Reactions of the platinum complexes cis-[PtCl₂(PPh₃)₂ ], [PtCl₂(dppf)], or [PtCl₂(dppe)], and the palladium complexes, [PdCl₂(PPh₃)₂] or [PdCl₂(dppe)], with bis(cyanomethyl)sulfone NCCH₂SO₂ CH₂CN in methanol with added trimethylamine base gave the new cyano-substituted metallathietane-3,3- dioxide complexes [M{CH(CN)SO₂ CH(CN)}L₂ ] (M = Pd, Pt) in good yields and purities. ¹H and ³¹P{¹H} NMR spectroscopy indicates that (in the majority of cases) the products are formed as a mixture of cis and trans isomers, with regard to the disposition of the cyano groups across the four-membered ring. An X-ray structure determination on a crystal of [Pd{CH(CN)SO₂CH(CN)}(PPh₃)₂] showed it to be of the cis isomer, the first structurally characterized cis isomer of this type of complex. The four-membered palladacyclic ring is highly planar, in contrast to other metallathietane-3,3-dioxide complexes that have puckered rings. Factors influencing the relative abundances of the two isomers of [Pd{CH(CN)SO₂CH(CN)}(PR₃)₂] have been investigated by means of NMR spectroscopy and DFT calculations.