The synthesis and reactivity of oxo-bridged bis(group 4 metal unsymmetric phosphonium-stabilized carbene) complexes are described. The reaction of [CH2RNRS] (1; RN = PPh2NSiMe3, RS = PPh2S) with 1 equiv of [M(NMe2)4] (M = Zr, Hf) afforded the group 4 metal unsymmetric phosphonium-stabilized carbene complexes [M(NMe2)2(CRNRS)] (M = Zr (4), Hf (5)). Their reactions with water in toluene afforded the oxo-bridged derivatives O[M(NMe2)(CRNRS)]2 (M = Zr (6), Hf (7)). Compound 6 underwent an insertion reaction with AdNCO to form O[Zr{OC(NMe2)NAd}(CRNRS)]2 (8; Ad = adamantyl). Compounds 4-8 were characterized by NMR spectroscopy and X-ray crystallography. (Chemical Presented). © 2015 American Chemical Society.