The compound [A([18]crown-6](2)[Pt(CO)(3)] . 10 NH3 (A=K, Rb, [18]crown- 6=1,4,7,10,13,16-Hexaoxacyclooctadecane) containing the anion [Pt(CO)(3)](2-) was the unexpected result of a reaction between K6Rb6Ge17, Pt(CO)(2)(PPh3)(2), [18]crown-6 and [2.2.2]-crypt. This compound represents the first example of a mononuclear carbonyl platinate and expands the list of known group 10 carbonyl metallates. The central anion has a trigonal planar shape with an approximate D-3h symmetry. Theoretical investigations confirm the trigonal planar structure of the carbonylate and give insight into the electronic structure. The calculations reveal a strong charge density at the central platinum atom, while the HOMO shows a dispersion of the residual electrons under and over the carbonyl plane.