The stable dianionic complexes Q2[cis-Pt(C6Fs)2CI4] (Q =NBu4 +, 1; PEtPh3, 2) have been obtained from reaction of the corresponding Q2[trans-Pt(C6F5)2C12] complexes with C12 in CCi4 solution. [PEtPh3]2[cis-Pt(C6Fs)2CI4 ] is the first dianionic organometallic complex of Pt(IV) structurally characterized by X-ray diffraction. It crystallizes with two CH2C12 solvent molecules per complex molecule. The crystal system is monoclinic and the space group is C2/c (No. 15), a=23.648(5), b=13.895(3), c = 17.651(4) /~, /3= 106.42(3) °, V=5563(2) ~3, Z=4 (20 °C). [NBu4]2[c/s-Pt(C6Fs)2CI4] has been used as starting material in the synthesis of [NBu4][mer-Pt(C6Fs)2CI3(NH2-p-C6H4CH3)], 3. The crystal structure of this new stable Pt(IV) compound has been determined. It crystallizes in the monoclinic system, space group P21/n (No. 14), a = 11.122(3), b =23.150(3), c = 16.086(3) A, B= 105.79(3) °, V= 3985(2) /~3, Z=4 (20 °C). The disposition of the two C6Fs groups is very similar in both molecules. This is a consequence of steric constraints imposed by the octahedral coordination of the platinum atom. The authors are grateful for financial support from the Comisión Interministerial de Ciencia y Tecnología, Spain (Project PB92-0364, PB92-0360), and for a grant to A.M. Peer reviewed