Everything from the 7 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A transition element is a d-block element that forms one or more stable ions with an incomplete d subshell. Scandium (only Sc3+, 3d0) and zinc (only Zn2+, 3d10) are d-block elements but not transition elements. Across Ti to Cu, electrons enter the inner 3d subshell, so the extra nuclear charge is largely shielded: atomic radii and first ionisation energies stay very similar. When ions form, the 4s electrons are lost first. Chromium (3d54s1) and copper (3d104s1) are exceptions to the filling order. Transition elements show variable oxidation states, act as catalysts, form coloured compounds and form complex ions.
When forming ions, remove the 4s electrons first: Fe2+ is [Ar]3d6, not [Ar]3d44s2. Chromium is 3d54s1 and copper 3d104s1. Zinc is in the d block but is not a transition element, because Zn2+ has a full 3d subshell; scandium is excluded because Sc3+ has no d electrons.
A complex is a central metal atom or ion surrounded by ligands. A ligand is a species with at least one lone pair that forms a dative covalent (coordinate) bond to the metal. Monodentate ligands (H2O, NH3, Cl−, CN−, OH−) form one bond; bidentate ligands (1,2-diaminoethane, en, and ethanedioate, C2O42−) form two; polydentate EDTA4− forms six. The coordination number is the number of dative bonds to the metal. Coordination number 6 gives an octahedral complex, 4 gives tetrahedral (large ligands such as Cl−) or square planar (Pt2+, as in cisplatin), and 2 gives linear, as in [Ag(NH3)2]+. The overall charge is the metal's oxidation state plus the charges of the ligands.
The coordination number counts dative bonds, not ligands: [Cu(en)2(H2O)2]2+ is 6-coordinate. Neutral ligands (H2O, NH3, en) add nothing to the charge, so [Co(NH3)6]3+ contains Co(+3). Four ligands can mean tetrahedral or square planar; Pt(II) complexes are square planar.
In ligand exchange one ligand in a complex is replaced by another. Pale blue [Cu(H2O)6]2+ reacts with excess aqueous ammonia to give deep blue [Cu(NH3)4(H2O)2]2+; NH3 and H2O are similar in size, so the coordination number stays 6. With concentrated HCl it gives yellow-green [CuCl4]2−: the large Cl− ions change the coordination number to 4 and the shape to tetrahedral. Pink [Co(H2O)6]2+ similarly gives blue [CoCl4]2−. Adding a little NaOH(aq) or NH3(aq) removes H+ from water ligands and precipitates the neutral hydroxide, e.g. blue Cu(OH)2(H2O)4; some precipitates redissolve in excess reagent.
A little ammonia acts as a base and gives the hydroxide precipitate; only excess ammonia acts as a ligand and gives the deep blue solution. Concentrated HCl changes the coordination number from 6 to 4 because Cl− is large. [CuCl4]2− has a 2− charge (+2 + 4 × −1).
Transition elements have variable oxidation states because the 3d and 4s electrons are close in energy. Acidified manganate(VII), MnO4−, is reduced to Mn2+ (purple to colourless) and is its own indicator: the end point is the first permanent pale pink. It reacts with Fe2+ in a 1 : 5 ratio and with ethanedioate in a 2 : 5 ratio. Acidified dichromate(VI), Cr2O72−, is reduced to Cr3+ (orange to green) and reacts with Fe2+ in a 1 : 6 ratio. E⊖ values predict feasibility: a reaction is feasible if E⊖cell is positive. Cu2+ oxidises I− to I2, which is titrated with thiosulfate: overall n(Cu2+) = n(S2O32−).
Get the ratios right: MnO4− : Fe2+ = 1 : 5, MnO4− : C2O42− = 2 : 5, Cr2O72− : Fe2+ = 1 : 6, Cu2+ : S2O32− = 1 : 1. Acidify manganate(VII) with sulfuric acid, never hydrochloric. The manganate(VII) end point is the first permanent pale pink, not colourless.
In a free ion the five 3d orbitals are degenerate (equal in energy). Ligand lone pairs repel the d electrons unequally, so the orbitals become non-degenerate: in an octahedral complex they split into three lower and two higher orbitals; in a tetrahedral complex into two lower and three higher, with a smaller gap. When white light passes through, an electron is promoted from a lower to a higher d orbital, absorbing a photon of energy ΔE = hf. The colour seen is the complementary colour of the light absorbed. Ions with an empty (d0) or full (d10) d subshell cannot make this transition, so they are colourless. A different ligand, oxidation state or coordination number changes ΔE and so changes the colour.
The colour you see is not the colour absorbed: a blue solution absorbs red-orange light. Octahedral splitting is 3 lower, 2 upper; tetrahedral is the reverse, 2 lower, 3 upper, with a smaller ΔE. Electrons are promoted within the 3d subshell, not from 3d to 4s. Colour needs a partly filled d subshell, so Zn2+ and Sc3+ compounds are white.
Stereoisomers have the same structural formula but a different arrangement of atoms in space. Cis-trans isomerism occurs in square planar complexes of the type MA2B2, such as Pt(NH3)2Cl2, and in octahedral complexes of the type MA4B2, such as [Co(NH3)4Cl2]+: in the cis isomer the two identical ligands are at 90°, in the trans isomer at 180°. Optical isomerism occurs when a complex is chiral: its mirror image is non-superimposable. Octahedral complexes with three bidentate ligands, such as [Ni(en)3]2+, or two bidentate ligands in a cis arrangement, exist as a pair of enantiomers. Cisplatin is an anticancer drug: it loses its Cl− ligands and binds to nitrogen atoms on DNA, stopping cancer cells from dividing.
Cis-trans isomerism needs square planar or octahedral geometry; a tetrahedral MA2B2 complex has no geometrical isomers. Cis means the identical ligands are at 90°, trans at 180°. In [Co(en)2Cl2]+ only the cis isomer is chiral, giving three stereoisomers in total. It is cisplatin that is the drug.
The stability constant, Kstab, is the equilibrium constant for the formation of a complex ion in a solvent from its constituent ions or molecules. For Cu2+(aq) + 4NH3(aq) ⇌ [Cu(NH3)4(H2O)2]2+(aq), Kstab = [complex] / ([Cu2+][NH3]4), units mol−4 dm12. Water is left out because it is the solvent and its concentration is effectively constant. Values are huge, so they are often quoted as log Kstab. A ligand exchange happens if the new complex has a larger Kstab: the larger the value, the more stable the complex. Multidentate ligands such as EDTA4− form very stable complexes (the chelate effect) because one ligand releases several water molecules, so the number of particles and the entropy increase.
Leave water out of Kstab expressions. Units depend on the number of ligands: four monodentate ligands give mol−4 dm12, but one EDTA4− gives mol−1 dm3. A larger Kstab means a more stable complex, and ligand exchange goes towards it. The chelate effect is mainly an entropy effect, not a stronger bond.