Why a complex needs a careful name
By now you can look at a [[complex-ion|complex ion]] and read its anatomy: a central metal, a cluster of ligands reaching in with their lone pairs, a coordination number counting how many donor atoms touch the metal, and an overall charge. But a structure you can only point at is not yet something you can write in a paper, order from a catalogue, or say to a colleague over the phone. 'The pink cobalt thing with six ammonias' will not do. We need a name that is unique, that anyone can decode back into the exact formula, and that no two chemists could write two different ways.
That is the job of [[coordination-nomenclature|coordination nomenclature]] — a small, strict rulebook agreed by IUPAC. The good news is that it is genuinely a *system*, not a list to memorise: master maybe seven rules and you can name compounds you have never seen, and reverse the process to write formulas from names. The whole thing rests on one idea you already own from the previous guides — that a complex is a metal surrounded by ligands inside square brackets, behaving as a single unit. Naming is just the grammar that describes that unit out loud.
Naming the ligands, in alphabetical order
Inside the brackets, the ligands are named first and the metal last — the reverse of the formula's writing order, which puts the metal first. Crucially, the ligands are listed alphabetically by their ligand name, not by how many there are and not by charge. Neutral ligands usually keep something close to the molecule's own name (ammonia bound to a metal is *ammine*, with two m's; water is *aqua*; carbon monoxide is *carbonyl*; nitric oxide is *nitrosyl*). Anionic ligands take an -o ending: chloride becomes *chloro*, cyanide *cyano*, hydroxide *hydroxo*, oxide *oxido*, and so on. So Cl- contributes 'chloro', NH3 contributes 'ammine'.
How many of each ligand is shown by a Greek prefix: *di-, tri-, tetra-, penta-, hexa-*. Here is the trap that catches everyone once: when the prefix would collide with a ligand whose own name already contains a counting word (like *ethylenediamine*, which contains 'di'), or with a long or complicated ligand name, you switch to the multiplying prefixes *bis-, tris-, tetrakis-* and wrap the ligand name in parentheses. So two chlorides give *dichloro*, but three ethylenediamines (en) give *tris(ethylenediamine)*, not 'triethylenediamine', which would be ambiguous. And one more subtlety worth burning in: these counting prefixes do not count for alphabetising. You alphabetise by the ligand's base name — 'ammine' files under A and 'chloro' under C — regardless of whether it is *tetraammine* or *diammine*.
The metal, its oxidation state, and the -ate ending
After the ligands comes the metal, and immediately after the metal's name comes its [[oxidation-state|oxidation state]] in Roman numerals inside parentheses, with no space: cobalt(III), iron(II), platinum(IV). You compute it from the arithmetic you already know — the charges on the ligands plus the metal's oxidation state must add up to the overall charge of the complex. In [Co(NH3)6]3+, the six ammines are neutral, so cobalt must be +3 to give the +3 overall: cobalt(III). In [Fe(CN)6]4-, the six cyanides each carry -1 (total -6), so iron must be +2 to land at -4: iron(II).
There is one twist for the metal's name. If the *whole complex* carries a net negative charge — an anionic complex — the metal's name takes an -ate suffix. So cobalt becomes *cobaltate*, zinc becomes *zincate*, aluminium becomes *aluminate*. A handful of metals revert to their Latin roots for this ending, which you simply have to know: iron becomes *ferrate*, copper *cuprate*, lead *plumbate*, tin *stannate*, silver *argentate*, gold *aurate*. Thus [Fe(CN)6]4- is named with 'ferrate', not 'ironate'. Neutral and cationic complexes use the plain English metal name with no suffix. The -ate ending is, in effect, a flag that says 'this complex is an anion'.
Cation, then anion — and a name read off in full
Many coordination compounds are salts: a complex ion paired with an ordinary counter-ion. The naming order for the whole salt follows the same convention as NaCl — cation first, then anion — separated by a space, exactly as you would say 'sodium chloride'. It does not matter whether the complex itself is the cation or the anion; you name whichever part is positive first. So in [Co(NH3)6]Cl3 the cationic complex is named first and the three chloride counter-ions second, giving 'hexaamminecobalt(III) chloride'. In K4[Fe(CN)6] the four potassium ions come first and the anionic complex second: 'potassium hexacyanoferrate(II)'.
Let us walk one formula all the way to a name, slowly, so the machinery is visible. Take [Pt(NH3)2Cl2], a neutral molecule — this is the famous anticancer drug cisplatin. The metal is platinum, sitting inside the brackets with two ammines and two chlorides. Work the steps in order and the name assembles itself.
- List and name the ligands. Two NH3 give 'ammine' and two Cl- give 'chloro'. There is no counter-ion outside the brackets, so the whole molecule is just this one neutral unit.
- Alphabetise by ligand base name. 'Ammine' (A) comes before 'chloro' (C), so ammine is written first regardless of the counting prefixes you are about to add.
- Add the counting prefixes. Two of each: 'diammine' and 'dichloro'. Neither ligand name contains a hidden number, so plain di- is fine — no need for bis-.
- Find the oxidation state. The complex is neutral overall; two chlorides total -2 and the two ammines are neutral, so platinum must be +2 to balance: platinum(II).
- Assemble: ligands (alphabetical, with prefixes) + metal + (oxidation state). The complex is not an anion, so no -ate. The full name is diamminedichloroplatinum(II) — written as one word for a neutral molecule.
Reading a name backwards into a formula
A good naming system runs in reverse, and being able to decode a name into a formula is the real test that you understand it. Take 'potassium hexacyanoferrate(II)'. Read it as a sentence. 'Potassium' is the cation, named first, so K+ ions sit outside the brackets. 'Ferrate' tells you the metal is iron *and* that the complex is an anion. The '(II)' fixes iron at +2. 'Hexacyano' means six cyanide ligands, each CN- carrying -1. So the complex ion is iron at +2 plus six times -1, an overall charge of 2 - 6 = -4: the anion is [Fe(CN)6]4-. To make a neutral salt you need four K+ to balance the -4, giving the formula K4[Fe(CN)6].
NAME -> FORMULA (worked from the name 'potassium hexacyanoferrate(II)') potassium ........ K+ (cation, named first -> outside brackets) hexa-cyano ....... 6 x CN- (6 ligands, each -1, total -6) -ferr-(II) ....... Fe as iron, oxidation state +2 -ate ............. the complex is an ANION charge of ion = (+2) + 6(-1) = -4 -> [Fe(CN)6]4- balance with K+: 4 x (+1) = +4 -> K4[Fe(CN)6] FORMULA -> NAME order: cation | [ligands alphabetical + metal(ox.state) ] anion
When you write a formula from scratch, mind the writing conventions, which differ slightly from the speaking order. Inside the brackets the metal comes first, then ligands; among the ligands, anionic ones are conventionally written before neutral ones, and within each of those groups they are ordered alphabetically by the first symbol of the ligand formula. The whole complex unit, ion or molecule, gets enclosed in square brackets, and the overall charge is written as a superscript outside the closing bracket. These are typographic habits, not chemistry — but following them is what keeps everyone's formulas instantly readable.
Honest edges and a few worked checks
A few honest caveats keep you from over-trusting the rules. First, nomenclature has been revised over the decades, so you will meet both old and new forms in the wild: 'chloro' versus the newer 'chlorido', 'cyano' versus 'cyanido'. Both decode to the same ligand; the rulebook is a living document, and older papers froze an older edition. Second, the names of polydentate and bridging ligands, and complexes with several metals, need extra machinery (kappa for donor atoms, eta for how many carbons of an organic ligand touch the metal, mu for bridges) that we leave for later guides — the rules here cover the vast majority of simple complexes you will name day to day. Tie this back to denticity: a chelating ligand like ethylenediamine grabs through two donor atoms, which is why it counts as one ligand worth two coordination sites.
Now run two quick checks to make the rules stick. The deep-blue complex [Cu(NH3)4]2+: four ammines (neutral), so copper is +2, the complex is a cation (no -ate). Name: tetraamminecopper(II), with copper(2+). The bright purple [Cr(H2O)4Cl2]+: 'aqua' (A) before 'chloro' (C) alphabetically, four waters and two chlorides; the two chlorides are -2 and the overall charge is +1, so chromium is +3, a cation (no -ate). Name: tetraaquadichlorochromium(III). Notice how in both you alphabetised by ligand base name, ignored the counting prefixes when ordering, computed the oxidation state from the charge balance, and reserved -ate for anions only.
A last reminder of why all this care is worth it. The very same atoms, [Co(NH3)5(NO2)]2+, can be assembled two ways — nitro (N-bound, yellow) or nitrito (O-bound, red) — and a coordination compound can also differ in which ligand sits inside the brackets versus outside, or in the geometric arrangement of identical ligands. Each of those differences is a real, separable substance with its own colour and reactions, and the name is the only thing that tells them apart on paper. That is exactly the world of isomerism you step into in the next guide — and you now have the vocabulary to name every isomer you will meet there.