New pentacoordinate bicyclodiazastannsulfide fused cyclopentadienyl M–Sn (M=Mo or W) bonded organometallic heterocycle {μ-[C 5 H 4 (CH 3 )CNNC(S)Ar]M(CO) 3 SnCl 2 } has been obtained by the condensation reaction of CH 3 COC 5 H 4 M(CO) 3 SnCl 3 with arylthiocarboxyhydrazide (ArCSNHNH 2 , Ar=2-furanyl, 2-thienyl, 2- or 4-hydroxyphenyl) in mild conditions. While the similar reaction of CH 3 COC 5 H 4 M(CO) 3 SnCl 3 with ArCONHNH 2 (Ar=2- or 4-hydroxyphenyl) only gives non-cyclic compound [C 5 H 4 (CH 3 )CNNHC(O)Ar]M(CO) 3 SnCl 3 , in which the tin atom remains tetracoordinate. In bicyclodiazastannsulfide the tin atom prefers to adopt pentacoordinate geometry, while in the corresponding bicyclodiazastannoxide the tin atom is hexacoordinate. In addition, phenylhydrazine, 2- or 4-hydroxyphenylcarboxyhydrazide is used to react with CH 3 COC 5 H 4 M(CO) 3 SnCl 3 , only tetracoordinate non-cyclic tin compound is obtained.