Cobalt complexes

Examine 4 complexes, each containing cobalt, and allow learners to devise structures for each of them. 

Introduction

Once the students have assimilated this problem, they will probably need further information – see suggested approaches.

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Transition metals and their complexes – which contain covalently bonded ligands. The problem is open-ended as far as properties are concerned and students are asked to write down what they know or what they can find in textbooks or other sources of information. If the students have not met the difference between chlorine bonded covalently in the complex and the ionic chloride outside the complex, and the fact that silver nitrate reacts with only the ionic chloride, you may wish to point this out or direct them toward an explanation in a textbook. A detailed knowledge of the concepts are unnecessary as students are encouraged to consult textbooks and data books during the exercise.

Resources

Inorganic textbooks and perhaps data books should be available for reference.

Possible strategies

Point out that a useful strategy is to start by examining the two extremes (compounds one and four) and fit in the intermediate compounds thereafter. In particular, you could feed in the following information about compounds one and four. Some explanation may be required but you can test whether the students understand the situation by letting them apply the information to compounds two and three.

• Co(NH₃)₃Cl₃ was written [Co(NH₃)₃Cl₃] in the textbook;
• Co(NH₃)₄Cl₃ was written [Co(NH₃)₄Cl₂]Cl in the textbook;
• Co(NH₃)₅Cl₃ was written [Co(NH₃)₅Cl]Cl₂ in the textbook; and
• Co(NH₃)₆Cl₃ was written [Co(NH₃)₆]Cl₃ in the textbook.

Suggested approach

During trialling the following instructions were given to students and proved to be extremely effective:

1. Working as a group, discuss the problem and how you are going to solve it. You can divide the work amongst you if you wish but keep each other informed of your progress.
2. Discussion can play a vital part in working out possible solutions to open-ended problems like this. Several minds working on a problem together can stimulate ideas that one on its own could not manage. 
3. Write a brief account of your ideas.
4. Working as a group, prepare a short (ca 5-minute maximum) presentation to give to the rest of the class. If possible all group members should take part: any method of presentation (such as a blackboard, overhead projector, etc) can be used.

Outline the problem, describe what you did and explain your predictions. After the presentation, be prepared to accept and answer questions and to discuss what you did with the rest of the class.