Low-co-ordination arsenic and antimony compounds: Synthesis and characterisation of 2-arsa- and 2-stiba-1,3-dionatolithium(I) complexes, [Li{OC(R)EC(R)O}L](E = As or Sb; R = Bu^t, C₆H₂Prⁱ³-2,4,6 or C₆H₂Bu^t₃-2,4,6; L = Et₂O or MeOCH₂CH₂OMe)

Reactions of [Li{E(SiMe₃)₂}(dme)] (E = As or Sb, dme = MeOCH₂CH₂OMe), with a range of acid chlorides, RCOCl, afforded the novel 2-arsa-1,3-dionatolithium(I) complexes [Li{OC(R)AsC(R)O}L] (R = Bu^t, L = 1/2 dme or Et₂O; R = C₆H₂Prⁱ₃-2,4,6, L = Et₂O; R = C₆H₂Bu^t₃-2,4,6, L = dme), and the first example of a 2-stiba-1,3-dionatolithium(I) complex [{[Li{OC(Bu^t)SbC(Bu^t)O}(dme)_0.5] ₂}_∞]. X-Ray crystal structural analyses for the complexes with R = Bu^t, L = 1/2 dme or Et₂O and the antimony compound show them to be dimeric in the solid state with respect to the metal and anionic ligands, the dimeric units being linked in an infinite polymeric chain by bridging dme molecules or solvated with Et₂O. A dimeric structure can also be inferred when R = C₆H₂Pr_i3-2,4,6, L = Et₂O. In solution these four compounds display fluxional behaviour. Treatment with HCl of the monomeric compound having R = C₆H₂Bu^t₃-2,4,6, L = dme yields the known diacylarsane [As{C(C₆H₂Bu^t₃-2,4,6)0} {C(C₆H₂Bu^t₃-2,4,6)OH}] the crystal structure of which reveals the enol form with the alcoholic proton intramolecularly hydrogen bonded to the opposing oxygen centre of the ligand.