Metallaheteroborane chemistry: Part 14. Synthesis of the {MS2B7}-cluster compounds [9,9-(PX3)2-9,6,8-PtS2B7H7] (where X=Ph or OMe), [9,9-(PPh3)2-9,6,8-RhS2B7H8] and [5,5-(PPh3)2-5,6,10-RhS2B7H8], and the {M2S2B7}-cluster compound [(PPh3)2HRh(PPh3)ClRhS2B7H7]