). When 8a(+) and its molybdenum analogue were reacted with 2,5-dimethoxy-2,5-dihydrofuran in the presence of a catalytic amount of acid, trans-[M(NNCH=CHCH2CO)(dppe)(2)](+) (11(+); M=Mo, W) was formed as the kinetic product, which gradually isomerized to the thermodynamically more stable compound trans-[MF(NNCH2CH=CHCO)(dppe)(2)](+) (12(+)). Both 11(+) and 12(+) (M=W) were crystallographically characterized, and the mechanism for the isomerization of 11(+) to 12(+) was proposed based on the results of the H-1 NMR measurements.