Substitution in Octahedral Complexes: Dissociative or Associative

September 15, 2026

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Mechanism Substitution in Octahedral Complexes: Dissociative or Associative Square planar substitution is associative; octahedral substitution is usually dissociative. The reason is simply how crowded the metal already is. BSc & MSc · Inorganic Chemistry · Concept The short answer: An octahedral complex is sterically crowded, so an incoming ligand cannot easily approach. Substitution therefore usually proceeds by a ligand leaving first, giving a five-coordinate intermediate. The evidence is that rates depend strongly on the leaving group and only weakly on the entering one. The two limiting mechanisms Dissociative Associative First step A ligand leaves, giving five-coordinate A…

Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra

September 13, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Spectra Orgel and Tanabe–Sugano Diagrams: Interpreting d–d Spectra Two diagrams for the same purpose, differing in whether they can handle a spin state change. BSc & MSc · Inorganic Chemistry · Concept The short answer: Both plot the energies of electronic terms against ligand field strength. Orgel diagrams cover weak-field high-spin complexes only. Tanabe–Sugano diagrams cover the whole range including the high-spin to low-spin crossover, and take the ground state as the horizontal axis so transition energies are read directly. What the diagrams show A free ion’s electronic states are described by term symbols. Placing that ion…

The Jahn–Teller Effect: Why Some Complexes Distort

September 11, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Structure The Jahn–Teller Effect: Why Some Complexes Distort A degenerate electronic state is unstable against distortion. Knowing which configurations are degenerate tells you immediately which complexes distort and how strongly. BSc & MSc · Inorganic Chemistry · Concept The short answer: A non-linear molecule in a degenerate electronic state will distort to remove that degeneracy, because doing so lowers the energy. In octahedral complexes the effect is strong for unevenly occupied eg orbitals and weak for uneven t2g occupation, because eg orbitals point directly at ligands. The statement Any non-linear molecule in a degenerate electronic state is…

Stability Constants and the Chelate Effect

September 5, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Coordination Stability Constants and the Chelate Effect Why a ligand that bites twice binds far more tightly than two ligands that bite once — and why the answer is entropy, not bond strength. BSc & MSc · Inorganic Chemistry · Concept The short answer: Complex formation proceeds stepwise, each step with its own constant, and the overall constant is their product. Chelating ligands give far larger overall constants than comparable monodentate ligands. The dominant reason is entropic: one chelate molecule replaces several monodentate ones, increasing the number of free particles. Stepwise and overall constants Ligands add one…

Electron Transfer: Inner Sphere versus Outer Sphere

September 5, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Mechanism Electron Transfer: Inner Sphere versus Outer Sphere Two mechanisms distinguished by one question — does a ligand bridge the two metals during transfer, or not? BSc & MSc · Inorganic Chemistry · Concept The short answer: In outer sphere transfer the coordination shells stay intact and the electron tunnels between them. In inner sphere transfer a bridging ligand connects the two metals and the electron passes through it. The classic evidence is ligand transfer: if the bridging ligand ends up on the other metal, the mechanism was inner sphere. The two mechanisms Outer sphere Inner sphere…

The Trans Effect: Predicting the Product of Square Planar Substitution

September 4, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Reaction Mechanism The Trans Effect: Predicting the Product of Square Planar Substitution A synthesis question in coordination chemistry usually reduces to one thing — knowing which ligand directs the incoming group to the position opposite itself. BSc & MSc · Inorganic Chemistry · Concept The short answer: In square planar complexes, certain ligands strongly labilise the position trans to themselves. Arranging synthesis steps so the right ligand is present at the right time is how specific geometric isomers are made deliberately rather than as a mixture. What the effect is In a square planar complex, a ligand…

Crystal Field Theory: Splitting, CFSE and the Spectrochemical Series

August 30, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Coordination Crystal Field Theory: Splitting, CFSE and the Spectrochemical Series Coordination chemistry carries more entrance-exam weight than any other inorganic topic, and crystal field theory is the engine underneath most of it. BSc & MSc · Inorganic Chemistry · Concept The short answer: An octahedral field raises the two eg orbitals and lowers the three t2g orbitals, splitting them by Δo. Whether electrons pair up or occupy the upper set depends on whether Δo exceeds the pairing energy — which is what decides high-spin versus low-spin, and therefore colour, magnetism and stability. The model in one paragraph…

Haemoglobin and Myoglobin: Coordination Chemistry Doing a Job

August 29, 2026

| No Comments

| Chemical Science Exams, Inorganic Chemistry

Inorganic Chemistry · Bioinorganic Haemoglobin and Myoglobin: Coordination Chemistry Doing a Job Bioinorganic questions are coordination chemistry questions wearing biological clothing. The reasoning is the same — geometry, spin state and ligand field. BSc & MSc · Inorganic Chemistry · Concept The short answer: Both proteins carry iron in a porphyrin ring. Myoglobin has one such site and binds oxygen with a simple hyperbolic curve; haemoglobin has four and binds cooperatively, giving a sigmoidal curve. The cooperativity is a structural consequence of a spin-state change moving the iron into the porphyrin plane. The haem group Both proteins are built around…