Systematic Inorganic Chemistry

Systematic Inorganic Chemistry

Group IVA:   C, Si, Ge, Sn and Pb

The elements in group IVA; carbon-C, silicon-Si, germanium-Ge, tin-Sn and lead-Pb. All members of group IVA exhibit an oxidation state of +4, but the +2 oxidation state increases in stability down the group.The Ge(II) and Sn(II) states are well established, and Pb(II) is more stable than Pb(IV) state. The elements possess the outer-shell electron configuration of ns2np2.The electronegativities of the elements are generally low  Formation of 4+ ions by electron loss is not observed for any of these elements; the ionization energies are too high

 Group IVA: General Properties

	Property
	Atomic radius (Å)	1st Ionization energy (kJ/mol)	X-X single bond (kJ/mol)
C	0.77	1086	346
Si	1.17	786	226
Ge	1.22	762	186
Sn	1.40	709	151
Pb	1.46	716	-

C – strictly non-metallic Ge – metalloid

 Si – Essentially non-metallic Sn & Pb – metallic 

 Carbon differs from the other group 4A elements in its pronounced ability to form multiple bonds both with itself and with other non-metals, especially N, O, and S.the strength of a bond between two atoms of a given element decreases as we go down the group.Carbon-carbon bonds are quite strong. As a consequence, carbon has a striking ability to form compounds in which carbon atoms are bonded to one another.This property permits the formation of extended chains and rings of carbon atoms and accounts for the large number of organic compounds that exist. CARBON: Natural isotopes: 12C = 98.89%   13C = 1.11%

Radioisotope  14C

C4+ does not exist in any normal chemical process.

C4- may possibly exist in some carbides of the most electropositive metals (eg beryllium carbide, Be2C, and aluminium carbide, Al4C3) in which the high charge density cations Be2+ and Al3+ can form stable lattices with such highly charged anion.

C ® Is22s2 2p2 ® Is2 2s1 2px1 2py1 2pz1 (sp3 hybridization)

In general carbon forms covalent compounds

Group IVA: Occurrence Carbon is the only one to occur in the elemental state as DIAMOND and GRAPHITE.Silicon – 2nd  most abundant element in the earth’s crust forming about 27.7%.  (second to oxygen).

   Silicates are present in rocks.

   Common Sand = impure form of silica.

   Glass = mixture of silicates.

Germanium is a rare element. It occurs in traces in coal, in rare mineral  argyrodite, 4Ag2S.GeS2, in germinate, Cu3(Ge.Fe)S4, and as a mixture in zinc and tin ores.Tin occurs mainly as cassiterite or tin stone,SnO2.Lead as galena, PbS

Group IVA: General Properties

Coordination number:Maximum coordination number of carbon = 4. Because it can never accommodate more than 8 electrons in its valence shell.Other members:

  Can expand their octet due to the accessibilty of d- orbitals. Thus, they can have higher coordination numbers and  form complex ions e.g., SiF62-, PbCl42- etc

Catenation - Carbon

Formation of chains or rings of carbon atoms, not only with single but also with multiple bonds; C-C-C, -C=C-C, -CºC- Conditions for catenation:

1. An element must have a valence of at least two.

2. Must form strong bonds to itself.

3. A kinetic inertness of the catenated compounds towards other molecules or ions

NOTE: Carbon has the highest tendency to catenate.

Si & S ® Next to carbon in catenation but far inferior to it. Thermal stability of –C-C-C– chains is due to the intrinsic strength of the C-C bond.C-C bond is stable towards oxidation due to comparable energy between C-C and C-O bondsTherefore given the necessary activation energy Si-Si bonds will be converted to Si-O bonds ,S-S bonds will also be converted to S-O bonds

C-C = 346 kJ/mol Si-Si = 226 kJ/mol S-S = 226 kJ/mol

C-O = 336 kJ/mol Si-O = 368 kJ/mol S-O = 330 kJ/mol

Some Group Trends

1. Catenation – Important feature of the Group.

Extensive chains occur in Si & Ge hydrides, up to Si6H14 and Ge9H20, Silicon halides and in Ge2Cl6.For Sn & Pb catenation occurs only in organo-compounds.There is a smooth decrease in the tendency to catenate – the order is C>> Si>Ge=Sn>>Pb

This may be ascribed partly due to the diminishing strength of the M-M bonds (M = C, Si, Ge, Sn & Pb) 

2. Down the group there is a steady decrease in the M-C and M-H bond energies.

3. Strength of covalent bonds (M-X) with other atoms decrease from Si to Pb

NOTE:

Inspite of high Si-Cl or Si-F bond energies, compounds containing these bonds are highly reactive.

Example:Si-Cl bonds are much more reactive than Si-C bonds because, though stronger, they are much polar [Sid+-Cld-] rendering the silicon more susceptible to attack by a nucleophile such as OH-.

Allotropes of Carbon Allotropy occurs because of different crystal structures and shapes in the solid state and/or combination of different numbers of atoms into molecules.Carbon has several allotropes including Diamond, Graphite and Fullerenes

Diamond and Graphite

Diamond and Graphite are allotropes of Carbon ie. Different forms of the same element

They differ in their physical and chemical properties due to difference in the arrangement and bonding of the atoms.

Diamond:One of the hardest substances known:Denser than graphite [D = 3.51 gcm-3; G = 2.22gcm-3]At 300 K and 1 Atm Graphite is the more stable allotrope. 

Diamond structureEach carbon atom is tetrahedrally surrounded by four other carbon atoms at a distance of 1.54Å covalently bonded to it.

  Each C atom is sp3 hybridized.The structure possesses no free electrons.Behaves as an insulator and is inert.The rigid, strong three dimensional linkages make diamond one of the hardest substances known.

Graphite:

The structure of graphite is quite different from that of diamond Amorphous carbons such as charcoal and soot are very tiny particles of graphite ,Graphite consists of layers of carbon atoms Within the layers, covalent bonds hold the carbon atoms in six-membered rings.The distance between the carbon layers is very large (335 pm or 3.35 Å) Hence the attraction between layers is very weak .The layered structure of graphite accounts for its ability to conduct electricity,,Observed softness and lubricity is attributed to the easy slippage of these layers over one another 

STRUCTURE OF DIAMOND AND GRAPHITE

Note: common structure of graphite = ABAB stacking

 = Hexagonal form = MOST STABLE  Every second layer is superimposable

Rhombohedral form – stacking order = ABCABC

  ie. Every third layer is superimposable.

Successful conversion of GRAPHITE ® DIAMOND

  at 3000 K and pressures above 125 kbar.To obtain useful rates of conversion, a Transition metal

   catalyst [Cr, Fe or Pt] is used.

NOTE;40% of world’s supply of Industrial quality diamonds are synthetic ones.Chemical reactivity of Diamond is much lower than that of graphite due to the separation of the carbon sheets in graphite

Some reactions of Graphite.

1. Mellitic acid (C6(CO2H)6: Obtained by treating graphite with hot concentrated HNO3

2. Graphite Oxide: Graphite reacts with a suspension of KClO4 in a 1:2 mixture (by volume) of conc. HNO3/H2SO4 to give graphite oxide.

This is an unstable pale lemon coloured product of variable stoichiometry and structure. It decomposes slowly at 70 oC

3. Graphite monofluoride (CFx): Graphite reacts with an atmosphere of fluorine at 400-500oC to give graphite monofluoride CFx (x = appr. 0.68-0.99).

  The reaction is catalysed by HF and can then occur at much lower temperatures.

  At ca. 600oC the reaction proceeds with explosive violence to give a mixture of CF4, C2F6, and C5F12.

 

  The colour of CFx depends on the reaction temp. and on the fluorine content (ie. Black (x ≈ 0.7), Grey ( x ≈ 0.8), silver (x ≈ 0.9) and transparent white (x > 0.98)). The structure has not been definitely established.

4.  INTERCALATION COMPOUNDS OF GRAPHITE

Intercalation is the reversible inclusion of a molecule (or group) between two other molecules (or groups). Example, graphite intercalation compounds.

Graphite may undergo reactions in which the carbon layers move further apart and molecules or ions are accumulated between them; the products of such reactions are known as intercalation or lamellar compounds.The reactions are often reversible and the graphite nature of the host lattice is retained.Graphite, which has good electrical conductivity ®the conductivity remains and is sometimes enhanced

How are intercalation compounds of graphite formed

Formed by insertion of various atoms, molecules or ions between the layers of graphite. Eg. K, Na, F-, Br-, FeCl3, BrF3, TiF4, etc.eg. C8K is formed by direct interaction of graphite with K vapour at 300oC. Rb and Cs behave similarly. In this compound, the graphite layers remain intact.

Note: The electrical resistance of graphite intercalation compounds is lower than for graphite itself. In contrast to graphite, which is diamagnetic, the compounds have a temperature-independent paramagnetism.The graphite intercalation compounds formed by the halides of different elements (ie. HF, ClF3, BrF3, XeF6, TiF4, FeCl3, etc) are prepared by heating a mixture of the reactants. 

INORGANIC COMPOUNDS OF CARBON

1. CARBIDES:These are solid binary compounds in which carbon combines with elements of similar or lower electronegativity, especially metals.

Obtained by direct interaction (or with metal oxides) at high temp.

  2Al2O3 + 6C ® Al4C3 + 3CO2

Carbides of the most electropositive elements such as the alkali and alkaline earth metals which mostly contain dicarbide ion C22- (called acetylide) exhibit ionic bonding. They react with water to produce ethyne, C2H2;

  Na2 C2 + 2 H2O  →  2NaOH + C2H2

  CaC2 + 2H2O  →  Ca(OH)2 + C2H2

However, the beryllium carbide (Be2C) and aluminium carbide (Al4C3) which contain C4- ion react with water to produce methane, CH4;

  Al4C3 + 12H2O → 4Al(OH)3 + 3CH4   

 Whereas silicon and boron carbides (SiC & B4C) exhibit covalent bonding. This is due to the electronegativity differences, whereas most of nonmetals are more electronegative than carbon.

2. OXIDES:

Four binary oxygen compounds of carbon are known; CO, CO2, C3O2, C12O9: 

 i. Carbon monoxide (CO) is formed when elemental carbon burns in a deficiency of oxygen.

ii. Carbon dioxide (CO2) is formed when C burns in an excess of oxygen.

iii. Carbon suboxide (C3O2) formed by dehydration of malonic acid H2C(CO2H)2.

iv. Dehydration of benzenehexacarboxylic acid (mellitic acid) C6(CO2H)6 gives a carbon oxide (C12O9).

3. CARBON HALIDES:

  All contain the carbon atom tetrahedrally coordinatined to the four halogen atoms

  CF4 – Extraordinary stable compound.

  It is the end product in the fluorination of any carbon containing compound.

  CCl4  = Common solvent.

  CBr4 and CI4 are also known.

  There is steady decrease in the C–X bond energies i.e., C-F>C-Cl>C-Br>C-I. This is because of size factor.

4.  Carbon also forms compds with C-N bond, [eg. HCN, (CN)2] and C-S bond, [eg. CS2).

1. Hydrides, MH4 :  M = Si ®Pb are covalent.

The stability of the hydrides decreases down the group because of the decreasing M–H bond energies.PbH4 has not been properly characterised .

SILANES : compounds with Si-H bonds.

Silanes, SinH2n+2 are chains of up to n = 8 and as cyclic compounds SinH2n (n = 5, 6)

Thermal stability decreases with increasing chain length and only SiH4 (monosilane) and Si2H6 are indefinitely stable at 25oC. Higher silanes decompose. Above 500⁰C all silanes decompose to Si and H2

The silanes react vigorously with oxygen and spontaneously ignite or explode in air.

Silanes do not react with pure water. However, In the presence of base, Si-H bonds are rapidly hydrolyzed;

  SiH4 + (n +1)H2O → SiO2.nH2O + 2H2

GERMANES:

Germanes are less flammable than silanes, although still rapidly oxidized in air.

Resistant to hydrolysis:

GeH4 is unaffected by 30% NaOH.

Stannanes are also known [SnH4 (stannane) is the lowest member].

2. Halides MX4: M = Si ®Pb.

  X = F, Cl.  Are the most important.

SiF4  only partially hydrolyzed by water

  2SiF4 + 4H2O ® SiO2 + 2(H3O)+ + [SiF6]2- + 2HF

Whereas SiCl4 is rapidly hydrolyzed by water;

  SiCl4 + 2H2O ® SiO2 + 4HCl

3. Oxygen compounds.

  SiO2, GeO2, SnO2 and PbO2 are known.

  SiO2 = purely acidic, GeO2 = less acidic

  SnO2 = amphoteric, PbO2 = somewhat more basic.

 

ie Basicity increases from SiO2 ® PbO2.

SiO2 – Unreactive towards Cl2, H2, acids and most metals at 25oC, but is attacked by F2, aqueous HF, alkali hydroxides and fused carbonates

Silicon dioxide (SiO2) 

Silicon dioxide, SiO2, is commonly known as silica.

Forms a giant or macromolecular (three dimensional)  structure in which each silicon atom is surrounded tetrahedrally by four covalently bonded oxygen atoms.

Each oxygen is bonded to two silicon atoms.

However, the overall stoichiometry or the silicon – oxygen ratio remains the same, i.e. SiO2. 

Note; silicon dioxide and carbon dioxide share the same type of formula, yet their properties are very different;CO2 is a colourless gas at RT whereas SiO2 is a solid which melts at 1600⁰C.C-O bond is much weaker than Si-OTheir differences can be explained by their chemical structures; CO2 consists of  individual molecules with double bonds between the oxygen atoms and the carbon, O=C=O so as to satisfy the valences of both C and O atoms WhereasSiO2 is a giant covalent structure consisting of an extended network of Si and O atoms, single bonded to achieve their valences

4. Some anionic species: MF62-, M = Si, Ge, Sn, Pb.

  ie. SiF62-, GeF62-, SnF62- are known.

Silicon only forms fluoro anions, normally SiF62-. It is very stable and accounts for the incomplete hydrolysis of SF4 in water.

2SiF4 + 2H2O ® SiO2 + SiF62- +2H+ + 2HF

THE DIVALENT STATE:

Si – divalent Si ® Thermodynamically unstable. SiF2, SiO, SiS, SiH2 & SiCl2 have been identified in high temperature reactions.Ge forms stable dihalides; GeF2, GeCl2 and GeBr2.

  Disproportionate on heating.

  2GeX2 ® GeX4 + Ge

Pb – only Pb has a well defined cationic chemistry.

 REFERENCES

Bailar , J. C. , Emeleus , H. J. , Nyholm , R. , and Trotman-Dickinson , A. F. ( 1973 ). Comprehensive Inorganic Chemistry ,

Vol. 1 . Pergamon Press , Oxford . The chemistry of metals is extensively covered in this fi ve-volume set .

Burdett , J. K. ( 1995 ). Chemical Bonding in Solids . Oxford University Press , New York . An advanced book that treats

many of the aspects of structure and bonding in solids .

Cotton, F. A., Wilkinson, G., Murillo, C. A., and Bochmann, M. (1999). Advanced Inorganic Chemistry , 6th ed. This

book is the yardstick by which other books that cover the chemistry of the elements are measured. Several

chapters present detailed coverage of the chemistry of metals.

Everest , D. A. ( 1964 ). The Chemistry of Beryllium . Elsevier , Amsterdam . A general survey of the chemistry, properties,

and uses of beryllium .

Flinn , R. A. , and Trojan , P. K. ( 1981 ). Engineering Materials and Their Applications , 2nd ed. Houghton Miffl in ,

Boston . Chapters 2, 5, and 6. This book presents an excellent discussion of the structures of metals and the

properties of alloys .

Greenwood , N. N. , and Earnshaw , A. ( 1997 ). Chemistry of the Elements . Butterworth-Heinemann , Oxford . Chapters

20–29. This book may well contain more descriptive chemistry than any other single volume, and it contains

extensive coverage of transition metal chemistry .

Jolly , W. L. ( 1972 ). Metal-Ammonia Solutions . Dowden, Hutchinson & Ross , Stroudsburg, PA . A collection of

research papers that serves as a valuable resource on all phases of the physical and chemical characteristics of

these systems that involve solutions of group IA and IIA metals .