Reactions of trinuclear platinum clusters with electrophiles: ionisation isomerism with [Pt3(?2-I)(?-PPh2)2(PPh3)3]I and [Pt3(?-PPh2)2I2(PPh3)3]. Structures of [Pt3(?2-Cl)(?-PPh2)2(PPh3)3]PF6, [Pt3(?-PPh2)2I2(PPh3)3] and of the Pt-Ag cluster [Pt3?3-AgBF4(?2-I)(?-PPh2)2(PPh3)3]BF4.