In chemistry, the term transition metal (or transition element) has three possible meanings:
- The IUPAC definition defines a transition metal as "an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell".
- Many scientists describe a "transition metal" as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table.Petrucci, R.H.; Harwood, W.S. and Herring, F.G. (2002) General Chemistry, 8th ed, Prentice-Hall, pp. 341-2Housecroft, C.E. and Sharpe, A.G. (2005) Inorganic Chemistry, 2nd ed, Pearson Prentice-Hall, pp. 20-21 In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called "inner transition metals".
- Cotton and Wilkinson expand the brief IUPAC definition (see above) by specifying which elements are included. As well as the elements of groups 4 to 11, they add scandium and yttrium in group 3 which have a partially filled d subshell in the metallic state. These last two elements are included even though they do not (so far) seem to possess the catalytic properties which are so characteristic of the transition metals in general. Lanthanum and actinium in Group 3 are however classified as lanthanides and actinides respectively.
ClassificationIn the d-block the atoms of the elements have between 1 and 10 d electrons. The elements of groups 4-11 are generally recognized as transition metals, justified by their typical chemistry, i.e. a large range of complex ions in various oxidation states, coloured complexes, and catalytic properties either as the element or as ions (or both). Sc and Y in Group 3 are also generally recognized as transition metals. However, the elements La–Lu and Ac–Lr and Group 12 attract different definitions from different authors.
- Many chemistry textbooks and printed periodic tables classify La and Ac as Group 3 elements and transition metals, since their atomic ground-state configurations are s2d1 like Sc and Y. The elements Ce-Lu are considered as the " lanthanide" series (or "lanthanoid" according to IUPAC) and Th-Lr as the " actinide" series.Petrucci, R. H. et al. (2002), "General Chemistry", 8th edn, Prentice-Hall, pp. 49–50, 951Miessler, G. L. and Tarr, D. A. (1999) Inorganic Chemistry, 2nd edn, Prentice-Hall, p. 16 The two series together are classified as f-block elements, or (in older sources) as "inner transition elements".
- Some inorganic chemistry textbooks include La with the lanthanides and Ac with the actinides.Cotton, F.A. and Wilkinson, G. (1988) Inorganic Chemistry, 5th ed., Wiley , pp. 625–7 Housecroft, C. E. and Sharpe, A. G. (2005) Inorganic Chemistry, 2nd ed., Pearson Prentice-Hall, p. 741 This classification is based on similarities in chemical behaviour, and defines 15 elements in each of the two series even though they correspond to the filling of an f subshell which can only contain 14 electrons.
- A third classification defines the f-block elements as La-Yb and Ac-No, while placing Lu and Lr in Group 3. This is based on the Aufbau principle (or Madelung rule) for filling electron subshells, in which 4f is filled before 5d (and 5f before 6d), so that the f subshell is actually full at Yb (and No) while Lu (and Lr) has an s2f14d1 configuration. However La and Ac are exceptions to the Aufbau principle with electron configuration s2d1 (not s2f1 as the Aufbau principle predicts) so it is not clear from atomic electron configurations whether La or Lu (Ac or Lr) should be considered as transition metals.Scerri, E.R. (2011) A Very Short Introduction to the Periodic Table, Oxford University Press.
Electronic configurationThe general electronic configuration of the d-block elements is gas (n − 1)d1–10n s0–2. The period 6 and 7 transition metals also add (n − 2)f0–14 electrons, which are omitted from the tables below. The Madelung rule predicts that the typical electronic structure of transition metal atoms can be written as gasns2(n − 1)dm where the inner d orbital is predicted to be filled after the valence-shell s orbital. This rule is however only approximate – it only holds for some of the transition elements, and only then in their neutral ground state. The d-sub-shell is the next-to-last sub-shell and is denoted as -sub-shell. The number of s electrons in the outermost s sub-shell is generally one or two except palladium (Pd), with no electron in that s-sub shell in its ground state. The s-sub-shell in the valence shell is represented as the ns sub-shell, e.g. 4s. In the periodic table, the transition metals are present in eight groups (4 to 11), with some authors including some elements in groups 3 or 12. The elements in group 3 have an ns2(n − 1)d1 configuration. The first transition series is present in the 4th period, and starts after Ca (Z = 20) of group-2 with the configuration Ar4s2, or scandium (Sc), the first element of group 3 with atomic number Z = 21 and configuration Ar4s23d1, depending on the definition used. As we move from left to right, electrons are added to the same d-sub-shell till it is complete. The element of group 11 in the first transition series is copper (Cu) with an untypical configuration Ar4s13d10. Despite the filled d subshell in metallic copper it nevertheless forms a stable ion with an incomplete d subshell. Since the electrons added fill the orbitals, the properties of the d-block elements are quite different from those of s and p block elements in which the filling occurs either in s or in p-orbitals of the valence shell. The electronic configuration of the individual elements present in all the d-block series are given below: A careful look at the electronic configuration of the elements reveals that there are certain exceptions, for example Cr and Cu. These are either because of the symmetry or nuclear-electron and electron-electron force. The orbitals that are involved in the transition metals are very significant because they influence such properties as magnetic character, variable oxidation states, formation of colored compounds etc. The valence and orbitals have very little contribution in this regard since they hardly change in the moving from left to the right in a transition series. In transition metals, there is a greater horizontal similarities in the properties of the elements in a period in comparison to the periods in which the d-orbitals are not involved. This is because in a transition series, the valence shell electronic configuration of the elements do not change. However, there are some group similarities as well.
Characteristic propertiesThere are a number of properties shared by the transition elements that are not found in other elements, which results from the partially filled d shell. These include
- the formation of compounds whose colour is due to d–d electronic transitions
- the formation of compounds in many oxidation states, due to the relatively low energy gap between different possible oxidation states
- the formation of many paramagnetic compounds due to the presence of unpaired d electrons. A few compounds of main group elements are also paramagnetic (e.g. nitric oxide, oxygen)
Coloured compoundsColour in transition-series metal compounds is generally due to electronic transitions of two principal types.
- charge transfer transitions. An electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer (LMCT) transition. These can most easily occur when the metal is in a high oxidation state. For example, the colour of chromate, dichromate and permanganate ions is due to LMCT transitions. Another example is that mercuric iodide, HgI2, is red because of a LMCT transition.
- d-d transitions. An electron jumps from one d-orbital to another. In complexes of the transition metals the d orbitals do not all have the same energy. The pattern of splitting of the d orbitals can be calculated using crystal field theory. The extent of the splitting depends on the particular metal, its oxidation state and the nature of the ligands. The actual energy levels are shown on Tanabe-Sugano diagrams.
Oxidation statesA characteristic of transition metals is that they exhibit two or more oxidation states, usually differing by one. For example, compounds of vanadium are known in all oxidation states between −1, such as , and +5, such as . Main group elements in groups 13 to 18 also exhibit multiple oxidation states. The "common" oxidation states of these elements typically differ by two. For example, compounds of gallium in oxidation states +1 and +3 exist in which there is a single gallium atom. No compound of Ga(II) is known: any such compound would have an unpaired electron and would behave as a free radical and be destroyed rapidly. The only compounds in which gallium has a formal oxidation state of +2 are dimeric compounds, such as , which contain a Ga-Ga bond formed from the unpaired electron on each Ga atom. p. 240 Thus the main difference in oxidation states, between transition elements and other elements is that oxidation states are known in which there is a single atom of the element and one or more unpaired electrons. The maximum oxidation state in the first row transition metals is equal to the number of valence electrons from titanium (+4) up to manganese (+7), but decreases in the later elements. In the second row the maximum occurs with ruthenium (+8), and in the third row, the maximum occurs with iridium (+9). In compounds such as and the elements achieve a stable octet by forming four covalent bonds. The lowest oxidation states are exhibited in metal carbonyl complexes such as (oxidation state zero) and (oxidation state −2) in which the 18-electron rule is obeyed. These complexes are also covalent. Ionic compounds are mostly formed with oxidation states +2 and +3. In aqueous solution the ions are hydrated by (usually) six water molecules arranged octahedrally.
MagnetismTransition metal compounds are paramagnetic when they have one or more unpaired d electrons. In octahedral complexes with between four and seven d electrons both high spin and low spin states are possible. Tetrahedral transition metal complexes such as are high spin because the crystal field splitting is small so that the energy to be gained by virtue of the electrons being in lower energy orbitals is always less than the energy needed to pair up the spins. Some compounds are diamagnetic. These include octahedral, low-spin, d6 and square-planar d8 complexes. In these cases, crystal field splitting is such that all the electrons are paired up. Ferromagnetism occurs when individual atoms are paramagnetic and the spin vectors are aligned parallel to each other in a crystalline material. Metallic iron and the alloy alnico are examples of ferromagnetic materials involving transition metals. Anti-ferromagnetism is another example of a magnetic property arising from a particular alignment of individual spins in the solid state.
Catalytic propertiesThe transition metals and their compounds are known for their homogeneous and heterogeneous catalytic activity. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes. Vanadium(V) oxide (in the contact process), finely divided iron (in the Haber process), and nickel (in catalytic hydrogenation) are some of the examples. Catalysts at a solid surface ( nanomaterial-based catalysts) involve the formation of bonds between reactant molecules and atoms of the surface of the catalyst (first row transition metals utilize 3d and 4s electrons for bonding). This has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowered). Also because the transition metal ions can change their oxidation states, they become more effective as catalysts.
Physical propertiesAs implied by the name, all transition metals are metals and thus conductors of electricity. In general, transition metals possess a high density and high melting points and boiling points. These properties are due to metallic bonding by delocalized d electrons, leading to cohesion which increases with the number of shared electrons. However the group 12 metals have much lower melting and boiling points since their full d subshells prevent d–d bonding, which again tends to differentiate them from the accepted transition metals. Mercury has a melting point of and is a liquid at room temperature.
- Inner transition element, a name given to any member of the f-block
- Main group element, an element other than a transition metal
- Ligand field theory a development of crystal field theory taking covalency into account
- Crystal field theory a model that describes the breaking of degeneracies of electronic orbital states
- Post-transition metal, a metallic element to the right of the transition metals in the periodic table