First-row d-block elements
13.1 First-row d-block elements (2 hours)
Pause for thought
Paramagnetism
It is good to see that the magnetic properties of transition metal complexes are now on the syllabus. However if you are not careful it could be a case of ' a little knowledge is a dangerous thing'. Paramagnetism is associated with unpaired electrons as a spinning unpaired electron creates a small magnetic field. This will line up in an applied electric or magnetic field to make the transition metal complex weakly magnetic when the field is applied, i.e. they reinforce the external magnetic field. The more unpaired electrons there are in the complex ion the stronger will be the paramagnetic effect. It is tempting to ask questions such as; "Will Fe2+ complexes be paramagnetic?"
The electron configuration of iron is 1s22s22p63s23p64s23d6 so, when the two 4s electrons have been lost to form the ion, Fe2+ will have the configuration [Ar]3d6. In an octahedral complex the five d orbitals are split with three of them going to lower energy and two to higher energy. Applying the Aufbau principle of filling the lower energy orbitals first the six electrons will form three pairs of electrons in the lower three 3d orbitals and so the complex ion will exhibit no paramagnetism, i.e. it will be diamagnetic. This seems straightforward logic but it ignores the effect of the ligands and in particular their position in the spectrochemical series. Ligands high in the series such as CN¯ cause large splitting (see diagrams where ΔE' > ΔE) and the complex [Fe(CN)6]4¯ is indeed diamagnetic as it has no unpaired electrons. However when water is the ligand the splitting is much less and now it is energetically favourable to apply Hund's rules and the complex ion [Fe(H2O)6]2+ has four unpaired electrons and is consequently paramagnetic.
High and low spin complexes are not specifically on the syllabus but it does mean that asking students to deduce whether a compound will be paramagnetic or not could cause unforeseen problems.
Nature of Science
Science looks for trends and the discrepancies within these trends. The behaviour of the transition elements follows certain patterns. The d-block elements zinc, chromium and copper do not always follow these patterns and so can be considered anomalous in the first-row d-block.
Learning outcomesAfter studying this topic students should be able to: Understand
Apply their knowledge to:
| Clarification notesCommon oxidation states of the transition metal ions are listed in Sections 9 and 14 of the data booklet. International-mindednessBecause of their properties and uses transition metals are important international commodities. For some countries, mining for precious metals is a major economic factor. |
Teaching tipsThere is quite a lot to cover in this sub-topic and not a lot of time (4 hours in total for Topics 13.1 and 13.2) so stick to the syllabus carefully and do not go into extra detail about individual transition/d-block elements/metals/complexes/compounds (note that they are called all of these in the syllabus!) unless you particularly want to. The questions students will be asked in the exams are very much on the general principles rather than on specific detailed chemistry of particular compounds. Much of the chemistry is related to the electron configuration (remember that the particular cases for chromium and copper should be known). After defining what is (and what is not) a transition element I focus in turn on the different properties they need to cover. Variable oxidation states are relatively easy and there is a good practical involving compounds of vanadium (although that is not one of the elements covered on the syllabus list but it also illustrates E For the complex ions I demonstrate to students the reaction of dilute copper(II) sulfate solution with first concentrated ammonia then concentrated hydrochloric acid and then go back and forth with water in between. I put the beaker on an old OHP and project the reaction onto a screen. There are a lot of fumes so it is spectacular but it illustrates equilibrium well in addition to the formation of different complex ions of copper. Give them practice at working out the total charge on an ion when different ligands are used. If I have a good class I do mention high and low crystal field splitting when I cover paramagnetism (see ‘Pause for thought’ above) but this is going beyond the syllabus. Although catalytic properties are mentioned the ‘Guidance’ does not provide any help as to what should be covered. It is probably enough to mention the difference between homogeneous and heterogeneous catalysis and briefly discuss the importance of variable oxidation states in the formation of intermediate compounds and the ability of transition metals to adsorb gases on their surface. Examples could be limited to iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in hydrogenation and the use of precious metals such as palladium or platinum in catalytic converters. | Study guide
Pages 20-21 QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC test: First-row d-block elements. For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see First row d-block elements questions.Vocabulary listFor short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers seevariable oxidation state (or number) IM, TOK, Utilization etc.See separate page which covers all of Topics 3 & 13 Practical work |
Teaching slides
Teachers may wish to share these slides with students for learning or for reviewing key concepts.
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
First row d-block elements:
Other resources
1. Paramagnetism and diamagnetism in general explained well by the Department of Physics and Astronomy, UCLA
Paramagnetism and diamagnetism
2. A video by Flinn Scientific showing the strong paramagnetism of the Mn2+ ion which contains five unpaired electrons.
Paramagnetic transition metal ions
3. A catalytic converter deconstructed from the Science Channel. An excellent video showing how a catalytic converter works. It stresses the importance of both surface area and temperature for the efficient functioning of the platinum, rhodium and palladium catalysts.