The hydrazido(2-) complexes [WCl2(NNH2)(L)(PMe2Ph)(2)] (L = PMe2Ph, CO), derived from the dinitrogen complex cis-[W(N-2)(2)(PMe2Ph)(4)] (1), reacted with beta-diketones (RCOCHRCOR3)-C-1-C-2 (R-1 = Me, Bu-t, Ph; R-2 = H, Me; R-3 = Me, Et, Bu-t, Ph) to afford a series of diazoalkane complexes [WCl2(NN=(CRCHRCOR3)-C-1-C-2)(L)(PMe2Ph)(2)]. The detailed structure of cis,mer-[WCl2(NN=CMeCH2COPh)(PMe2Ph)(3)] has been determined by X-ray analyses. These diazoalkane complexes were treated with a KOH/EtOH mixture to produce pyrazoles in moderate yields, whereas the reaction of 1 with a MeCOCH2COPh/KOH/EtOH mixture resulted in the direct formation of 5-methyl-3-phenylpyrazole.