Chemistry
Coordination compounds
Ligands, coordination number, crystal field, colour, and an 18-electron sketch.
Basics
Metal and ligands
A coordination compound is a Lewis-acid metal ion bound to Lewis-base ligands. Coordination number is how many atoms attach directly (often 4 or 6). Monodentate ligands occupy one site; chelates (en, EDTA) occupy several and often gain stability (the chelate effect). Charge balance and oxidation number keep the formula honest. IUPAC names exist; in class, first count the ligands and the metal.
Geometry and isomers
Four-coordinate complexes are tetrahedral or square planar; six-coordinate ones are usually octahedral. The same formula can hide linkage isomers (SCN⁻ vs NCS⁻), ionisation isomers, and optical isomers of chiral chelates. Cis–trans shows up in octahedral Ma₄b₂ and square-planar Ma₂b₂. Stereochemistry changes catalysis, colour, and magnetism. Draw wedges and dashes for front and back.
Crystal field and colour
Ligands split the d set by a gap Δ. In octahedral geometry t₂g lies below e_g. A large Δ favours pairing (low spin); a small Δ favours high spin. The colour you see is the complement of the absorbed gap. The spectrochemical series (I⁻ < … < CN⁻) ranks how much a ligand raises Δ. Magnetism tracks unpaired electrons.
18-electron sketch
In organometallic chemistry, counting the metal valence electrons plus those donated by ligands often lands near 18 (a krypton-like shell). Exceptions are common; catalytic cycles may swing 16–18. Ni(CO)₄ and Fe(CO)₅ are textbook cases. Pick one counting convention (ionic vs covalent) per problem. Toxic carbonyls have their own lab rules.
Formulas
Complex charge
Sum the metal oxidation number and ligand charges.
Symbols
-
ox_metalmetal oxidation number -
q_ligandcharge of each ligand
Octahedral crystal-field split
If absorption sits near this energy, a colour appears.
Symbols
-
Δ_ooctahedral splitting
Crystal-field stabilisation (octahedral)
Occupancy plus pairing energy P. Sign conventions vary slightly by text.
Symbols
-
n_{t₂g}electrons in t₂g -
Ppairing energy
Key table
| Haemoglobin | Fe coordination; O₂ binding changes colour and shape — a bio-complex |
|---|---|
| Square planar | common for d⁸ (Ni²⁺, Pd²⁺, Pt²⁺) |
| Strong-field ligands | CN⁻, CO, en — large Δ, low-spin tendency |
In this field
Atoms and the periodic table
Atomic structure, nucleus and electrons, periods and groups.
Chemical bonding
Ionic, covalent, and metallic bonds, plus molecular shape.
The mole and stoichiometry
Moles, molar mass, empirical formulas, and yield.
Gases and acids–bases
Ideal gas, pH, and buffers.
Thermo, equilibrium, kinetics
Enthalpy, Gibbs energy, K, and rate laws.
Electrochemistry and organic basics
Oxidation numbers, cells, Nernst, functional groups.
Nuclear chemistry
Radioactivity, half-life, binding energy.
Solutions and kinetics
Concentration, colligative properties, rate laws.
Analytical chemistry
Titration, spectroscopy, and chromatography — measuring what is present and how much.
Polymers and solids
Crystals and glasses, metal–ionic–covalent solids, polymer chains and the glass transition.
Electrochemical cells
Galvanic versus electrolytic, Nernst, Faraday, and a corrosion sketch.
Phase equilibria and diagrams
Gibbs phase rule, unary diagrams, eutectics, and the lever rule.