A series of new bis (pyrazol-1-yl) methanes with organotin groups on the methine carbon have been synthesized by the reaction of bis (pyrazol-1-yl) methyllithium with organotin halide. Their reactions with W (CO) 5THF have been investigated in detail, indicating that these new ligands display versatile reactivities upon treatment with W(CO)5THF, depending on the properties of the substituents on the tin atom. Treatment of triarylstannylbis(pyrazol-1-yl)methanes, Ar3SnCHPz2 (Ar) phenyl or p-tolyl; Pz) substituted pyrazole), with W(CO)5THF results in the oxidative addition of the tin-carbon(sp3) bond to the tungsten (0) center to yield novel metal-metal bonded complexes CHPz2(CO)3W-SnAr3, in which ourmembered metallacycles are found and bis (pyrazol-1-yl) methide acts as a tridentate monoanionic 3-[N,C,N] chelating ligand. When there are alkyl substituents in the 4-position of pyrazole rings, some decarbonylation intermediates Ar3SnCHPz2W (CO) 4 can be isolated. Sequentially heating these intermediates can also result in the oxidative addition of the tin-carbon (sp3) bond to the tungsten (0) center, implying that the oxidative addition reaction of the tin-carbon(sp3) bond may form these decarbonylation intermediates. While the reactions of trialkylstannylbis (pyrazol-1-yl) methanes, R3SnCHPz2 (R) Et or iPr), with W (CO) 5THF yield only the decarbonylation heterodinuclear complexes R3SnCHPz2W(CO)4, in which bis(pyrazol-1-yl)methanes act as N,N-chelating bidentate ligands, no analogous oxidative addition products are obtained. However, treatment of phenyldi(tert-butyl)-stannylbis (pyrazol-1-yl) methanes, (tBu) 2PhSnCHPz2 (Pz) 3,5-dimethylpyrazole or 3, 4, 5-trimethylpyrazole), with W (CO) 5THF results in the oxidative addition of the tin-carbon (sp2) bond instead of the tin-carbon (sp3) bond to the tungsten(0) center to yield metal-metal bonded complexes (tBu)2SnCHPz2W(CO)3(Ph), in which a novel 3-[N,Sn,N] coordination mode as well as asymmetric semibridging carbonyl between the W-Sn bond is observed. In addition, the reactions of trivinylstannylbis (pyrazol-1-yl) methanes, (CH2dCH) 3SnCHPz2, with W(CO)5THF result in a vinyl group bonded to the tungsten center in