Transcript Group 14 - University of Ottawa

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.
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.
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.
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
Diars Ligand
Finally, this can metathesise with 1,2-dichlorobenzene to make the
bindentate diars ligand:
AsMe2
Cl
+ 2 Me2AsNa
+ 2 NaCl
AsMe2
Cl
This ligand is the proper hardness to coordinate well to a Pd(IV) center:
Me2
As
As
Me2
Cl
Pd
Cl
Me2
As
As
Me2
2+
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)
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
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.
Mixed Alkyl-Halide Compounds for +3
Monoorgano dihalides – RMX2
Prepared by reaction of R3M with MX3
These compounds commonly exhibit associated structures:
Me
Me
I
Sb
Sb
I
X
I
O
X
I
X
X
Sb
X
Sb X
X
X
X = Br, I
X
Bi
X
O
Bi
X
X
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.
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+]XThe 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:
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.
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
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.