The Group 15 Elements

Part A: The Essentials

The properties of the Group 15 elements are diverse and more difficult to rationalize in terms of atomic radii and electron configuration than the p-block elements encountered so far. The usual trends of increasing metallic character down a group and stability of low oxidation states at the foot of the group are still evident but they are complicated by the wide range of oxidation states available.

15.1 The Elements

All members of the group other than N are solids under normal conditions. However, the trend to increasing metallic character down the group is not clear-cut because the electrical conductivities of the heavier elements actually decrease from As to Bi.

Properties Table

Allotropes of Phosphorus

Arsenic exists in two solid forms: yellow arsenic (tetrahedral As₄ molecules) and grey/metallic arsenic (more stable, puckered hexagonal layers). Bismuth has recently been found to be radioactive, decaying by α emission with a half-life of 1.9 × 10¹⁹ years.

Natural Occurrence: Nitrogen makes up 78% by mass of the atmosphere. Phosphorus is found as fluorapatite, Ca₅(PO₄)₃F, and hydroxyapatite, Ca₅(PO₄)₃OH. The chemically softer elements As, Sb, and Bi are often found in sulfide ores such as realgar (As₄S₄), orpiment (As₂S₃), stibnite (Sb₂S₃), and bismuthinite (Bi₂S₃).

15.2 Simple Compounds

The wide variety of possible oxidation states can be understood by considering the valence-electron configuration ns²np³. This configuration suggests the highest oxidation state should be +5. According to the inert-pair effect, the +3 oxidation state should be more stable for Bi.

Common Oxidation States

Key distinctions of nitrogen:

• High electronegativity (exceeded only by O and F)
• Small atomic radius
• Absence of accessible d orbitals
• Coordination numbers rarely exceed 4 in simple molecular compounds

Heavier elements frequently reach coordination numbers of 5 and 6, as in PCl₅ and AsF₆⁻.

Nitrides

Nitrogen forms binary compounds (nitrides) with almost all elements. They are classified as:

Phosphides

P atoms may be arranged in rings, chains, or cages, for example P₇³⁻, P₈²⁻, and P₁₁³⁻. Formulas range from M₄P to MP₁₅.

Hydrides

All elements form simple hydrides EH₃. Ammonia (NH₃) is a pungent, toxic gas and excellent solvent for Group 1 metals. The other hydrides—phosphine (PH₃), arsine (AsH₃), and stibine (SbH₃)—are all poisonous gases.

Halides

Trihalides are known for all Group 15 elements. Pentafluorides exist for P to Bi, but pentachlorides only for P, As, and Sb. The pentabromide is known only for P. Nitrogen cannot form NF₅ (atom too small), but NF₄⁺ exists.

15.3 Oxides and Oxoanions of Nitrogen

Nitrogen forms oxo compounds and oxoanions in all oxidation states from +5 to +1.

Nitrogen Oxides Summary

Nitric Acid and the Ostwald Process

Nitric acid (HNO₃) is a major industrial chemical produced by the Ostwald process—an indirect route from N₂ to HNO₃ via NH₃:

The NO₃⁻ ion is a moderately strong oxidizing agent but reactions are generally slow in dilute acid. Concentrated HNO₃ undergoes faster reactions. A strong reducing agent like Zn can reduce HNO₃ all the way to NH₄⁺ (oxidation state −3):

A weaker reducing agent like Cu yields NO₂ or NO:

Aqua Regia

Aqua regia ("royal water") is a mixture of concentrated HNO₃ and HCl, yellow due to NOCl and Cl₂. It can dissolve gold and platinum:

NO₂/N₂O₄ Equilibrium

Nitrogen(IV) oxide exists as an equilibrium mixture:

The N−N bond in N₂O₄ is long and weak because the unpaired electron is delocalized over all three atoms in NO₂ rather than concentrated on N.

Biological Role of Nitric Oxide

NO is generated in vivo and performs functions such as:

• Reduction of blood pressure
• Neurotransmission
• Destruction of microbes

Part B: The Detail

In this section we review the detailed chemistry of the Group 15 elements. We shall see the wide variety of oxidation states achieved by the elements, particularly nitrogen and phosphorus.

15.4 Occurrence and Recovery

Phosphorus production:

Elemental phosphorus is produced by carbon arc reduction at 1500°C:

Arsenic is extracted from flue dust of copper and lead smelters or by heating ores:

Antimony is extracted by heating stibnite with iron:

15.5 Uses

Nitrogen uses:

• Inert atmosphere in metal processing, petroleum refining, food processing
• Laboratory inert atmosphere
• Liquid nitrogen as refrigerant (b.p. −196°C, 77 K)
• Production of ammonia (Haber process) and nitric acid (Ostwald process)

Phosphorus uses:

• Pyrotechnics, smoke bombs, steel making
• Striking strip on matchboxes (red P + sand)
• Cleaning agents, water softeners (sodium phosphate)
• Detergent builders (condensed phosphates)
• Fertilizers (85% of phosphoric acid production)
• Biological: bones, teeth, cell membranes, DNA, RNA, ATP

15.6 Nitrogen Activation

N₂ is strikingly unreactive due to:

• Strong N≡N triple bond (950 kJ mol⁻¹) → high activation energy
• Large HOMO−LUMO gap → resistant to electron-transfer processes
• Low polarizability → discourages polar transition states

The Haber process:

Fritz Haber won the Nobel Prize in 1918 for developing this process; Carl Bosch in 1931 for engineering the first plants. The process had major impact on civilization—before it, fertilizers came from guano and saltpetre from South America.

N₂ complexes form with metals:

Direct reduction to ammonia at room temperature has been achieved with a Mo catalyst containing a triamidoamine ligand, cycling between Mo(III) and Mo(VI).

15.10 Hydrides

(a) Ammonia

Properties of NH₃:

• Boiling point −33°C (higher than other Group 15 hydrides due to H-bonding)
• Excellent nonaqueous solvent (dissolves 330 g Cs in 100 g liquid NH₃ at −50°C)
• Weak base: pK_b = 4.75

Autoprotolysis equilibrium in liquid ammonia:

Ammonium salts decompose on heating. When the anion is oxidizing (NO₃⁻, ClO₄⁻, Cr₂O₇²⁻), the NH₄⁺ is oxidized to N₂ or N₂O:

(b) Hydrazine and Hydroxylamine

Hydrazine (N₂H₄) is manufactured by the Raschig process:

Hydrazine is a weaker base than ammonia (pK_b1 = 7.93). Major uses: rocket fuel, foam-blowing agent, boiler water treatment.

(c) Phosphine, Arsane, and Stibane

Bond angles decrease down the group:

This decrease is attributed to reduced sp³ hybridization from NH₃ to SbH₃.

Commercial synthesis of PH₃ uses disproportionation:

Organophosphines (PR₃) and organoarsanes (AsR₃) are soft ligands used in metal coordination chemistry, stabilizing metals in low oxidation states.

15.11 Halides

(a) Nitrogen Halides

• NF₃: Only exergonic binary halogen compound of N. Pyramidal, not a Lewis base (F atoms withdraw electron density from lone pair)
• NCl₃: Highly endergonic, explosive yellow oil
• NBr₃: Explosive deep red oil
• NI₃: Explosive solid

NF₃ can be converted to NF₄⁺:

(b) Halides of the Heavy Elements

Trihalides range from gases (PF₃, b.p. −102°C) to solids (BiF₃, m.p. 649°C). Common preparation is direct reaction of element with halogen.

The trichlorides are useful for synthesis:

PF₃ resembles CO—weak σ donor but strong π acceptor. Complexes include [Ni(PF₃)₄], analogous to [Ni(CO)₄].

Pentahalides:

• PF₅, AsF₅: gases, trigonal bipyramidal in gas phase
• SbF₅: highly viscous liquid (cyclic tetramer due to F-bridging)
• PCl₅: solid exists as [PCl₄⁺][PCl₆⁻]
• SbF₅ + HF forms superacid: H₂F⁺ + SbF₆⁻

The alternation effect explains why AsCl₅ is very unstable (poor shielding of 3d electrons increases effective nuclear charge).

15.15 Oxoanions of Phosphorus, Arsenic, Antimony, and Bismuth

Phosphorus Oxoanions

White phosphorus disproportionates in base:

H₂PO₂⁻ is used in "electrodeless plating" to reduce Ni²⁺:

15.16 Condensed Phosphates

Heating H₃PO₄ above 200°C causes condensation, forming P−O−P bridges:

Most commercially important: sodium tripolyphosphate (Na₅P₃O₁₀)—used in detergents, water treatment, food industry.

15.17 Phosphazenes

PN is structurally equivalent to SiO. Phosphazenes (R₂PN units) are analogous to siloxanes (R₂SiO units).

Cyclic phosphazene dichlorides are good starting materials:

At ~290°C the trimer changes to polyphosphazene. Cl atoms are readily displaced by Lewis bases:

The PPN⁺ cation ([Ph₃P═N═PPh₃]⁺) is useful for forming salts of large anions, soluble in polar aprotic solvents.

15.18 Organometallic Compounds of Arsenic, Antimony, and Bismuth

Oxidation states +3 and +5 are encountered. Examples: As(CH₃)₃ (+3) and As(C₆H₅)₅ (+5).

(a) Oxidation State +3

Preparation methods include Grignard reagents or organolithium compounds:

The M−C bond strength decreases for a given R group: As > Sb > Bi. All compounds act as Lewis bases; basicity decreases As > Sb > Bi.

The bidentate ligand diars (C₆H₄(As(CH₃)₂)₂) is useful. Many complexes of soft metals like Rh(I), Ir(I), Pd(II), Pt(II) have been prepared.

Synthesis of diars starts from (CH₃)₂AsI:

Polyarsane compounds (RAs)ₙ can be prepared. Polymethylarsane exists as cyclic pentamer or ladder-like structure. MᐩM bond strength: As > Sb > Bi.

Arylometals include arsabenzene (C₅H₅As, stable to 200°C), stibabenzene (isolated but polymerizes), and bismabenzene (very unstable). These exhibit aromatic character.

(b) Oxidation State +5

Trialkylarsanes act as nucleophiles:

Phenyllithium on tetraphenylarsonium gives pentaphenylarsenic:

AsPh₅ is trigonal bipyramidal (VSEPR), but SbPh₅ is square pyramidal!

15.7 Nitrides and Azides

Azides

Azides (N₃⁻) are synthesized by oxidation of sodium amide:

The azide ion is isoelectronic with N₂O and CO₂, and is linear. It is a good ligand but heavy-metal complexes (Pb(N₃)₂, Hg(N₃)₂) are shock-sensitive detonators:

NaN₃ is used in airbags: ~50 g liberates ~26 dm³ N₂ at room temperature.

Polynitrogen cation N₅⁺ has been synthesized—a powerful oxidizing agent that ignites organic material:

15.9 Arsenides, Antimonides, and Bismuthides

GaAs (gallium arsenide) is used for integrated circuits, LEDs, and laser diodes. It has higher electron mobility than Si and produces less electronic noise. Used in mobile phones, satellite communications, radar systems.

Exercises