Geoffrey Pass

The elements of the first transition series

The reactions described below are designed to illustrate some of the chemistry of the elements of the first transition series. The majority of the reactions selected can be carried out on a test tube scale. The relative stabilities of the different oxidation states for a given element in aqueous solution open to the atmosphere are demonstrated and the changes in the stability of the oxidation states for each element. The reactions of a number of the elements with a given anion are included to illustrate differences which occur in the products of the reaction. Reactions in which complex ions are produced are included, but the use of complexes to stabilize the less common oxidation states is not described at this stage (see page 71). Reactions which can be considered as characteristic of a given metal ion are also included.

The stabilization of oxidation states

When an element can exist in more than one oxidation state in aqueous solution each oxidation state will have a different thermodynamic stability. The relative stability of two oxidation states in aqueous solution is most conveniently expressed in terms of the electrode potential for the reaction .

High vacuum techniques in chemistry

The work described in this chapter is designed to introduce students to the use, advantages, and limitations of high vacuum techniques in chemistry.

Coordination chemistry II: stereochemistry

The study of isomerism in coordination compounds is of considerable interest since by determining the number and type of isomers for a variety of different compounds Werner was able to make his classic deductions regarding the shapes of such complexes. Subsequent work has shown Werner’s conclusions to be essentially correct.

Coordination chemistry I: typical compounds

The reaction between two or more independently stable molecules to give a stable product with its own characteristic properties has been known for many years. The ammine complexes formed between ammonia and cobalt(II) chloride are typical examples. In many instances the complex does not give reactions in solution characteristic of the uncomplexed metal ion or ligand. However, thermodynamic and kinetic stabilities vary so widely that this is not a general criterion of the formation of a coordination compound. The development of the ideas as to the nature of these compounds should be known to the student.

Inert atmosphere techniques

The preparation and manipulation of certain inorganic compounds requires the use of an inert atmosphere. Inert often implies that the atmosphere is free from water and oxygen. The reactions are carried out in a dry nitrogen atmosphere. In other cases nitrogen must also be excluded and then an argon atmosphere is used. Such inert atmospheres are conveniently maintained in a glove box. Glove boxes are available in a variety of shapes and sizes, constructed from a variety of materials. They all possess the following basic features: (1) A gas-tight box in which the experimental work is performed. (2) A smaller gas-tight box fitted to the side of the main unit, which functions as an air-lock. All materials are introduced into the main unit through this lock. (3) A transparent window in the front of the box through which the manipulations may be observed. (4) A pair of arm-length rubber gloves sealed to two ports in the front of the box. (5) A variety of tubes sealed through the wall of the main box, to supply standard services, e.g. water, gas.

Oxo-acids and oxo-acid salts

The oxo-acids and oxo-acid salts of boron and the elements of groups IV–VII of the periodic table, from the second short period onwards, contain anions which can be considered as based on MO x units, where M is the group element and x can have values from 1–6.

High temperature reactions

The general type of reaction considered here involves heating an element or compound in a gas stream. The solid material, in a suitably inert container, is placed in a reaction tube which is heated by a tube furnace, and a measured flow of gas is passed through the tube. A gas absorption system may be incorporated on the inlet side of the reaction tube in which undesirable impurities in the gas stream can be removed by absorption in suitable solutions. A smaller absorption system may also be placed on the exit side of the tube to prevent any back absorption of air. This system also provides a useful visual check on the flow of gas from the reaction tube. It is also advisable to have a mercury-sealed lute on the inlet side of the reaction tube to release any pressure build-up should the gas flow be blocked in any way. This will eliminate the possibility of sealed joints being forced open, with subsequent spraying out of the contents of the Drechsel bottles, see Figure 4, p. 34.

Chemistry in non-aqueous solvents

Many chemical reactions require a suitable solvent. Water is the solvent of choice in inorganic chemistry, although in organic chemistry other solvents are more often used.

The preparation of some manganese compounds

A characteristic property of the transition elements is the ability of an element to exist in a number of different oxidation states. Manganese has oxidation states from — III to +VII. The lower oxidation states are only formed with π-bonding molecules (see p. 73). For the change from one oxidation state to another, the oxidation potentials when available, give some information as to the type of oxidizing or reducing agent which will bring about the required reaction. By suitable adjustment of reaction conditions a transition element in one oxidation state may be converted to a different oxidation state.