Group 15

1. Group 15 Found with oxidation states of (III) and (V). However, these are purely formal. Can form catenated and ring structures Can form multiple bonds R3M: - these compounds have a lone pair and are very weak soft Lewis bases due to their diameter.They act as ligands to softer metals (low-valent Rh, Ir, Pt, Pd), and this activity diminishes greatly down the group. R5M – these compounds are coordinatively and electronically saturated. They tend to be less stable than the +3 oxidation state due to the inert pair effect.

2. R3E Alkyls The most common method of synthesis is metathesis: ECl3 + 3 CH3MgBr (THF) (CH3)3E + 3 MgClBr Another method is to employ alkali metal derivatives with organic electrophiles: These compounds are pyramidal.

3. R3E as Ligands Through the lone pair, R3E compounds of group 15 are often Lewis base ligands for the transition metals. Their affinity for transition metals follows the group trend and can be interpreted as HSAB behaviour: NR3 > PR3 > AsR3 > SbR3 >> BiR3 Because of this, tuning of the hardness of a group 15 ligand is possible. A good example of this is the use of diars in noble metal chemistry.

4. Diars Ligand The synthesis of diars ligands is instructive, because it incorporates three distinctively organometallic steps: The first step is the direct reaction of arsenic with methyl iodide to afford the monoiodo species: 4 As + 6 MeI  3 Me2AsI + AsI3 Subsequent purification allows separation of the Me2AsI, which can undergo direct reaction with sodium: Me2AsI + 2 Na  Me2AsNa + NaI

5. Diars Ligand Finally, this can metathesise with 1,2-dichlorobenzene to make the bindentate diars ligand: This ligand is the proper hardness to coordinate well to a Pd(IV) center:

6. R3E stability Triorganoarsines are generally stable at room temperature but the stability of triorganobismuthines varies with structure. e.g. 3 Me2BiAr → 2 Me3Bi + BiAr3 R3E are readily oxidized to give R3MO (M = As, Sb) and R2BiOR aryl compounds less sensitive High inversion barrier (chiral arsines R1R2R3As have been isolated)

7. Mixed Alkyl-Halide Compounds for +3 The mixed alkyl-halo compounds of E3+ can be made by limiting the metathesis reaction (difficult to control!), or by redistribution: 2 MeMgCl + AsCl3 Me2AsCl + 2 MgCl2 3-x R3P + x PI3  3 R3-xPIx Direct reaction can result in a mixture of alkyl halides: 2 As + 3 RCl  R2AsCl + RAsCl2 Or, similarly, by reaction with a halide acid: Me2PhAs + HI  Me2AsI + PhH Thermolysis of R3MX2

8. Mixed Alkyl-Halide Compounds for +3 The monohalides of group 15 are important to inorganic chemistry. Synthesis of ligands of the type R2E- is possible either through direct reaction or by direct reaction to a lithium reagent: R2BiCl + TePh- R2BiTePh + Cl R2PCl + 2 Li  R2PLi + LiCl The mixed compounds are also usually pyramidal with no bridging interactions between the halides and neighboring molecules.

9. Mixed Alkyl-Halide Compounds for +3 Monoorgano dihalides – RMX2 Prepared by reaction of R3M with MX3 These compounds commonly exhibit associated structures:

10. Reaction of R3E With a Dihalide mixed alkyl-halides of +5 Because the there is an accesible +5 oxidation state,, the group 15 compounds can react directly with dihalides to eliminate an alkyl halide: R3E + X2 R3EX2 (heat) R2EX + RX This intermediate is one route (metathesis) to ER5. The homoleptic +5 alkyls decompose on heating to produce the +3 alkyls, producing alkanes, alkenes, hydrogen. Aryl analogues are more stable. The R5E is not synthesized by metathesis from EX5.

11. Structures of ER5-nXn Generally not ionic with pseudo-TBP geometries. Note that in some cases (e.g. Me3AsX2 X = Br, I) an ionic formulation is more correct [Me3AsX+]X- The arsenic compounds of this type are trigonal bipyramidal, staying monomeric. The stibines of this type can allow bridging halide interaction, based on the size of the central atom (sterics) rather than its electrophilicity:

12. Routes to R5E Making the homoleptic +5 compounds is always indirect, the stability of either the octahedral coordination as a complex cation or anion does not allow direct synthesis of R5E from X5E: Ph3SbCl2 + 3 PhLi  Li+[SbPh6]- + 2 LiCl Li+[SbPh6]- + H2O  LiOH + PhH + SbPh5 Addition of two equivalents in the above system leads to a mixture of products, so the reaction is driven stoichiometrically to [SbPh6]- and then hydrolysed back to SbPh5.

13. Routes to R5E Notice that this group also exhibits amphoteric behaviour, due to the similarity between its electronegativities and that of carbon. Ph3As + PhI  [Ph4As]+I- + PhLi  Ph5As + LiI The C-E bonds are long and weak, due to the radii of the centres, but have a high covalency. The most common example of R5E synthesis is the oxidation of R3E species with dihalides and subsequent metathesis: R3E + X2 R3EX2 R3EX2 + 2 LiR  R5E + 2 LiCl

14. Alkyl Phosphines Trimethylphosphine is pyrophoric, but this reactivity diminishes as the R group increases in steric bulk. Phosphine oxides are very stable in air, and can be made by metathesis or oxidation: O=PCl3 + 3 MeMgCl  3 MgCl2 + O=PMe3 H2O2 + PMe3  O=PMe3 + H2O This reactivity extends to oxygen and allows the stabilization of the more reactive +3 oxidation state with the less reactive +5 oxidation state. This reactivity generalizes over the group, but with the reactivity diminishing extremely down the group. This is due to the larger radius and inert pair effect.