Homonuclear multiple bonding in heavier main group elements

Recent developments in multiple bonding between heavier main group atoms 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 Bi; R = alkyl or aryl ligand) display double bonding throughout the group. For the Group 14 species R2EER2 (E = Si, Ge, Sn or Pb, R = organo or related group), it is argued that, at present, only the silicon and certain germanium derivatives merit designation as ‘dimetallenes’. Data for multiply bonded heavier Group 13 compounds are currently very scarce. Nonetheless, the available structures of compounds 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 gallium, indium and thallium compounds where monomeric species are obtained readily. The M—M bond order in the dimers RMMR is apparently less than 1 but can be increased by reduction to give RMMR2– but it is probable that the overall M—M bond order remains less than 2.