The metalladithiolene ring in [CpM(S 2 C 2 Z 2 )]-type complexes is a very unique metallacycle; a variety of organic species, such as alkynes, 1 quadricyclane, 2 and azides, 3 are known to insert into the M-S bond. Diazo compounds N 2 CXY react with cobaltadithiolene complexes [CpCo(S 2 C 2 Z 2 )] to give alkylidene-bridged complexes [CpCo(S 2 C 2 Z 2 )(CXY)], containing a Co-C-S three-membered ring (cobaltathiirane). 4 We have reported the structural changes in alkylidene-bridged complexes due to two types of bond cleavage, as shown in Scheme 1: one is the cleavage of the Co-C bond in the reactions with protic acids to give a 1:1:1 adduct having an S-alkyl moiety; 5 the other is that of the Co-S bond in the reaction of [CpCo(S 2 C 2 (CN) 2 )(CH 2 )] with a Lewis base (P(OMe) 3 ) to give a 1:1:1 adduct, in which the addition of the P(OMe) 3 to the Co atom causes ring expansion to give a six-membered metallacycle (see Scheme 1). 6 Here we report a novel type of 1:1:1 adduct, (η 5cyclopentadienyl){2-[(bis(methoxycarbonyl)methylene)-. 7 This compound has a sulfonium ylide moiety resulting from the cleavage of a Co-S bond in the reaction of P(OMe) 3 with the alkylidene-bridged complex [CpCo(S 2 C 2 Z 2 )(CZ 2 )], which is formed by the reaction of (η 5 -cyclopentadienyl)[1,2-bis-(methoxycarbonyl)ethenedithiolato-κS,S′]cobalt(III) (1) with dimethyl diazomalonate. 8 One previous example of a similar ylide complex was reported by Seyferth et al. 9 In this complex, there is a carbon atom between the sulfur and the metal, whereas in our complex the sulfur atom of the sulfonium group is bonded to the metal. To our knowledge, this type of adduct is a very rare example of an organometallic compound with a sulfonium group bonded through sulfur to the metal. In this study, the structure of the novel 1:1:1 adduct was determined by X-ray crystal structure analysis, together with the original alkylidene-bridged complex 2a and another type of 1:1:1 adduct (a ring-expanded complex, 4b).The reaction of the dithiolene complex 1 with dimethyl diazonmalonate, to give the alkylidene-bridged † Suetsugu, T.; Takagi, T.; Akiyama, T.; Sugimori, A.; Aoki, K.; Yamazaki, H. J. Organomet. Chem. 1995, 487, C8. (2) Hatano, H.; Eguchi, Y.; Okumachi, T.; Fujita, T.; Nagao, H.; Kajitani, M.; Akiyama, T.; Sugimori, A. J. Organomet. Chem. 1992, 430, C64. (3) Sakurada, M.; Kajitani, M.; Akiyama, T.; Sugimori, A. Chem. Express 1991, 6, 759. (4) Sakurada, M.; Kajitani, M.; Dohki, K.; Akiyama, T.; Sugimori, A. J. Organomet. Chem. 1992, 423, 141. (5) Kajitani, M.; Kawakita, F.; Chikuma, E.; Sakurada, M.; Akiyama, T.; Sugimori, A. Chem. Lett. 1995, 85. (6) Sakurada, M.; Kajitani, M.; Hatano, H.; Matsudaira, Y.; Suetsugu, T.; Ono, S.; Akiyama, T.; Sugimori, A. Organometallics 1992, 11, 2337. (7) [CpCo{S2C2(COOMe)2C(COOMe)2}(P(OMe)3)] (3a): 1 H NMR (270 MHz, CDCl3, TMS) δ 3.63 (s, 3H, OCH3), 3.74 (s, 3H, OCH3), 3.76 (s, 3H, OCH3), 3.83 (s, 3H, OCH3), 3.92 (d, 9H, JPH ) 10.98 Hz, P(OCH3)3), 5.22 (s, 5H, C5H5); 13(8) A solution of 1 and dimethyl diazomalon...