The reductive carbonylation of either platinum(1V) fluorosulfate, Pt(SO,F),, or palladium(II1) fluorosulfate, Pd(S03F),, either in solution of fluorosulfuric acid, HS03F, or in the solid state with CO at pressures of 0.5 to 2 atmospheres and at 25"C, affords in high yield complexes of the type M(C0)2(S03F),, M = Pd or Pt. The square-planar complexes are readily converted by solvolysis in liquid antimony(V) fluoride in the presence of CO at a pressure of about I atmosphere to white, thermally stable, salts of the composition [M(CO)4][Sb2F11]2r M = Pd or Pt, which contain the novel, square-planar cations [Pd(C0)4]Z+ and [Pt(CO),12+. The two cations exhibit the highest C O stretching frequencies reported so far, about 120 cm-' higher than in free CO, which suggests the absence or near absence of =-back donation for the M-CO bond.