Coordination complexes and applications

Naming rules, in order of application. IUPAC nomenclature is a fixed procedure, not a set of independent conventions — apply the rules in sequence and the name falls out: Ligands are cited before the metal, in alphabetical order by ligand root (ignoring any multiplying prefix). Simple ligands take di-, tri-, tetra-; ligands whose own name already contains a multiplier, or is itself complex/polydentate (e.g. ethylenediamine), take bis-, tris-, tetrakis- instead, with the ligand name in parentheses — bis(ethylenediamine). Anionic ligands always end in -o (chloro, cyano, oxalato, hydroxo); neutral ligands mostly keep their common name (aqua, ammine, pyridine), with $\text{CO}$ = carbonyl and $\text{NO}$ = nitrosyl as memorised exceptions. The metal's oxidation state is written as a Roman numeral in parentheses directly after the metal name — this is the step students skip and lose marks on. If the whole complex ion is an anion, the metal name gets the suffix -ate (ferrate, cobaltate, platinate); cationic and neutral complexes keep the plain metal name. For polynuclear (bridged) complexes, a shared ligand connecting two metal centres is cited with the prefix $\mu$- and alphabetised alongside the terminal ligands, e.g. $[(\text{NH}_3)_5\text{Cr}(\mu\text{-OH})\text{Cr}(\text{NH}_3)_5]^{5+}$ is $\mu$-hydroxo-bis[pentaamminechromium(III)] ion.

Isomerism — same formula, different molecules. Coordination compounds split isomerism into two families that are easy to confuse if you only memorise definitions rather than picture what moves. Structural isomers have the same atoms connected differently: ionisation isomers swap a ligand with an outside counter-ion, $[\text{Co(NH}_3)_5\text{Br}]\text{SO}_4$ versus $[\text{Co(NH}_3)_5\text{SO}_4]\text{Br}$; hydrate isomers move water in or out of the coordination sphere, $[\text{Cr(H}_2\text{O})_6]\text{Cl}_3$ versus $[\text{Cr(H}_2\text{O})_5\text{Cl}]\text{Cl}_2\cdot\text{H}_2\text{O}$; linkage isomers bind an ambidentate ligand through a different donor atom, nitro $\text{–NO}_2$ (N-bound) versus nitrito $\text{–ONO}$ (O-bound); and coordination isomers redistribute ligands between the cation and anion of a salt built from two complex ions, $[\text{Co(NH}_3)_6][\text{Cr(CN)}_6]$ versus $[\text{Cr(NH}_3)_6][\text{Co(CN)}_6]$. Stereoisomers share identical connectivity but differ in 3D arrangement: geometrical isomerism gives cis/trans pairs in square planar (CN 4) and octahedral (CN 6) complexes, and facial(fac)/meridional(mer) arrangements for octahedral $\text{MA}_3\text{B}_3$ complexes; optical isomerism gives non-superimposable mirror images (enantiomers), most commonly in octahedral complexes bearing chelating ligands — $[\text{Co(en)}_3]^{3+}$ has no internal mirror plane and is chiral, while $[\text{Co(NH}_3)_6]^{3+}$, with six identical monodentate ligands, is not. The oxidation-state step above is worth isolating as its own tool, because every naming and every isomer-classification problem starts by pinning it down from a simple charge-balance sum.

Problem. Name the complex $[\text{Co(NH}_3)_4(\text{H}_2\text{O})\text{Cl}]\text{Cl}_2$ by IUPAC rules, and determine the oxidation state of cobalt.

Step 1. Identify $Q$, the charge on the bracketed complex ion. Two $\text{Cl}^-$ counter-ions sit outside the bracket, so by charge balance the complex ion itself must carry $Q = +2$.

Step 2. List every ligand and its charge $q_i$: four $\text{NH}_3$ ($q = 0$ each), one $\text{H}_2\text{O}$ ($q = 0$), one $\text{Cl}^-$ ($q = -1$). The total ligand charge is $\sum_i n_i q_i = 4(0) + 1(0) + 1(-1) = -1$.

Step 3. Apply Equation 1: $x + (-1) = +2$, so $x = +3$. Cobalt is in the $+3$ oxidation state.

Step 4. Build the name. Alphabetise the ligand roots (ignoring multiplying prefixes): ammine, aqua, chloro — that order is already alphabetical, so the name reads tetraammine (4 $\text{NH}_3$), aqua (1 $\text{H}_2\text{O}$), chloro (1 $\text{Cl}^-$), followed by the metal name with its Roman-numeral oxidation state, and finally the outer counter-ion named as "chloride":

Final answer: cobalt is +3, and the full IUPAC name is tetraamminaquachlorocobalt(III) chloride.

Competitive stretch

Extend the same logic to geometry with the classic cisplatin pair. $[\text{Pt(NH}_3)_2\text{Cl}_2]$ is square planar, giving two geometric isomers: cis (the two $\text{NH}_3$ adjacent, 90 degrees apart) and trans (each pair diagonally opposite, 180 degrees apart). Neither is optically active — the molecular plane itself is a mirror plane, so both are achiral. Yet only cis is the drug cisplatin: its chloride ligands, displaced by water inside a cell, bridge two adjacent guanine bases on DNA because the labile sites sit 90 degrees apart, distorting the double helix and blocking replication. Trans-platin's labile sites are 180 degrees apart, unable to make that cross-link, so it is essentially inactive. Geometry alone, not electronic structure, is the difference.

• Alphabetising by the multiplying prefix instead of the ligand root. "Diammine" is filed under "a" (ammine), not "d" — strip di/tri/tetra/bis/tris/tetrakis before ordering ligand names.
• Dropping or mis-signing the oxidation-state Roman numeral. Always apply Equation 1 explicitly — sum the ligand charges, then solve for the metal — rather than recalling a metal's "usual" oxidation state, which can be wrong for that specific complex.
• Assuming all cis/trans pairs behave alike. Cisplatin versus trans-platin shows that identical formula and identical bonding can still mean the difference between a life-saving drug and a biologically inert isomer, purely from geometry.
• Confusing coordination isomerism with ionisation isomerism. Coordination isomerism swaps ligands between two different complex ions in a salt built from a complex cation and a complex anion; ionisation isomerism swaps a ligand with a simple counter-ion outside a single complex ion.

Ask the tutor: want to work through why $[\text{Co(en)}_3]^{3+}$ is chiral while $[\text{Co(NH}_3)_6]^{3+}$ is not, or practice naming a tricky polynuclear bridged complex?