A comparison of binuclear dimethylplatinum(II) complexes with the bridging ligands X(PPh2)2, X = CH2 or NH

The reaction of [Pt2Me4((mu)-SMe2)2], 1, with dppa = Ph2PNHPPh2 gave the complex [Pt2Me4((mu)-SMe2)((mu)-dppa)] and, with more dppa, a mixture of the binuclear complex [Pt2Me4((mu)-dppa)2] and the mononuclear complex [PtMe2(dppa)]. In contrast, reaction of [Pt2Me4((mu)-SMe2)((mu)-dppa)] with dppm = Ph2CH2PPh2 or of [Pt2Me4((mu)-SMe2)((mu)-dppm)] with dppa gave only the binuclear complex [Pt2Me4((mu)-dppm)((mu)-dppa)]. This unusual complex, which has both bridging dppm and dppa ligands in equivalent positions in an organometallic complex, is more inert to rearrangement to mononuclear complexes, [PtMe2(dppm)] and [PtMe2(dppa)], than either symmetrical complex [Pt2Me4((mu)-dppm)2] or [Pt2Me4((mu)-dppa)2]. A combination of structural and NMR evidence indicates that all binuclear dimethylplatinum(II) complexes adopt a twist boat conformation, and the activation energy for fluxionality via a boat intermediate follows the series [Pt2Me4((mu)-dppm)2] >> [Pt2Me4((mu)-dppm)((mu)-dppa)] > [Pt2Me4((mu)-dppa)2].