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Transition metals definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

45 terms

Transition metal characteristics and complex formation

Transition metal: An element that forms at least one stable ion with an incomplete d subshell.
d-block element: An element whose highest-energy electron is in a d subshell; scandium to zinc in Period 4.
Ligand: A molecule or ion that forms a coordinate bond with a transition metal ion by donating a pair of electrons.
Coordinate bond: A covalent bond in which both electrons of the shared pair come from the same atom.
Complex: A central metal atom or ion surrounded by ligands bonded to it by coordinate bonds.
Coordination number: The number of coordinate bonds from the ligands to the central metal atom or ion.
Monodentate ligand: A ligand that forms one coordinate bond to a metal ion, such as water, ammonia or the chloride ion.
Bidentate ligand: A ligand that forms two coordinate bonds to the same metal ion, such as ethane-1,2-diamine or the ethanedioate ion.
Multidentate ligand: A ligand that forms more than two coordinate bonds to the same metal ion, such as EDTA⁴⁻, which forms six.

Octahedral complex: A complex with six coordinate bonds pointing to the corners of an octahedron, with bond angles of 90°.
Tetrahedral complex: A complex with four coordinate bonds pointing to the corners of a tetrahedron, with bond angles of 109.5°.
Square planar complex: A complex with four coordinate bonds pointing to the corners of a square round the metal, with bond angles of 90°.
Ligand substitution: A reaction in which one or more ligands in a complex are replaced by different ligands.
Stereoisomers: Compounds with the same structural formula but a different arrangement of their atoms in space.
Cis–trans isomers: Stereoisomers of a complex in which two identical ligands are adjacent, 90° apart, in the cis isomer and opposite, 180° apart, in the trans isomer.
Optical isomers: Stereoisomers that are non-superimposable mirror images of each other.
Chelate effect: The replacement of monodentate ligands by bidentate or multidentate ligands, favoured because the number of particles increases and so the entropy increases.
Stability constant: The equilibrium constant for the formation of a complex ion from the aqua ion in aqueous solution; the larger the value, the more stable the complex.

Complementary colours: Two colours of light that together make white light; a substance that absorbs one appears the other.
Transmitted light: The light that passes through a sample without being absorbed; its colour is the colour the sample appears.
d-orbital splitting: The separation of the five d orbitals of a metal ion into groups of different energy by the lone pairs of the ligands; the gap between the groups is ΔE.
Ground state: The lowest-energy arrangement of the electrons in an atom or ion.
Excited state: An arrangement of the electrons at higher energy than the ground state, reached when an electron absorbs energy and moves to a higher orbital.
Colorimeter: An instrument that measures the absorbance of a solution for light of a chosen colour, selected by a filter.
Absorbance: A measure of how much light of a given wavelength a sample absorbs, proportional to the concentration of the absorbing species.
Calibration curve: A graph of a measured quantity, here absorbance, against the known concentrations of standard solutions, used to find an unknown concentration.

Oxidation state: The charge an atom would have if all its bonds were ionic, used to follow the electrons in a redox reaction.
Oxidising agent: A species that accepts electrons from another species and is itself reduced.
Reducing agent: A species that donates electrons to another species and is itself oxidised.
E°cell: The EMF of a cell under standard conditions, equal to E° of the couple being reduced minus E° of the couple being oxidised; a positive value means the reaction is feasible.
End point: The point in a titration at which a colour change shows that the reaction is just complete.
Self-indicating titration: A titration in which a reactant changes colour at the end point, so no separate indicator is needed.
Concordant titres: Titres that agree within 0.10 cm³ of each other; their mean is used in the calculation.
Standard electrode potential: The EMF of a half-cell measured against the standard hydrogen electrode under standard conditions of 298 K, 100 kPa and 1.00 mol dm⁻³ solutions.
Amphoteric: Able to react both as an acid and as a base, as chromium(III) hydroxide does with acids and with excess sodium hydroxide.

Catalyst: A substance that increases the rate of a reaction by providing an alternative route with a lower activation energy, and is chemically unchanged at the end.
Heterogeneous catalyst: A catalyst in a different phase from the reactants.
Homogeneous catalyst: A catalyst in the same phase as the reactants.
Adsorption: The formation of weak bonds between reactant molecules and atoms on the surface of a solid.
Desorption: The breaking of the bonds between product molecules and a catalyst surface, which releases the products.
Active site: A place on the surface of a heterogeneous catalyst where reactant molecules are adsorbed and react.
Support medium: An unreactive material with a large surface area over which a thin layer of catalyst is spread, to increase the surface area for each gram of catalyst.
Catalyst poisoning: The loss of catalytic activity when an impurity is strongly adsorbed onto the active sites, blocking them to the reactants.
Intermediate: A species formed in one step of a reaction and used up in a later step, so it does not appear in the overall equation.
Autocatalysis: Catalysis of a reaction by one of its own products, so that the rate increases as the product forms.