Chapter 14: The Group 14 Elements

Part A: The Essentials

The elements of Group 14 (the carbon group) are of fundamental importance in industry and nature. We discuss carbon in many contexts throughout this text, including organometallic compounds and catalysis. The focus of this section is on the essential aspects of the chemistry of Group 14.

14.1 The Elements

Key Points: The lightest elements of the group are nonmetals; tin and lead are metals. All the elements except lead exist as several allotropes.

The lightest members of the group, carbon and silicon, are nonmetals, germanium is a metalloid, and tin and lead are metals. This increase in metallic properties on descending a group is a striking feature of the p block and can be understood in terms of the increasing atomic radius and associated decrease in ionization energy down the group.

As the valence configuration ns²np² suggests, the +4 oxidation state is dominant in the compounds of the elements. The major exception is lead, for which the most common oxidation state is +2, two less than the group maximum. The relative stability of the low oxidation state is an example of the inert-pair effect.

Properties of Group 14 Elements

Property	C	Si	Ge	Sn	Pb
Melting point/°C	3730 (sublimes)	1410	937	232	327
Atomic radius/pm	77	117	122	140	154
Ionic radius (M²⁺)/pm	—	—	73	93	119
Ionic radius (M⁴⁺)/pm	—	—	53	69	78
First ionization energy/kJ mol⁻¹	1090	786	762	707	716
Pauling electronegativity	2.5	1.9	2.0	1.9	2.3
Electron affinity/kJ mol⁻¹	154	134	116	107	35
E°(M⁴⁺,M²⁺)/V	—	—	—	+0.15	+1.46
E°(M²⁺,M)/V	—	—	—	−0.14	−0.13

The electronegativities of carbon and silicon are similar to that of hydrogen and they form many covalent hydrogen and alkyl compounds. Carbon and silicon are strong oxophiles and fluorophiles, having high affinities for hard anions O²⁻ and F⁻ respectively. Their oxophilic character is evident in the extensive series of oxoanions: carbonates and silicates. In contrast, Pb²⁺ forms more stable compounds with soft anions, such as I⁻ and S²⁻.

💎

Diamond

3D tetrahedral framework

Hardest natural substance, electrical insulator

✏️

Graphite

Stacked planar sheets

Soft, good conductor, slippery

⚽

Fullerene C₆₀

Soccer-ball structure

Discovered 1985, Nobel Prize 1996

🧪

Carbon Nanotubes

Rolled graphene sheets

High strength, unique electronic properties

Occurrence and Applications

Carbon: Found as diamond, graphite, and in coal/coke. Used as fuel, reducing agent, lubricant (graphite), and cutting tools (diamond).
Silicon: Makes up 26% of Earth's crust. Found in sand, quartz, feldspar, clays. Used in semiconductor devices, solar cells, and glass.
Germanium: Low abundance, found in germanite ore and zinc ores. First widely used transistor material.
Tin: Occurs as cassiterite (SnO₂). Used to plate steel (tin cans), in bronze alloys, and solder.
Lead: Occurs as galena (PbS). Used historically in plumbing (now restricted), ammunition, radiation shielding, and batteries.

14.2 Simple Compounds

Key Points: All the Group 14 elements form simple binary compounds with hydrogen, oxygen, the halogens, and nitrogen. Carbon and silicon also form carbides and silicides with metals.

All the Group 14 elements form tetravalent hydrides, EH₄. Carbon forms an enormous range of hydrocarbon compounds, while silicon forms silanes. The stability of long-chain catenated hydrocarbons is due to the high C−C and C−H bond enthalpies.

Selected Mean Bond Enthalpies (kJ mol⁻¹)

Bond	C	Si	Ge	Sn	Pb
E−H	412	318	288	250	<157
E−O	360	466	350	—	—
E=O	743	642	—	—	—
E−E	348	226	186	150	87
E=E	612	—	—	—	—
E≡E	837	—	—	—	—
E−F	486	584	466	—	—
E−Cl	322	390	344	320	301

The data illustrate how the E−E bond enthalpy decreases on descending the group. As a result, the tendency to catenation decreases from C to Pb. Silicon forms silanes up to Si₇H₁₆, but these are much more reactive than alkanes.

Example 14.2: Formation of Catenated Species

Using bond enthalpy data, calculate the standard enthalpy of formation of C₂H₆(g) and Si₂H₆(g):

Δ_fH°(C₂H₆,g) = [2(715) + 3(436)] − [348 + 6(412)] = −82 kJ mol⁻¹

Δ_fH°(Si₂H₆,g) = [2(439) + 3(436)] − [326 + 6(318)] = −48 kJ mol⁻¹

The more negative value for ethane is due to the greater C−H bond enthalpy compared to Si−H.

Oxides of Carbon

Carbon forms CO, CO₂, and the suboxide O=C=C=C=O. The bond in CO is short and strong (bond enthalpy 1076 kJ mol⁻¹), consistent with a triple bond :C≡O:. Carbon dioxide has double bonds and is the acid anhydride of carbonic acid.

Oxide	m.p./°C	b.p./°C	ν(CO)/cm⁻¹	k(CO)/N m⁻¹	Bond length (CO)/pm
CO	−199	−192	2145	1860	113
CO₂	sublimes	−78	2449, 1318	1550	116
OCCCO	−111	7	2290, 2200	—	128 (CC), 116 (CO)

14.3 Extended Silicon–Oxygen Compounds

Key Point: As well as forming simple binary compounds with oxygen, silicon forms a wide range of extended network solids that find a range of applications in industry.

Aluminosilicates

Aluminosilicates are formed when Al atoms replace some of the Si atoms in a silicate. They occur naturally as clays, minerals, and rocks. Because Al occurs as Al(III), its presence in place of Si(IV) in an aluminosilicate renders the overall charge more negative by one unit. An additional cation, such as H⁺, Na⁺, or ½Ca²⁺, is required for each Al atom that replaces a Si atom.

🏺

Kaolinite

Al₂(OH)₄Si₂O₅

China clay, used in ceramics and medicine

✨

Talc

Mg₃(OH)₂Si₄O₁₀

Slippery feel, neutral layers

📄

Muscovite Mica

KAl₂(OH)₂Si₃AlO₁₀

Charged layers, readily cleaves into sheets

🔷

Zeolites

Microporous frameworks

Molecular sieves, catalysts, ion exchangers

Zeolites and Molecular Sieves

The molecular sieves are crystalline microporous aluminosilicates having open structures with apertures of molecular dimensions. A subclass, the zeolites, have an aluminosilicate framework with cations trapped inside tunnels or cages.

Molecular Sieve	Composition	Bottleneck/pm	Properties
A	Na₁₂[(AlO₂)₁₂(SiO₂)₁₂]·xH₂O	400	Small molecules, hydrophilic
X	Na₈₆[(AlO₂)₈₆(SiO₂)₁₀₆]·xH₂O	800	Medium molecules, hydrophilic
ZSM-5	Na₃[(AlO₂)₃(SiO₂)₉₃]·xH₂O	550	Moderately hydrophilic
ALPO-5	AlPO₄·xH₂O	800	Moderately hydrophobic
Silicalite	SiO₂	600	Hydrophobic

Uses of Zeolites

Ion exchange: Water softeners in detergents
Absorption: Selective gas separation, gas chromatography
Solid acid catalysis: Cracking hydrocarbons for fuel
Shape-selective reactions: Alkylation and isomerization of aromatics

Part B: The Detail

14.5 Diamond and Graphite

Key Points: Diamond has a cubic structure. Graphite consists of stacked two-dimensional carbon sheets; oxidizing agents or reducing agents may be intercalated between these sheets with concomitant electron transfer.

Diamond Structure

Each C atom bonds to 4 neighbors at 154 pm in a tetrahedral arrangement. Density: 3.51 g cm⁻³

Graphite Structure

Planar sp² layers with 142 pm bonds. Layer separation: 335 pm. Density: 2.26 g cm⁻³

Diamond has the highest known thermal conductivity because its structure distributes thermal motion in three dimensions efficiently. The measurement of thermal conductivity is used to identify fake diamonds. Diamond is one of the most highly prized gemstones due to its durability, clarity, and high refractive index.

The electrical conductivity of graphite perpendicular to the planes is low (5 S cm⁻¹ at 25°C) indicating semiconductor behavior. Parallel to the planes, conductivity is much higher (30 kS cm⁻¹) indicating semimetal behavior.

📦 BOX 14.1: Synthetic Diamonds

Diamonds were first synthesized in 1955 using graphite and a d metal heated to 1500–2000 K at 7 GPa. The d metal (typically nickel) dissolves the graphite and the less soluble diamond phase crystallizes.

High-pressure synthesis: Uses metal catalyst at high T and P
Shock synthesis (Du Pont method): Explosive pressure at 30 GPa for milliseconds
Chemical vapor deposition: Low-pressure process using methane pyrolysis
SiC extraction: Under Cl₂ and H₂ at 1300 K

Graphite Intercalation Compounds

Graphite can act as either an electron donor or electron acceptor towards atoms that penetrate between its sheets. For example, K atoms reduce graphite by donating electrons to the π* band:

Graphite + K → KC₈ (potassium ions between layers)

The different stoichiometries are associated with staging, where metal ions insert between neighboring layers, every other layer, etc. Examples include KC₈, KC₁₆, and KC₃₆.

Graphite bisulfates form when graphite is heated with H₂SO₄/HNO₃ mixture. Electrons are removed from the π band and HSO₄⁻ ions penetrate between sheets. This oxidative intercalation leads to higher conductivity than pure graphite.

14.6 Other Forms of Carbon

(a) Carbon Clusters - Fullerenes

Key Point: Fullerenes are formed when an electric arc is discharged between carbon electrodes in an inert atmosphere.

The discovery of the soccer-ball-shaped C₆₀ cluster in the 1980s created great excitement. The 1996 Nobel Prize in Chemistry was awarded to Richard Smalley, Robert Curl, and Harold Kroto for this discovery. The molecule consists of five- and six-membered carbon rings with overall icosahedral symmetry.

Fullerenes can be reduced to form fulleride salts, C₆₀ⁿ⁻ (n = 1 to 12). The structure of K₃C₆₀ consists of a face-centered cubic array of C₆₀ ions with K⁺ in octahedral and tetrahedral sites. It is a metallic conductor at room temperature and a superconductor below 18 K.

Fullerene-Metal Complexes

C₆₀ undergoes five electrochemically reversible electron transfer steps in nonaqueous solvents. Electron-rich Pt(0) phosphine complexes attack C₆₀, with Pt spanning a pair of C atoms. The [Ru₃(CO)₉] cluster can cap a six-fold face of C₆₀.

Endohedral fullerenes (M@C₆₀) contain atoms inside the cage, such as H₃@C₆₀, La@C₈₂, and La₃@C₁₀₆.

(b) Carbon Nanotubes

Carbon nanotubes consist of one or more concentric cylindrical tubes formed by rolling graphene sheets. The ends are often capped by fullerene-like hemispheres.

🧵

Single-Walled (SWNT)

~1 nm diameter

Properties depend on how graphene is rolled

📜

Multi-Walled (MWNT)

Concentric tubes

"Russian-doll" or "parchment" model

🔗

Nanobuds

CNT + fullerene

Fullerene anchors on nanotube wall

Applications include hydrogen storage, catalysis, and high-strength materials like body armor.

(c) Graphene

📦 BOX 14.2: Graphene, the Wonder Material

Graphene is monolayer graphite: a single layer of hexagonally arranged carbon atoms. The 2010 Nobel Prize in Physics was awarded to Andre Geim and Konstantin Novoselov for groundbreaking experiments on graphene.

Strength: ~200× stronger than structural steel
Thermal conductivity: Highest recorded
Elasticity: Stretches up to 20%
Electrical conductivity: Very high (but no band gap)
Production methods: "Scotch tape" exfoliation, CVD, electric discharge

(d) Partially Crystalline Carbon

Key Points: Amorphous and partially crystalline carbon in the form of small particles are used as adsorbents and strengthening agents; carbon fibers impart strength to polymeric materials.

Carbon black: Finely divided carbon from hydrocarbon combustion. Used as pigment, in printer's ink, and as filler for rubber (tires).
Activated carbon: From controlled pyrolysis of organic material. High surface area (>1000 m² g⁻¹), excellent adsorbent.
Carbon fibers: From pyrolysis of asphalt or synthetic fibers. High tensile strength for tennis rackets, aircraft components.

14.7 Hydrides

(a) Hydrocarbons

Key Point: The stability of catenated hydrocarbons can be attributed to high C−C and C−H bond enthalpies.

Methane, CH₄, is the simplest hydrocarbon—an odourless, flammable gas found in large underground deposits as natural gas:

CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(g) Δ_combH° = +882 kJ mol⁻¹

Methane is not very reactive: it is not hydrolyzed by water and reacts with halogens only under UV radiation.

(b) Silanes

Key Points: Silane is a reducing agent; it forms Si(OR)₄ with alcohols and undergoes hydrosilylation with a platinum complex as catalyst.

Silane, SiH₄, is prepared commercially by reduction of SiO₂ with Al under high pressure of H₂:

6 H₂(g) + 3 SiO₂(g) + 4 Al(s) → 3 SiH₄(g) + 2 Al₂O₃(s)

Silanes are much more reactive than alkanes. SiH₄ is spontaneously flammable in air due to the large atomic radius of Si (open to nucleophilic attack), greater polarity of Si−H bond, and availability of low-lying d orbitals.

Hydrosilylation is the addition of SiH across multiple bonds of alkenes/alkynes:

CH₂=CH₂ + SiH₄ → CH₃CH₂SiH₃ (Pt catalyst)

📦 BOX 14.3: Methane Clathrates

Methane clathrates are crystalline solids formed at low temperatures when ice crystallizes around CH₄ molecules. 1 m³ of clathrate liberates up to 164 m³ of methane gas.

Found under ocean floor sediments and in permafrost
Oceanic reservoir estimated at 10,000+ Tt of carbon
Potential energy source but risk of greenhouse gas release

(c) Germane, Stannane, and Plumbane

Germane (GeH₄) and stannane (SnH₄) are synthesized by reaction of tetrachlorides with LiAlH₄. Plumbane (PbH₄) is extremely unstable. Stability varies: SiH₄ < GeH₄ > SnH₄ > PbH₄ (alternation effect). Alkyl/aryl groups stabilize the hydrides—trimethylplumbane survives for hours at room temperature.

14.8 Compounds with Halogens

(a) Halides of Carbon

Key Points: Nucleophiles displace halogens in carbon-halogen bonds; organometallic nucleophiles produce new M−C bonds; mixtures of polyhalocarbons and alkali metals are explosion hazards.

Carbon tetrahalides: CF₄ (colorless gas), CCl₄ (dense liquid), CBr₄ (pale yellow solid), CI₄ (red solid). Stability decreases from CF₄ to CI₄.

Property	CF₄	CCl₄	CBr₄	CI₄
Melting point/°C	−187	−23	90	171 dec
Boiling point/°C	−128	77	190 sub	—
Δ_fG°/kJ mol⁻¹	−879	−65	148	>0

All tetrahalomethanes are thermodynamically unstable with respect to hydrolysis, but the reaction for C−F bonds is very slow—fluorocarbon polymers like PTFE are highly resistant to water.

Phosgene (OCCl₂) Reactions:

CO + Cl₂ → OCCl₂
OCCl₂ + NH₃ → (H₂N)₂CO (urea)
OCCl₂ + ROH → (RO)₂CO (carbonates)
OCCl₂ + H₂O → CO₂ + 2 HCl

(b) Silicon and Germanium Halides

Key Point: Because silicon can form hypervalent intermediate states, substitution reactions of silicon halides occur more readily than those of carbon halides.

Silicon tetrachloride is prepared by direct reaction of elements or chlorination of silica with carbon. Silicon halides are mild Lewis acids:

SiF₄(g) + 2 F⁻(aq) → SiF₆²⁻

Hydrolysis of Si and Ge tetrahalides is fast via aqua complex intermediates, unlike kinetically resistant carbon tetrahalides.

(c) Tin and Lead Halides

Key Points: Tin forms dihalides and tetrahalides; for lead, only the dihalides are stable.

Sn(II) solutions are useful mild reducing agents:

Sn²⁺(aq) + ½O₂(g) + 2H⁺(aq) → Sn⁴⁺(aq) + H₂O(l) E° = +1.08 V

Lead tetrachloride is an unstable yellow oil (inert-pair effect) that decomposes to PbCl₂ + Cl₂. PbBr₄ and PbI₄ are unknown. The [SnCl₃]⁻ ion has a pyramidal structure due to a stereochemically active lone pair.

14.9–14.11 Oxides

Compounds of Carbon with Oxygen and Sulfur

Key Points: Carbon monoxide is a key reducing agent in iron production and a common ligand in d-metal chemistry; carbon dioxide is the acid anhydride of carbonic acid.

CO uses include reduction of metal oxides in blast furnaces and the water gas shift reaction for H₂ production:

CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g)

CO is an excellent ligand towards d-metal atoms in low oxidation states (its toxicity comes from binding to Fe in hemoglobin). CO₂ is a weak Lewis acid but at higher pH, OH⁻ coordinates to form HCO₃⁻. The enzyme carbonic anhydrase accelerates this reaction by 10⁹×.

The sulfur analogues CS and CS₂ are known; CS is unstable while CS₂ is endergic (Δ_fG° = +165 kJ mol⁻¹).

📦 BOX 14.4 & 14.5: Carbon Cycle and CO₂ Sequestration

The carbon cycle involves photosynthesis (reducing CO₂) and respiration/combustion (releasing CO₂). Over geological time, buried organic matter formed fossil fuels while O₂ accumulated in the atmosphere.

Carbon sequestration uses amine solutions to capture CO₂ from flue gases. The CO₂ is then compressed and stored underground. Current technology reduces power plant output by 25–40% and increases costs significantly.

14.10 Simple Compounds of Silicon with Oxygen

Key Point: The Si−O−Si link is present in silica, a wide range of metal silicate minerals, and silicone polymers.

Silicate structures are built from tetrahedral SiO₄ units. Charge balance: terminal O contributes −1, shared O contributes 0.

Orthosilicate: [SiO₄]⁴⁻ (no sharing)
Disilicate: [O₃SiOSiO₃]⁶⁻ (one shared O)
Chain metasilicate: [SiO₃²⁻]_n (two shared O per Si)
Cyclic metasilicate: [Si₆O₁₈]¹²⁻ in beryl

Silicate glasses form when melts cool without crystallizing. Adding basic oxides (Na₂O, CaO) converts Si−O−Si links into terminal SiO groups, lowering the softening temperature.

14.11 Oxides of Germanium, Tin, and Lead

Key Point: The +2 oxide becomes more stable going down the group from Ge to Pb.

GeO: Disproportionates to Ge + GeO₂
SnO: Blue-black form with stereochemically active lone pair; disproportionates to Sn + SnO₂ when heated
SnO₂: Rutile structure (cassiterite mineral)
PbO: Red and yellow forms, stable due to inert-pair effect
PbO₂: Maroon, rutile structure, oxidizing (reduced to stable Pb(II))
Pb₃O₄: "Red lead" - mixed valence Pb(II)/Pb(IV)

📦 BOX 14.7: The Lead-Acid Battery

In fully charged state: cathode = PbO₂, anode = Pb, electrolyte = dilute H₂SO₄

Cathode: PbO₂ + HSO₄⁻ + 3H⁺ + 2e⁻ → PbSO₄ + 2H₂O
Anode: Pb + SO₄²⁻ → PbSO₄ + 2e⁻

Voltage ~2V is remarkably high for aqueous system—success hinges on high overpotentials for water oxidation on PbO₂ and reduction on Pb.

14.12–14.14 Nitrogen Compounds, Carbides, and Silicides

14.12 Compounds with Nitrogen

Key Points: The cyanide ion, CN⁻, forms complexes with many d-metal ions; its coordination to active sites of enzymes like cytochrome c oxidase accounts for its high toxicity.

HCN is produced by catalytic partial oxidation of CH₄ and NH₃. It is highly volatile (b.p. 26°C) and highly poisonous. Unlike CO which targets hemoglobin, CN⁻ targets Fe in cytochrome c oxidase, causing rapid collapse of energy production.

Cyanogen (CN)₂ is a pseudohalogen that dissociates to ·CN radicals. CN⁻ is a pseudohalide ion.

Silicon nitride (Si₃N₄) is produced from Si + N₂ at high temperature. It is very hard and inert, used in high-temperature ceramics.

Trisilylamine (H₃Si)₃N has very low Lewis basicity and a planar structure (unlike trimethylamine), attributed to electronic effects from the more electropositive Si atoms.

14.13 Carbides

Binary compounds of carbon with metals/metalloids are classified as:

🧂

Saline Carbides

Groups 1, 2, Al

Ionic: intercalates (KC₈), dicarbides (CaC₂), methides (Be₂C)

⚙️

Metallic Carbides

d-block metals

Hard, metallic conductivity: WC, Fe₃C (cementite)

💎

Metalloid Carbides

B, Si

Hard covalent solids: B₄C, SiC (carborundum)

The dicarbide C₂²⁻ ion is triply bonded [C≡C]²⁻, isoelectronic with N₂. Calcium carbide reacts with water to produce acetylene:

CaC₂(s) + 2 H₂O(l) → Ca(OH)₂(s) + HC≡CH(g)

14.14 Silicides

Key Points: Silicon-metal compounds (silicides) contain isolated Si, tetrahedral Si₄ units, or hexagonal nets of Si atoms.

Ferrosilicon (Fe₃Si) plays an important role in steel manufacture. K₄Si₄ contains [Si₄]⁴⁻ tetrahedral clusters isoelectronic with P₄. Many f-block elements form MSi₂ with hexagonal AlB₂ structure.

14.16–14.17 Organosilicon and Organometallic Compounds

14.16 Organosilicon and Organogermanium Compounds

Key Points: Methylchlorosilanes are important starting materials for silicone polymers; properties depend on degree of cross-linking. Tetraalkyl and tetraaryl germanium(IV) compounds are chemically and thermally stable.

Silicon tetraalkyls and tetraaryls are monomeric with tetrahedral Si. The C−Si bond is strong and compounds are fairly stable. The unreactive Si(CH₃)₃ group is widely used in organic synthesis.

The Rochow process provides an industrial route to methylchlorosilanes:

n MeCl + Si → Me_nSiCl_4-n (Cu catalyst)

Hydrolysis of methylchlorosilanes produces silicones (polysiloxanes):

Me₃SiCl + H₂O → Me₃SiOH + HCl
2 Me₃SiOH → Me₃SiOSiMe₃ + H₂O

Most silicon polymers have a Si−O−Si backbone (reflecting strong Si−O bonds), whereas carbon polymers have C−C backbones (strong C−C bonds).

Silicone Applications

Liquids: Lubricants, hydraulic fluids (viscosity stable with temperature)
Gels: Personal care products (shampoos, conditioners)
Resins: Sealants, waterproofing, synthetic rubber
Other: Prosthetic implants, dry-cleaning fluids

Organogermanium(IV) compounds (R₄Ge) are tetrahedral and stable. Uses limited by high cost but include CVD precursors for GeO₂. Germylenes (R₂Ge) are stabilized by bulky R groups and insert into C−X and M−C bonds (analogous to carbenes).

14.17 Organometallic Compounds

Key Points: Tin and lead form tetravalent organo compounds; organotin compounds are used as fungicides and pesticides.

Organotin compounds have the widest range of uses of all main-group organometallics (>50 kt annual production):

PVC stabilization: Scavenge labile Cl⁻ to prevent degradation
Biocides: Fungicides, algaecides, wood preservatives
Antifouling: Preventing barnacle attachment (now restricted to vessels >25m)

R₄Sn compounds are tetrahedral monomers. Halide derivatives R₃SnX often contain Sn−X−Sn bridges forming chains. Bulky R groups favor monomeric structures.

Tetraethyl lead was formerly used as an antiknock agent in petrol but is now phased out due to environmental concerns. Alkyllead compounds (R₄Pb) can be made via Grignard reagents:

2 PbCl₂ + 4 RLi → R₄Pb + 4 LiCl + Pb

Halide derivatives like Pb(CH₃)₃Cl have chain structures with bridging Cl atoms; bulky substituents favor monomers.

Exercises and Problems

Exercise 14.1: Correct the Inaccuracies

Identify errors in these statements about Group 14 chemistry:

(a) None of the elements in this group is a metal. False: Sn and Pb are metals.
(b) At very high pressures, diamond is thermodynamically stable. True: Diamond is denser than graphite.
(c) CO₂ and CS₂ are weak Lewis acids with hardness increasing from CO₂ to CS₂. False: CS₂ is softer.
(d) Zeolites are layered materials exclusively composed of aluminosilicates. False: They are 3D frameworks and can include pure silica or AlPO₄.
(e) Reaction of calcium carbide with water yields ethyne reflecting the presence of highly basic C₂²⁻. True

Exercise 14.5: SiF₄ + (CH₃)₄NF

SiF₄ reacts with (CH₃)₄NF to form [(CH₃)₄N][SiF₅].

(a) Cation: Tetrahedral (CH₃)₄N⁺

(b) Anion: Trigonal bipyramidal SiF₅⁻

The ¹⁹F-NMR shows two environments because the trigonal bipyramid has 3 equatorial and 2 axial F atoms.

Example 14.1: Bonding in Diamond and Boron

Question: Each B atom in elemental boron is bonded to five other B atoms but each C atom in diamond is bonded to four nearest neighbors. Explain.

Answer: Both B and C have four orbitals (one s, three p). Carbon has four valence electrons—one for each orbital—so forms 2c,2e bonds with four neighbors. Boron has only three electrons, so uses 3c,2e bonds to utilize all four orbitals, bringing an additional B atom into bonding distance.

Example 14.4: Bond Order of C₂²⁻

Question: Predict the bond order of C₂²⁻.

Answer: Using the MO energy-level diagram with 10 electrons gives configuration 1σ_g² 1σ_u² 1π_u⁴ 2σ_g².

Bond order = ½(n − n*) = ½(8 − 2) = 3 (triple bond)

Self-test: If C₂²⁻ is oxidized to C₂⁻, bond length increases and strength decreases (bond order drops to 2.5).

Self-test 14.1: Describe how the electronic structure of graphite is altered when it reacts with (a) potassium, (b) bromine.

Self-test 14.2: Use bond enthalpy data to calculate the standard enthalpy of formation of CH₄ and SiH₄.