Synthesis and structural characterization of the Pt2(II,III) complex (NBu4)[(C6F5)2Pt(.mu.-C6F5)2Pt(C6F5)2] and the Pt2(III,III) complex (NBu4)[(C6F5)2Pt(.mu.-C6F5Cl)(.mu.-C6F5)Pt(C6F5)2]. Novel ligand reactivity of a bridging C6F5 group