In the present study we have isolated MX42- (M = Pd, Pt; X = Cl, Br) in a room temperature Penning trap under virtually collision-free, ultra high vacuum conditions and have probed for metastable decay on a time scale of up to 1000 s. While PdCl42-, PdBr42- and PtBr42- do not react appreciably on this time scale, PtCl42- undergoes tunneling electron loss to form PtCl4-; with a half-life of 5 s. We provide kinetic data and ab-initio quantumchemical calculations of relevant initial and final states as well as Coulomb barriers. All four species are metastable towards halide loss on the basis of these calculations.