Transition Metals: Properties & Complex Ions (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about the properties of transition metals and the shapes and isomerism of their complex ions.

Everything distinctive about transition metals — their variable oxidation states, coloured complexes, and catalytic activity — comes from their incomplete d-subshell, and that same subshell is what lets them form complex ions with ligands, whose size and number set the complex's shape and even its isomerism.

A transition metal is a d-block element that forms at least one stable ion with a partially filled d-subshell. This is why scandium and zinc, whose ions have empty or full d-subshells, don't count, leaving titanium to copper as the first transition series. That incomplete d-subshell is behind everything characteristic about these metals: their variable oxidation states, their ability to form coloured complex ions, and their use as catalysts. A complex ion forms when ligands — molecules or ions with a lone pair to donate — bond to the central metal ion, and the size and number of those ligands determines both the shape of the complex and whether it can show isomerism.

This video covers what makes an element a transition metal first, then their key properties, then how ligands form complex ions, then the shapes those complexes take, and finishes with the isomerism some of them show.

A transition metal is a d-block element that forms at least one stable ion with an incomplete d-subshell. Scandium only forms Sc 3 plus, with an empty 3d subshell, and zinc only forms Zn 2 plus, with a full 3d10 subshell, so neither counts. The first transition series runs from titanium to copper. Two elements break the expected pattern of filling 4s before 3d: chromium is Ar in square brackets 3d5 4s1 rather than 3d4 4s2, and copper is Ar in square brackets 3d10 4s1 rather than 3d9 4s2, because a half-full or full d-subshell is more stable.

Transition metals can form more than one positive ion, giving them variable oxidation states — iron, for example, forms both Fe 2 plus and Fe 3 plus. That ability to adopt different oxidation states is also what lets them form complex ions and coloured compounds. The same metal in the same oxidation state can also give completely different colours depending on the ligand attached to it. Their variable oxidation states are also why transition metals make good catalysts, since the metal can gain or lose electrons to and from other species during the reaction.

A ligand is a molecule or ion that forms a dative covalent bond with a transition metal by donating a lone pair of electrons. Monodentate ligands form just one dative bond — water, ammonia and chloride ions are common examples. Bidentate ligands, like 1,2-diaminoethane and the ethanedioate ion, each form two dative bonds, while multidentate ligands can form several — EDTA 4 minus is hexadentate, forming all six dative bonds to the metal ion by itself.

The shape of a complex depends on the number of dative bonds and the size of the ligands involved. Six dative bonds to small ligands like water or ammonia gives an octahedral shape, with 90-degree bond angles. Four dative bonds usually gives a tetrahedral shape when the ligand is large, like chloride, but can give a square planar shape instead with smaller ligands like cyanide. Two dative bonds, as in the silver or copper(I) ion bonded to two ammonia molecules, gives a linear complex with a 180-degree bond angle.

Square planar with two pairs of different ligands or octahedral complexes with a two to four ratio of different ligands can show cis-trans isomerism, even without a double bond present. Cisplatin, the anti-cancer drug, is the cis isomer of a square planar complex, and it's only that cis form that has the beneficial medical effect. Octahedral complexes with bidentate ligands can also show optical isomerism, forming two non-superimposable mirror images that differ only in which way they rotate polarised light.

The word "dentate" comes from the French word for teeth, and tells you how many bonds — or "teeth" — the ligand bites onto the metal ion with: monodentate for one, bidentate for two, and multidentate for several.

Transition metals are defined by their incomplete d-subshell, which is behind their variable oxidation states, coloured complexes, and catalytic activity. That same incomplete d-subshell lets them form complex ions with ligands, and it's the size and number of those ligands that sets the complex's shape — octahedral, tetrahedral, square planar, or linear — and whether it can show cis-trans or optical isomerism.