Transition Elements notes

MDCAT Chemistry

This chapter covers the electronic structure, oxidation states, magnetic behaviour, colour and complex formation of transition elements. It also includes important properties of d-block and f-block elements, coordination compounds, selected industrial uses and optical isomerism.

Position and General Electronic Structure of d-Block Elements

The d-block elements are placed between the s-block and p-block elements in the periodic table. They are found in Groups 3 to 12. In these elements, the last electron enters a d-subshell of the penultimate shell.

The general electronic configuration is (n−1)d1 to 10 ns0 to 2. The 3d series extends from Sc to Zn, the 4d series from Y to Cd, and the 5d series from Hf to Hg. The 4f series occurs between La and Hf, while the 5f series occurs between Ac and Rf.

Sc is the first element of the 3d series.
The general configuration of d-block elements is (n−1)d1 to 10 ns0 to 2.
The 3d series contains Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.
The 4d series begins with Y and the 5d series begins with Hf in the usual modern arrangement.
The d-subshell contains five orbitals and can hold a maximum of 10 electrons.
In a free atom, 4s orbitals are filled before 3d orbitals, but 4s electrons are removed first during ion formation.

Transition Elements and Their Electronic Configurations

A transition element is an element that forms at least one stable ion with a partially filled d-subshell. This definition explains why some d-block elements are not true transition elements.

Scandium forms Sc3+, whose 3d subshell is empty, but Sc is commonly included at the beginning of the transition series. Zinc, cadmium and mercury usually form ions with completely filled d10 subshells, so they are non-typical transition elements.

A transition element forms at least one stable ion with a partially filled d-subshell.
Sc: [Ar] 3d1 4s2.
Cr: [Ar] 3d5 4s1, not [Ar] 3d4 4s2.
Cu: [Ar] 3d10 4s1, not [Ar] 3d9 4s2.
Cr has five unpaired d electrons in its atom.
Cu has a completely filled 3d10 subshell, so all its d electrons are paired.
Zn: [Ar] 3d10 4s2 and Zn2+: [Ar] 3d10.
Cr and Cu provide the pair in which one has all unpaired d orbitals and the other has all paired d orbitals.

Oxidation States of d-Block Elements

Transition elements show variable oxidation states because the energies of ns and (n−1)d electrons are similar. Both types of electrons can participate in bonding. The number of oxidation states generally increases towards the middle of a transition series and then decreases.

The +3 oxidation state is characteristic of the Group 3 elements. Scandium readily forms Sc3+ and its chloride is ionic. Zinc does not show a stable variable oxidation state in the same way as typical transition elements.

Scandium commonly shows the +3 oxidation state.
All the elements of Group IIIB show the +3 oxidation state in the usual FSc classification.
Sc forms ionic ScCl3 and readily forms Sc3+.
Manganese shows oxidation states from +2 to +7 in different compounds.
The highest oxidation state of manganese is +7, as in MnO4−.
Zn mainly shows the +2 oxidation state and is a non-typical transition element.
Variable oxidation states result from the comparable energies of ns and (n−1)d electrons.
The oxidation state of a metal in a complex is calculated by using the charge of ligands and the charge of the complex ion.

Magnetic Properties and Unpaired Electrons

The magnetic behaviour of a transition metal or ion depends on the number of unpaired electrons. A substance with one or more unpaired electrons is paramagnetic. A substance with all electrons paired is diamagnetic.

The d orbitals are singly occupied according to Hund's rule before pairing begins. The number of unpaired electrons may change when a metal ion forms or when ligands cause pairing in a complex.

Paramagnetic substances contain one or more unpaired electrons.
Diamagnetic substances contain only paired electrons.
Cr atom has the configuration 3d5 4s1 and has six unpaired electrons in total, with five in the d-subshell.
Cu atom has 3d10 4s1; its d electrons are all paired.
Fe3+ has a 3d5 configuration and has five unpaired electrons when no pairing occurs.
MnO4− contains Mn(VII), which is d0 and has no d electrons.
The number of unpaired electrons can be used to compare the magnetic behaviour of transition metal ions.
A weak-field ligand may leave electrons unpaired, while a strong-field ligand may cause pairing.

Colour of Transition Metal Ions

Many transition metal ions and compounds are coloured because electrons absorb visible light and move between split d orbitals. This is called a d to d transition. The colour seen is complementary to the colour absorbed.

Ions with a d0 or d10 configuration generally do not show d to d colour. Their compounds may still be coloured for other reasons, such as charge transfer, but the usual d-block colour explanation is absent.

The colour of many transition metal ions is due to electrons in d orbitals.
The electronic transition responsible is usually from one d orbital to another d orbital.
Cu2+ salt solutions are commonly blue because of d to d electronic transitions.
MnO4− is coloured mainly because of charge-transfer absorption, not a d to d transition, since Mn(VII) is d0.
d0 ions, such as Ti4+ and Mn7+, have no d electrons for a d to d transition.
d10 ions, such as Zn2+ and Cu+, have completely filled d orbitals and generally do not show d to d colour.
The colour depends on the metal ion, oxidation state, ligand type and arrangement of ligands.

Coordination Compounds and Ligands

A coordination compound contains a central metal atom or ion surrounded by ions or molecules called ligands. A ligand donates a lone pair of electrons to the central metal ion. The bond formed is a coordinate covalent bond.

The part inside square brackets is the coordination sphere. The species outside the brackets are counter ions. Coordination number is the number of donor atoms directly attached to the central metal ion, not necessarily the number of ligand molecules.

The bond between a transition metal and a ligand is coordinate covalent.
A ligand donates a lone pair of electrons to the central metal ion.
In [Cu(NH3)4]SO4, the coordination sphere is [Cu(NH3)4]2+.
The coordination number of Cu in [Cu(NH3)4]SO4 is four.
The coordination number of Co in [Co(NH3)6]3+ is six.
NH3, H2O, Cl−, CN− and F− can act as ligands.
A monodentate ligand attaches through one donor atom.
A bidentate ligand attaches through two donor atoms.
The central metal and its attached ligands lie inside the square brackets.

Complex Ions, Naming and Stability

A complex ion may be positively charged, negatively charged or neutral. If the complex ion is negative, it is called a complex anion. The oxidation state of the central metal is written in Roman numerals when a complex is named.

Complexes containing multidentate ligands are often more stable because one ligand forms several bonds with the same metal ion. This is called the chelate effect. Oxalate ion, C2O4 2−, is a bidentate ligand.

K2[Cu(CN)4] contains the complex anion [Cu(CN)4]2−.
In K2[Cu(CN)4], the central atom Cu is present in the anionic sphere.
K2PtCl6 contains the complex anion [PtCl6]2−.
In [Cu(NH3)4]SO4, SO4 2− is outside the coordination sphere.
[Co(NH3)6]3+ has six NH3 ligands and coordination number six.
The most stable oxidation state of copper for complex formation in the given comparison is Cu2+.
[Pt(C2O4)2]2− is more stable than a comparable complex with only monodentate ligands because oxalate is bidentate.
A complex anion has a negative charge on the whole coordination sphere.

Inner Orbital and Outer Orbital Complexes

In valence bond theory, complexes are described as inner orbital or outer orbital complexes. Inner orbital complexes use inner d orbitals and usually involve pairing of d electrons. Outer orbital complexes use outer d orbitals and generally do not require pairing of the original d electrons.

The nature of the ligand affects pairing. Strong-field ligands tend to cause pairing, while weak-field ligands usually do not. Fluoride ion is a weak-field ligand in the standard FSc treatment.

An outer orbital complex uses outer d orbitals in hybridisation.
[FeF6]3− is hexafluoroferrate(III).
Fe3+ has the configuration 3d5.
F− is a weak-field ligand, so [FeF6]3− is treated as an outer orbital complex.
[FeF6]3− has five unpaired electrons.
The coordination number of Fe in [FeF6]3− is six.
Strong-field and weak-field ligands can produce different numbers of unpaired electrons in complexes.
The number of unpaired electrons determines whether the complex is paramagnetic or diamagnetic.

Important Elements, f-Block and Selected Uses

The f-block elements are placed separately at the bottom of the periodic table. Their differentiating electron enters an f-subshell. The lanthanides involve the 4f subshell and the actinides involve the 5f subshell.

Some transition and related elements have important applications. Silver bromide is light-sensitive and is used in photographic plates. Fluorine is present in the polymer Teflon, whose repeating unit is based on fluorinated carbon chains.

Cerium has atomic number 58 and belongs to the f-block.
The lanthanides are associated with filling of the 4f subshell.
The actinides are associated with filling of the 5f subshell.
Photographic plates are coated with a thin film of AgBr, silver bromide.
Teflon contains fluorine in its repeating units.
The halogen that reacts spontaneously with gold to form gold fluoride is fluorine, F2.
Open hearth furnaces are associated in the given FSc treatment with the production of wrought iron.
La is identified in the group comparison as the element without unpaired valence electrons in the d-subshell.

Isomerism in Coordination Compounds

Coordination compounds may show different types of isomerism. Geometrical isomerism occurs when the same ligands occupy different relative positions around the metal ion. Cis means that similar groups are adjacent, while trans means that they are opposite.

Optical isomers are non-superimposable mirror images called enantiomers. A chiral complex has no plane of symmetry or other suitable symmetry element that makes it superimposable on its mirror image.

In an octahedral complex of the type M(aa)2b2, aa is a bidentate ligand.
The trans form of M(aa)2b2 is achiral and optically inactive.
The cis form of M(aa)2b2 is chiral.
The cis form exists as two enantiomeric forms.
Enantiomers have identical physical properties in an achiral medium but rotate plane-polarised light in opposite directions.
Cis and trans forms have the same molecular formula but different spatial arrangements.
Geometrical isomerism is caused by different positions of ligands around the central metal.

Key terms

d-block element: An element in which the differentiating electron enters a d-subshell of the penultimate shell.
Transition element: An element that forms at least one stable ion with a partially filled d-subshell.
d-subshell: A subshell containing five orbitals that can hold a maximum of 10 electrons.
Variable oxidation state: The ability of an element to form ions or compounds in more than one oxidation state.
Paramagnetic: A property of substances containing one or more unpaired electrons.
Diamagnetic: A property of substances whose electrons are all paired.
Ligand: An ion or molecule that donates a lone pair of electrons to a central metal ion.
Coordination sphere: The central metal ion and its directly attached ligands written inside square brackets.
Coordination number: The number of donor atoms directly bonded to the central metal ion.
Coordinate covalent bond: A covalent bond in which both bonding electrons are donated by one atom or ion.
Complex ion: A charged species containing a central metal ion surrounded by ligands.
Complex anion: A negatively charged complex ion, such as [PtCl6]2−.
Chelate: A complex formed when a multidentate ligand attaches to a metal ion through two or more donor atoms.
Inner orbital complex: A complex that uses inner d orbitals in its hybridisation.
Outer orbital complex: A complex that uses outer d orbitals in its hybridisation.
d to d transition: The absorption of energy that promotes an electron from one split d orbital to another.
Chirality: The property of a structure that cannot be superimposed on its mirror image.
Enantiomers: A pair of non-superimposable mirror-image forms of a chiral compound.

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