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CHEM 1101 | CHEMISTRY AND PERIODICITY OF GP 15 ELEMENTS

VSEPR

The shapes of molecules containing a central p-block element tend to be controlled by the no of electrons in the valence shell

Each valence shell electron pair on the central atom E in a molecule EXn containing E–X single bond is stereochemically significant and repulsions between them determine the molecular shape

Electron-electron repulsions decrease in the order

Lone pair-lone pair > Lone pair-bond pair > Bond pair-bond pair

When the central atom E is involved in multiple bond formation to atoms X, electron-electron repulsions decrease in the order

Triple bond-single bond > Double bond-single bond > Single bond-single bond

Repulsions between bonding pairs in EXn depend on the differences between the electronegativities of E and X; electron-electron repulsions are less the more E–X bonding density is drawn away from the central atom E

In other words, electrons are drawn away from the central atom and are hence further apart and less repulsion occurs. The more electronegative the atom, the less the electron repulsion and the bond angle decreases

Note: The model does not account for steric factors, in which case steric repulsion is outweighed by electronic effects

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If the presence of the lone pair has no geometric consequence, it is termed stereochemically inactive

Stereochemical inert pair effect refers to the tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion

When this effect operates, VSEPR will fail

For trigonal bipyrimidal, the bonds on the axial group would be longer

For 8 electron pairs, a square anti-prism shape is obtained

Distortions of tetrahedral molecule

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Stereochemically inactive lone pairs are usually observed for the heaviest members of a periodic group

Example: [SbCl6]- and [SbCl6]3-

[SbCl6]- is predicted to be octahedral (6 electron pairs) [SbCl6]3- is predicted to be pentagonal bipyrimidal (7 electron pairs) but it turns out to be octahedral

Group Trends

The valence shells of N, P, As and Sb are all similar and there are some similar stoichiometries e.g. EH3 but there is little resemblance between the characteristics of the compounds of P, As, Sb and Bi when compared to N

Sum of first 3 IE decreases and the lower OS (+3) is increasingly stable wrt higher (+5)

Example: R3P R3P=O (cf amine and the oxidation process from +3 to +5 OS becomes less likely down the Group)

Group 14: organic compounds formally +4, Sn exists as +2 (more stable) and +4 state

Elements become increasingly electropositive down the Group (increasing metallic character). While compounds of P are covalent (as in N), As, Sb and Bi exhibit cationic behaviour

Atomic radii increases smoothly with increase in principal quantum no

While the oxides of P are acidic, the oxides become increasingly basic down the Group

Down the Group, E—H and E—C bond strength decreases (expected) but E—F and E—Cl bond strength exhibits anomalies, where bond to Cl is weaker than that to F

Reason - bonding only in EH3 and E(CH3)3: decrease in overlap integral down the Group leads to weaker bond, valence orbitals are more diffuse, smaller overlap integral, weaker bond - and bonding (down EF3 and ECl3) series: weaker bond partially compensated by overlap - Comparing EF3 and ECl3, bond weakens as heavier halides are descended

Consider P: [Ne]3s23p33d0 which contains an empty d orbital. If there is correct symmetry, overlap of P’s d orbitals with filled orbital of X leads to delocalisation and hence partial bonding. However, N’s d orbitals are too far away from valence electrons to be involved in any overlap

Nitrogen PhosphorusVery strong p-p bondsN2 is thermodynamically stable

Weak p-p bonds P2 is thermodynamically unstable and only observed at > 900oC

Example: P(OR)3 exists but N(OR)3 cannot be prepared. Instead, O=N(OR) is obtained as there is p-p bonding between N and O

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p-d bonding is rareN does not have proper symmetry (d orbitals are too far away from valence shell to be used)

Weak to moderate but important d-p and d-d bonding

Oxides with amines (R3N+—O-) and phosphines (R3P=O). Bond between phosphorus d and oxygen p orbital

Bonding to TMs:R3P stablises electron rich metals (i.e. M(0) with low/zero OS) through a electron system. R3N stablises metals in high OS (i.e. M(II)Reason: P can use d orbitals to accept electron density from electron-rich metals No valence expansionObeys octet rule and maximum coordination no = 4

Valence expansionCoordination no of 5 and 6 are common

N: max CN = 4 (e.g. NH4+)

P: CN in PF5 (trigonal bipyramidal) = 5 (sp3d hybridised) CN in PF6- (octahedral) = 6 (sp3d2 hybridised – using valence bond model)

Phosphorus (Phospho: light, Phorein: bringing)

Discovered in 1669 by Henning Brandt from reductive distillation of putrified horse urine

Patented in 1851 by Thomas Albright using bone + carbon

Used in 19th Century match industry – Lucifers (strike anywhere matches containing white P) vs safety matches (red P). Phossy jaw was an occupational disease of those who worked with white P

Today, P is produced commercially by 2Ca3(PO4)2 + 6SiO2 + 10C P4 + 6CaSiO2 + 10CO

Allotropes

Yellow / White phosphorus is a waxy solid and exists as discrete P4 molecules; a tetrahedral molecular solid / van der Waals’ solid. It is soluble, especially in CS2, volatile and pyrophoric (spontaneously combust)

Red phosphorus is air stable, insoluble and not volatile as it exists as a polymer. It is prepared by heating P4 in a vacuum

For other allotropes, heating to higher temperatures leads to increase in cross linkages in the polymer

The most stable allotrope is black phosphorus and is obtained by heating to 1600oC and 100 atm

P4 (g) 2P⇌ 2 (g) (at 900oC)

P2 vs N2

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2E2 E⇌ 4

Thermodynamically, if N forms N4 (3 single bonds), H = 480 kJ mol-1 compared to 956 kJ mol-1 if it forms N2 (1 triple bond). Hence, N2 is preferred

However, if P forms P4 (3 single bonds), H = 600 kJ mol-1 compared to 485 kJ mol-1 if it forms P2. Hence, P4 is favoured

Electronic reason

bonding overlap integrals are similar for N and P and varies proportionally less down the Group

overlap is much more effective in first row elements than those below (much reduced overlap in P than N, resulting in weaker bond as p orbitals become longer not fatter down the Group)

Catenation (tendency to form single bonds) increases down the Group (similarly for O2 and S8)

Isolating elemental As, Sb and Bi

As: by thermolysis of FeAsS ore to give FeS and As

Sb: Sb2S3 + Fe FeS + Sb

There are fewer allotropes for As, Sb and Bi

Yellow As and Sb are comparable to P4 while other allotropes of As, Sb and Bi are similar to ‘black phosphorus’ which contain multiple cross linkages

Comparing NH3 and PH3

NH3 PH3

Highly soluble in water Not very soluble in waterKb = 10-5 Kb = 10-28

(very poor base, unlikely to gain H+)

Ka = 10-29 (poor acid)

PH3 + H+ PH⇌ 4+

Gives alkaline solution Gives neutral solution

Note: the above differences are due to differences in electronegativities NH3 contains polar bonds while PH3 contains non-polar bonds

Synthesis of PH3

Lab

PCl3 + LiAlH4 nucleophilic substitution

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Ca3P2 + H2O hydrolysis

P4 + I2 + H2O PH4I oxidationPH4+ + KOH PH3 acid base

Industrially, P4 + 3KOH + H2O PH3 + KH2PO4 (alkaline hydrolysis)

Nomenclature

Trialkyl (or triaryl)phosphine Trialkyl (or triaryl)phosphine oxide

Trialkyl (or triaryl)phosphine sulphide

Phosphorus trihalidePhosphorus oxyhalide (aka

Phosphoryl halide)

Na+R2P-

Sodium dialkyl (or aryl) phosphide

Trialkyl(aryl)phosphite Phosphoric acid (aka orthophosphoric acid)

Trialkyl (or aryl)phosphate

Phosphinic acid (unknown)

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Phosphorous acid (aka Phosphonic acid)

Hypophosphorous acid

 

Tetra-alkyl phosphonium saltAlkylphosphinite Dialkylphosphonate

Trihalides EX3 (X=F, Cl, Br)

Synthesised directly from reaction of the elements with the halogens

E (s) + 3/2 X2 EX3

Exception: PF3 in which P is oxidised directly to +5 OS (in PF5) since F2 is highly oxidising

Volatile and rapidly hydrolysed by water

PCl3 + H2O H3PO3 + H2PO4 (phosphoric acids)

Structure of PX3, AsF3, AsCl3, SbCl3, SbBr3 are all trigonal pyramidal as predicted by VSEPR

However, BiF3 is an ionic lattice with CN=9 and SbF3 is a lattice with 3 Sb—F (highly distorted octahedral)

PCl3 is an important precursor to organic compounds and can convert alcohols to alkyl halides and carboxylic acids to acyl chlorides

PI3 is a powerful deoxygenating agent and can convert sulfoxides to sulphides

SbF3 (Schwartz’s reagent) is a powerful fluorinating agent (R3PS R3PF2)

AsCl3 is used as a non-aqueous solvent

Comparing NCl3 with NI3,

NH4Cl + Cl2 NCl3 is an explosive and the vapour is used to bleach / sterilise flour

I2 + NH3 I3N—NH3 and is shock sensitive since 2NI3 N2 (g) + 3I2, where the reaction is thermodynamically driven by NN bond formation (entropy)

Pentahalides EX5 (EN since N cannot undergo valance expansion)

EX3 + X2 EX5

PF5 is a gas, PCl5 and PBr5 are solids and PI5 is unknown

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All are off-white or yellow moisture-sensitive materials

Predicted structure: trigonal bipyramidal

NMR

Chemical shift (where the peak occurs) – information on the no of different chemical environments for protons e.g. CH3CH2Cl has 2 different peaks

Coupling pattern – information on no of equivalent protons on nearest neighbours e.g. CH3CH2Cl: the –CH3 group causes the –CH2 group to split thrice (quartet) and the –CH2 causes the –CH3 group to split twice (triplet)

Note: Since –CH2 is closer to the electronegative Cl, it lies more to the left of the spectrum

Since 31P and 19F have both I= ½ (same as 1H), same rules apply

For PF5, experiment shows that all F atoms are equivalent, unlike what is predicted from theory (trigonal bipyramidal, with axial and equatorial F atoms)

Reason: Fluxionality / dynamic behaviour – the axial and equatorial places exchange rapidly making the 2 sets of F atoms look the same. The mechanism of rotation is Berry pseudo–rotation

The process is very rapid as the activation energy between the interconversion (trigonal bipyramidal to square pyramidal) is small

D3h C4v D3h

(square based pyramid intermediate)

Common for AB5 systems e.g. Fe(CO)5

However, though electron diffraction in gas phase gives the D3h structure, PCl5 solutions conduct electricity, indicating the presence of ions

Since Cl does not have I = ½, there is no coupling (no other I=1/2 species to couple with) and there are 2 chemical environments (i.e. 2 phosphorus containing species)

Hence, the structure is phase dependent

X-ray crystallography reveals that the compound is a salt containing [PCl4]+ (tetrahedral, Td) and [PCl6]– (octahedral, Od)

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For PBr5, it is a canary yellow solid and is an electrolyte in solution (i.e. ions present) and 31P NMR shows only 1 environment (no coupling)

X-ray shows that it is a salt made up of [PBr4]+ and Br– (unlike PCl5, this is not a second P-containing species)

Note: PBr5 is thermally unstable (PBr5 ∆→

PBr3 + Br2) and PI5 is not well known

For AsF5, it is generally similar to PF5

SbF5 associates through F bridges (6 coordinated species) either as linear polymers in liquid state or cyclic tetramers in solid state

BiF5 consists of infinite linear chains with trans linkages

Other EX5 exhibit complex structural behaviour based on [EX4]+ ions

Reactions of PCl3

PH3 PCl3 S=PCl3

P(OR)3 PR3, PClR2, PCl2R PCl5

PF3 P(NR2)3

O=PCl3H3PO4 + HCl + H3PO4

O=P(NR2)3 O=PR3

O=P(OR)3

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Grignard Reagent

Prepared by Mg(s) + RCl I 2, ether→

RMgCl

Useful as a source of nucleophile R– (e.g. RCl, RCO2H, R(C=O)R), changes polarisation on C from + to –

3RMgCl + PCl3 R3P + 3MgCl2

Results in formation of tertiary phosphines

Reaction requires dry solvent (ether) to prevent protonation of R– to get alkane and reaction proceeds in a stepwise manner, allowing mixed phosphines to be prepared

Example: PCl3 MeMgCl→

MePCl2 EtMgCl→

MeEtPCl PhMgCl→

MeEtPhP (trigonal

pyramidal)

Grignard reagents are structurally complex in solution (not R-Mg-X) but an equilibrium mixture containing R2Mg, RMgX, MgX2 etc.

Tertiary Phosphines

Compounds of type R3P (a homologous series)

Where PMe3, PEt3, PBu3 are stinking liquids and are easily oxidised in air

And PPh3, PCy3, PBut3 are solids and are relatively slowly oxidised due to bulky groups and can be easily handled

Compounds are basic due to lone pair on P

Excellent ligands for TM, especially metals in low OS

E.g. PhCl3 + excess PPh3 Rh(PPh3)3Cl

Note: coordinating ability depends on both basicity and steric factors

The greater the no of alkyl groups (electron-donating inductive effect), the more basic the phosphine

The presence of halogens or oxygen (electron-withdrawing) reduces the basicity of phosphines

Reason: The extent of basicity is related to the availability of the lone pair (i.e. PMe3 > PPh3 > PCl3 > PF3)

Tolman cone angle refers to the angle of the metal swept out by the van der Waals’ radius of the groups attached to the P atom

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Found by starting at the position of the metal and defining a cone which just touches the position of substituent groups

M—P distance is set at 2.28 Angstrom

Note: With bulky ligands, steric factors may dominate over electronic ones in determining the coordination to the metal

Hence, PBu3 > PPh3 > PMe3 > PH3 and PR3 > P(OR)3

Reason: Flexibility is introduced by O atom on P(OR)3 (rotation about O atom) as compared to PR3 and hence, corresponding phosphite is always smaller than the phosphine

Phosphines as Ligands

E.g. NiCl2 + 2PMe3 NiCl2(PMe3)2 (square planar)NiCl2 + 2PPh3 NiCl2(PPh3)2 (tetrahedral)

Larger substituent results in larger angle (109.5 compared to 90o)

Amines are more basic and ‘harder’, which are better ligands to metals with high OS

Phosphines are ‘soft’ and are better ligands to metals with low OS

‘Hard’ – electron density is non-polarisable whereas ‘soft’ – electron density is polarisable / easily distorted

M(0): easy to distort electron density M(II): becomes increasingly non-polarisable

Across the Period, metal/acid becomes harder. Down the Group, ligand (base) becomes softer

O2- is small and highly charged and it is difficult to distort electron cloud. S2- is easier to polarise

N-based ligands are hard while P-based ligands will be softer and thus the latter stabilises metals in low OS

Chiral Phosphines

Pyramidal inversion is fast for ammonia and amines but slower in phosphines

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Reason: The energy difference between the valence s and p orbital on N and P respectively. The activation energy to convert from trigonal pyramidal (sp3) to trigonal planar (sp2) is small for N but large for P. Hence, the rate is slower for P than N

Thus, it is possible to prepare chiral phosphines but not amines

Synthesis of Phosphites

PCl3 + 3ROH b⃗ase P(OR)3 + 3[BH]+Cl-

Structure of P(OR)3 – tetrahedral

All are stinking viscous liquids

Less basic than corresponding phosphine due to electronegative O atom

Smaller than corresponding phosphine due to flexibility of P—O—C linkage

Note: Under acidic conditions, PCl3 + 3ROH 3RCl + H3PO4

Oxidation Reactions of PCl3

Oxidation of PCl3 with O2 produces Cl3P=O (phosphoryl chloride) and oxidation with S8 produces Cl3P=S (thiophosphoryl chloride) [tetrahedral]

Cl3P=X can then undergo analogous substitutions to PCl3 while retaining the P=X bond (effectively inert)

E.g. Cl3P=O + 3MgCl R3P=O (phosphine oxide) + 3MgCl2

Cl3P=O + 3ROH b⃗ase (RO)3P=O (phosphite)

Organophosphorus Chemistry

Phosphines + Alkyl Halides (oxidative addition) Phosphonium salt

R3PIII + R’X [R3R’PV]+ + X-

Phosphonium salt formed (analogous to ammonium salt)

E.g. [Ph4P]+Br- :tetraphenulphosphonium bromide

Phosphonium salts are used as large cations to trap out large anions

WITTIG REAGENTS (requires -H to be present)

[Ph3P–CH3]+ + BuLi [Ph3P+–C–H2]

Ylid is formed, with opposite charges on adjacent atoms (subset of zwitterions)

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Phosphonium ylid is a zwitterion with opposite charges on neighbouring atoms

Ylids exhibit canonical forms between the charged species (C is sp3 hybridised) and R3P=CH2 (C is sp2 hybridised)

Orbital overlap diagram involves and bonding, where C donates electron density Cp Pd

Reason: C has an unhybridised p orbital and donates its electron density to the empty d orbital on P

Phosphonium Ylids

Converts carbonyl compounds to alkenes (>C=O to >C=CR2)

Example: Me2C=O + Ph3P=CH2 Me2C=CH2 + Ph3P=O

The reaction is an example of metathesis (exchange of bonds between the 2 reacting chemical species)

The driving force is the formation of the strong P=O bond

Note: Chemists apply different models for one compound depending on the context. The structure of the ylid is best understood using the P=C bond model but the reactivity is best understood using the charge separated model

Non-ylid P=C bond formed from elimination of HCl from a P–C bond

R2CH–PCl2 −⃗HCl R2C=PCl

Note: R groups have to be bulky groups to prevent polymerisation

Penta-alkyls (phosphoranes) are formed when reacting phosphonium salts with an organilithium compound

Only works if there is no -H atom

The reaction is adduct formation (adding something without removing anything, no substitution, no change in OS)

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[Ph4P]+I– + PhLi Ph5P + LiI

Predicted structure pf Ph5P: trigonal bipyramidal. However, SbPh5 is a square-based pyramid

However, P–C bonds can be cleaved when the carbon is aromatic to form a diaryl phosphide which is a useful reagent / nucleophile for making phosphines

Ph3P + 2Li T⃗HF Ph2P–Li+ + Ph–Li+

Ph2P– is bent / angular

Ph2PLi – lithium diphenyl phosphide is a strong nucleophile

Synthesis of Multidentate Phosphines

DPPE is the most common bidentate P ligand and is analogous to ethylenediamine

Making P–P bonds

Similar to Wurtz coupling of alkyl halides (C–C bond forming reactions)

Reagents: Na/NH3(l)

2R2PCl Na /NH3→

R2P–PR2 + 2NaCl

With small RPCl2, cyclic compounds are formed (e.g. enveloped structure with 4 P atoms in plane but 5th P atom sticking out)

However, if R is large, an unstable P=P bond is formed and the large R group shields P=P from any attack

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This is an example of a kinetic trap / steric shielding where large R groups cannot fit round a ring so the normal thermodynamic product is not obtained

Examples include benzene rings with t-butyl groups on both ortho positions and P=P bond attached to C1

Organo compounds of heavier elements

Exhibit many similar stoichiometries and properties but for arsonium salts, ylids, penta-aryl atimony,

E–C bond strength and ligating ability for TM decreases down the Group

PR3 > AsR3 > SbR3 > BiR3

Preparation of Organoarsenic compounds

AsCl3 is not used as the starting material due to its toxicity making it too dangerous to handle. Hence, oxide is used as the starting material

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Anomalous Properties of organostibines

Propensity to higher coordination no (due to valence expansion)

SbPh5 is predicted to be trigonal bipyramidal but is a square-based pyramid

Ph2SbF is expected to be trigonal pyramidal but it is an apex-shaped pseudo trigonal bipyramid as each Sb atom is coordinated to 4 groups

Me2SbCl3 is predicted to be trigonal bipyramidal but is octahedral about each Sb and 1 edge of the octahedron is shared by both antimoniy

As2O3

RX/NaOH

RAsO(ONa)2

H+

RAsO(OH)

2

SO2/HCl/I-

RAsCl2

RX/NaOH

R2AsO.OH

SO2/HCL/I-

R2AsCl

RMgX

R3As

RAsH2

RAs(NR2)2

R2AsCl3

(R2As)2

O

[R4As]+X- R5AsR5As R3AsX2 RMgX

RLiX2RLi

Cl

2

LiNR2LiAlH4

OH-

RMgX

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Phosphorus-nitrogen Compounds

May contain P–N and/or P=N

Phosphazines refer to unsaturated PN compounds

E.g. Ph3PCl2 + Ph’NH2 Ph3P=NPh’ + 2HCl

Typical r(P=N) = 1.60 Angstrom and typical P=N–C bond angle = 120o (sp2 hybridised about N)

Same bonding as seen in ylids (NpPd)

Pentavalent PN Compounds

PCl5 + NH4Cl P(NCl2)n + 4HCl (where n = 3-11)

P–Cl bond is very polar and hence is water-sensitive

Preparation requires dry hydrocarbon solvent

Small rings are stabilised by pd bonding

Cl can be substituted by nucleophilic substitution e.g. ROH, Grignard

For n=4, puckered conformation if halide is Cl. If halide is F, structure is planar

Inorganic Polymers

(NPCl2)3 150−300deg→

(NPCl2)n

For n=3,

Planar (similar to benzene) Aromatic

Contains P(V) and N(III)

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