Asl, ( R = p-FC,H, or Ph) and Ph,AslBr have been synthesised in anhydrous diethyl ether and characterised by elemental analysis and Raman spectroscopy. Single-crystal X-ray diffraction studies of Ph,AsBr, showed it t o possess a molecular trigonal-bipyramidal structure, in contrast to Ph,Asl, which adopts the molecular four-co-ordinate structure Ph,As-1-1. Both Me,Asl, and Me,AsBr, have the molecular four-co-ordinate structure, Me,As-X-X, thus illustrating that the structure of R,AsX, compounds isolated from diethyl ether is dependent on R and X. The structure of Ph,AslBr also revealed four-co-ordinate molecular geometry in contrast to Ph,AsBr,. The compound Ph,AslBr showed no dual occupancy of the halogen sites, and the heavier halogen is bound directly to the arsenic. The X-ray powder diffraction patterns of Ph,AsX, (X, = Br,, I, or IBr) have been recorded and are compared. Solution studies on R,Asl, (R = p-FC,H,, Ph or p-MeOC,H,) again showed the sensitivity of these molecules to the nature of R. For example, (p-FC,H,),Asl, was not formed in any solvent employed; instead, equimolar quantities of (p-FC,H,),Asl, and ( p -FC,H,),As were isolated: Ph,Asl, was only formed in non-polar solvents upon dropwise addition of a light petroleum solution of I , to a saturated solution of Ph,As in the same solvent. The compound (p-MeOC,H,),Asl, was the quantitative product from the direct reaction of the tertiary arsine with I, in diethyl ether. Its stability constant in CCI, is approximately 2.5 times greater than that calculated for Ph,Asl,.Compounds of stoichiometry R,AsX, (X, = F,, CI,, Br, or TBr) have not been the subject of extensive study, but have received sporadic attention over the past century. Several crystal structures are available including the trigonal-bipyramidal Ph,AsF,, M ~, A s C I , ~ and ( M ~, C C H , ) , A S B ~, . ~ However, Me,AsBr, was assigned an ionic structure from a preliminary X-ray study, despite a significant anion-cation interaction of 3.38 A (the van der Waals radius of dibromine is 3.9 A).,

Me,N),] have been prepared by the direct reaction of PR, and I, in diethyl ether solution. The compounds have been characterised by Raman and solid-state "P-{H} magic angle spinning (MAS) and solution N M R spectroscopy. Solid-state ,lP-{H} MAS N M R studies indicate that the predominant solid-state species is the molecular 'spoke' structure R3P-1-1; however, in some cases a minor peak was also assignable to the ionic species, [R,PI]I. Additionally, solid-state "P-{H} M A S N M R studies of (Me,N),PI, and (CH,=CHCH,),PhPI, indicate that, in contrast to all other compounds of stoichiometry R,PI, prepared in diethyl ether, they are exclusively ionic, [R,PI] I. The crystal structure of PhMe,PI, shows it to have the molecular 'spoke' geometry, PhMe,P-1-1, d(l-I) = 3.408 A, previously observed for Ph,PI,, in agreement with solid-state 31P-{H} MAS N M R results.

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