Chemistry 242 - Inorganic Chemistry II
Chapter 5 - The Chemistry of Selected Anions

Introduction

Chapter 4 of the text covered compounds which were simple ionic lattices: the cations and anions were derived from single atoms for example Al2O3 containing Al3+ and O2- ions. Many ionic compounds are up of a lattice of cations and anions where one or both are complex, that is made up of a number of covalently bound atoms.

Chapter 6 (which was partly covered in Chem 241 and will be more completely covered in Chem 341 deals with complex cations. The subject is called coordination chemistry. A coordination complex consists of a neutral metal atom or ion, usually a cation, bonded to a set of ligands which are neutral molecules or anions containing atoms with lone pairs.

This part of the course is concerned with various aspects of the chemistry of anions which are classified as follows in the chapter:

  1. Simple anions such as O2-, F- or CN-
  2. Discrete oxo anions such as NO3- or SO42-
  3. Polymeric oxo anions including the condensed silicates, phosphates and borates
  4. Complex halide anions such as TaF6- and other anionic ligands such as [Co(C2O4)3]3-
In each case we are concerned with the same kinds of thing:

A few types of anion are not covered: hydrides (containing H-), anionic metal hydrides (e.g. BH4-) and carbanions (e.g. C5H5-). They will be covered in later sections.

The Oxide, Hydroxide and Alkoxide Ions

Oxide

There are may solid metal oxides containing discrete O2- ions. Remember, the formation of this ion is endothermic and only the lattice energy makes its existence feasible. The oxide ion is unstable in solution. The oxides of the most electropositive metals, for example sodium or calcium oxide will react with water:
O2-(s)   +   H2O      2OH-(aq)
Others which do not react directly with water will react with acids, for example magnesium oxide:
MgO(s)   +   2H+(aq)      Mg2+(aq)   +   H2O
The product is not always "as expected" because some metal ions can also be reactive with respect to redox behaviour:
MnO2(s)   +   2H+(aq)   +   2Cl-(aq)      Mn2+(aq)   +   2H2O   +   Cl2(g)
The metal oxides, which are basic as described above, are to be contrasted with more covalent oxides, which react with water as acid anhydrides to give acids:
SO3(s)   +   H2O      H2SO4(aq)
or they dissolve in bases to give an anion of an acid:
Sb2O5(s)   +   2OH-(aq)   +   5H2O      2Sb(OH)6-(aq)
Some oxides are amphoteric such as zinc or aluminium oxide:
ZnO(s)   +   2H+(aq)      Zn2+(aq)   +   H2O
ZnO(s)   +   2OH-(aq)   +   H2O      Zn(OH)42-(aq)
Some oxides form several oxides, for example, CrO, Cr2O3 and CrO2. In such cases the oxide with the highest oxidation state of the other element will be the most acidic anyhydride:
CrO3(s)   +   H2O      H2CrO4
CrO is basic and produces an unstable hydroxide, Cr(OH)2. Cr2O3, the "normal" oxide, is amphoteric if it is prepared in a hydrated form; see below.

Finally, some oxides are inert to both acids and bases, for example, N2O or CO:

Hydroxides

While hydroxide is a common ligand (see below) discrete OH- exist only in combination with the most electroposive elements in the solid state e.g. NaOH. Such hydroxides are strong bases in water yielding a slovated metal ion and the OH-(aq) ion.

With more electropositive elements, the M-O bond becomes more covalent and the compounds are acids: It is difficult to come up with simple examples, but (HO)2SO2 and (HO)3PO are strong acids producing H+(aq) with water.

Amphoteric hydroxides lie between thse extremes:

MOH   +   H+(aq)      M+(aq)   +   H2O
MOH   +   OH-(aq)      MO-(aq)   +   H2O
Related to the above reactions is the acid dissociation of metal aquo complexes:
[M(H2O)x]n+      [M(H2O)x-1(OH)](n-1)+   +   H+(aq)
The higher the oxidation state of the metal and the smaller the ion, the more the equilibrium lies to the right.

In such dissociated complexes, hydroxide is acting as a ligand. It can also bridge two or three metal centres using two or three lone pairs on oxygen. All of this behaviour is displayed in the structures of the:

Hydrous Oxides

Some metals ions such as Fe3+, Cr3+ and Al3+ behave in a rather complicated way when their aqueous solutions are gradually made basic:
[Fe(H2O)6]3+   
pH < 0

[Fe(H2O)5(OH)]2+   
0 < pH < 2

[(H2O)4Fe(OH)2Fe(H2O)4]4+   
~2 < pH < ~3

colloidal Fe2O3.xH2O   
~3 < pH < ~5

precipitated Fe2O3.yH2O
~5 < pH

The last two vaguely specified formulae could be intepreted as more extended structures like the dimer. They could be written: {Fe2(OH)6.zH2O}n where z = x or y -3, but there are probably genuine oxide bridges present too.

As mentionned above, Cr2O3.nH2O is amphoteric:

Cr2O3.xH2O   +   acid      [Cr(H2O)6]3+
Cr2O3.xH2O   +   base      [CrO2]yn- (chromite ions)

Alkoxides

Alkoxide (RO-) is very analogous to hydroxide except that the ion is a stronger base than OH- and is therefore undergoes hydrolysis immediately in water to give ROH and OH-. Many metal alkoxides are know with frequent occurence of double or triple brides.

Mononuclear Oxo Anions

Oxo Anions of Carbon

The ions carbonate (CO32-) and bicarbonate (HCO3-, also called hydrogen carbonate) exist as ionic crystalline compounds with discrete ions, and can exist in neutral or alkaline solution. Carbonate can function as a monodentate, bidentate or bridging ligand also (e.g. [Co(NH3)5CO3]+ and [Co(NH3)4CO3]+).

The carbonates are mostly insoluble, except those of the alkali metals and ammonium. Some are important minerals such as limestone, CaCO3. They tend to be basic in solution by hydrolysis and the formation of bicarbonate:

CO32-(aq)   +   H2O      HCO3-(aq)   +   OH-(aq)
It is difficult to precipitate pure carbonates because of this hydrolysis and contamination by hydroxide ion for which many transition metal have a great affinity.

Oxalate (C2O42-) and carboxylate (RCO2-) are most commonly found as ligands. Oxalate is usually a chelating ligand and produces insoluble compounds in many cases. Carboxylate can be a monodentate, bidentate or bridging ligand (M2(RCO2)4 where M = Cu(II), Cr(II), Mo(II) or Rh(II), for example).

Oxo Anions of Nitrogen

Discrete nitrite ion (NO2-) is found only in the compounds of the most electropositve elements e.g. NaNO2. As a monodentate ligand, it can bind through oxygen (nitrito) or through nitrogen (nitro) e.g. [Co(NH3)5(ONO)]Cl2 and [Co(NH3)5(NO2)]Cl2 and also chelate via both oxygens, and bridge via one oxygen, or an oxygen and nitrogen.

Nitrates (NO3-) frequently contain the discrete nitrate ion, they are often hydrated and mostly soluble in water. Alkali metal nitrates give nitrites on heating. Other metals give an oxide plus nitrogen oxides and water.

Nitrate is not a very good ligand compared to water. It will complex highly charged metal ions as a monodentate ligand, as a chelating ligand and bridging two metals via two oxygens.

Oxo Anions of Phosphorus

The most important ones are derived from orthophosphoric acid, (OH)3PO, and contain PO43-, (OH)PO32- and (HO)2PO- Important compounds include ammonium phosphate, a fertilizer, and various phosphate buffers. Phosphates are also very prominent in biological situations. Phosphate is not a very good or important ligand although complexes are known. Note that while arsenates resemble phosphates, antimony(V) tends to be six-coordinate e.g. K[Sb(OH)6] in compounds called stibates.

Oxo Anions of Sulphur

The important sulphur(IV) ones are (pyramidal) sulphite, SO32-, and bisulphite, HOSO3-. Sulphur(VI) leads to the (tetrahedral) sulphate, SO42-, and bisulphate, HOSO3-, ions. Coordination compounds of sulphite are more common than those of sulphate, and neither are especially "important". A variety of coordination modes are possible, and examples are known.

Selenates are like sulphates, but tellurates tend to be six-coordinate e.g. K[TeO(OH)6] and derived from telluric acid, which is best written Te(OH)6.

Oxo Anions of the Halogens

Perchlorate, ClO4-, is perhaps the most important of the oxo halagen anions. Its compounds tend to be soluble with a few useful exceptions: ions such as Ce4+, Zr4+, Hf4+ and Th4+ can be separated from other metals by taking advantage of the insolubility of their perchlorates. Perchlorate is often used to precipitate larger complex cations, but it should be kept well away from organometallics, for example Fe(C5H5)+, because of the danger of explosions. It is a very poor ligand which can also be a useful property.

Perbromate was unknown until relatively recently, which is supposed another example of the effect of the d-orbital contraction. Periodate is known as IO4- but also in the six-coordinate forms, IO2(OH)4- and IO3(OH)32- following the trend noted above for groups 15 and 16.

Chlorates, bromates and iodates, XO3-, which are pyramidal are known mainly as their alkali metal salts. They are not very good ligands. The oxoanions in lower oxidation states become weaker acids and stronger oxidizing agents.

Oxo Anions of Transition Metals

One of the reasons that the original periodic tables had subgroups labelled A and B was the perceived similarity between such species as vanadate, VO43-, and phosphate, PO43-; chromate, CrO42-, and sulphate, SO42-, and permanganate, MnO4-, and perchlorate, ClO4-. Compounds of these ions are known with alkali metals. The ions are not known for their ligand properties.

Polynuclear Oxo Anions

These anions have at leat two "central" atoms and bridging oxygen(s). The text example is dichromate, Cr2O72- with a terminal Cr-O bond length of 1.61 Å, a bridging Cr-O bond length of 1.77 Å, and a Cr-O-Cr angle of 123o. In general the central atom polyhedra can share corners (one bridging oxygen or edges (two bridging oxygens). The following anion type illustrate some of these. The angle at oxygen can vary upto 180o.

Silicates and Borates

The silicates are based SiO4 tetrahedra while the borates are based on BO3 trigonal planar units. Remember that there will be one negative charge for each terminal oxygen atom in the formulae.

Silicates can involve:

The three-dimensional structure which involves all the oxygen atoms in bridging is the mineral silica which is not ionic, however, it is possible to replace some of the Si by Al- and various counter cations. The resulting minerals are called aluminosilicates, including natural ones e.g. felspar and the important synthetic class called zeolites which are characterized by various sized cavities. The composition of these compounds is of the type Mx/n[(AlO2)x(SiO2)y].zH2O where the cations are Mn+ (usually Na+, K+ or Ca2+).

Zeolites find use as: