Coordination chemistry
The idea
A coordination complex is a central metal ion holding a set of ligands — molecules or anions that donate electron pairs into the metal's empty orbitals, a textbook Lewis acid–base interaction. The number of donor atoms attached is the coordination number (six gives octahedral, four gives tetrahedral or square planar), and ligands range from monodentate (NH₃, Cl⁻, H₂O) to chelating polydentates like ethylenediamine or EDTA that grip through several atoms at once. Your ionic- and covalent-bonding background merges here: the complex ion has a net charge balanced by counterions outside the brackets.
Three bookkeeping skills unlock most problems: assign the metal's oxidation state from the overall charge minus the ligand charges; count d electrons from the resulting ion's configuration; and name systematically — ligands alphabetically with Greek prefixes, then the metal with its oxidation state in Roman numerals. Crystal field theory then explains color and magnetism: ligands split the d orbitals, electrons absorb visible light to jump the gap, and strong-field ligands can force pairing into low-spin arrangements.
Mind the color trap: a solution looks the color it transmits, not the color it absorbs. A complex absorbing orange-red light appears blue. And only the ligands written inside the square brackets are bonded to the metal — counterions outside dissociate freely in solution.
Worked example
For the compound [Co(NH₃)₅Cl]Cl₂, determine the oxidation state of cobalt, its d-electron count, the coordination number and geometry, and give the systematic name.
- Work out the complex ion's charge from the counterions: two free Cl⁻ outside the brackets mean the bracketed ion must be 2+ for neutrality.
- Assign cobalt's oxidation state inside the brackets: five NH₃ ligands are neutral and the bound Cl is −1, so Co + (−1) = +2 gives Co = +3.
- Count d electrons: neutral Co (Z = 27) is [Ar] 4s² 3d⁷; removing three electrons (both 4s, then one 3d) leaves Co³⁺ = [Ar] 3d⁶, a d⁶ ion.
- Count donor atoms for the geometry: five ammonia nitrogens plus one chloride make coordination number 6, so the complex is octahedral — and with NH₃ a reasonably strong-field ligand, the d⁶ set pairs up low-spin, predicting a diamagnetic ion.
- Name it: ligands in alphabetical order (ammine before chloro), counting prefixes, then the metal and its state — pentaamminechlorocobalt(III) chloride.
Answer. Cobalt is +3 and d⁶, in an octahedral complex of coordination number 6; the compound is pentaamminechlorocobalt(III) chloride.
Check your understanding
- Why does a chelating ligand like EDTA bind so much more tenaciously than six separate monodentate ligands, and what entropy argument explains it?
- How does the identity of the ligands decide whether a d⁶ complex is high-spin or low-spin, and what measurable property changes between the two?
- What would dissolve differently in water between [Co(NH₃)₅Cl]Cl₂ and [Co(NH₃)₄Cl₂]Cl, and how could a silver nitrate test tell them apart?
- Why does the observed color of a complex correspond to the light it does not absorb?
Build the foundations first
Coordination chemistry builds on these concepts. If any feel shaky, start there.