HOMONUCLEAR MULTIPLE BONDING IN HEAVIER MAIN-GROUP ELEMENTS

Recent developments in multiple bonding between heavier main group ste ms are the focus of this review. Emphasis is placed on compounds with homonuclear bonds. It is clear that the Group 15 derivatives REER (E = P,As, Sb or Ri; R = alkyl or aryl ligand) display double bonding thro ughout the group. For the Group 14 species R2EER2 (E = Si, Ge, Sn or P b, R = organo or related group), it is argued that, at present, only t he silicon and certain germanium derivatives merit designation as 'dim etallenes', Data for multiply bonded heavier Group 13 compounds are cu rrently very scarce. Nonetheless, the available structures of compound s such as (MR)(n) (M = Al, Ga, In or Tl; R = alkyl or aryl group; n = 1-6) indicate weakness of the M-M interaction especially for the galli um, indium and thallium compounds where monomeric species are obtained readily. The M-M bond order in the dimers RMMR is apparently less tha n 1 but can be increased by reduction to give [RMMR](2-) but it is pro bable that the overall M-M bond order remains less than 2.