Formulas and names

Formula rules: write the central atom first, then the ligands in alphabetical order whatever their charge. Put the whole entity in square brackets, polyatomic ligands and abbreviations in round brackets, and leave no spaces inside the sphere.

An abbreviated ligand is alphabetised by the first letter of its abbreviation. For a charged entity written alone, the charge goes outside the bracket as a right superscript with the number before the sign: [Co(CN)₆]³⁻, [Cr(H₂O)₆]³⁺.

Naming rules: name the cation first, whether or not it is the complex. Inside the entity, name the ligands in alphabetical order before the metal, which is the reverse of the order used when writing the formula.

Anionic ligands end in -o (the 2004 IUPAC draft makes this -ido, so chloro becomes chlorido). Neutral ligands keep their names except aqua (H₂O), ammine (NH₃), carbonyl (CO) and nitrosyl (NO).

Use di, tri, tetra for numbers of simple ligands. When a ligand's own name already holds a number, use bis, tris, tetrakis with the ligand in brackets: [NiCl₂(PPh₃)₂] is dichloridobis(triphenylphosphine)nickel(II).

Give the metal's oxidation state as a Roman numeral in brackets. In a cationic or neutral complex the metal keeps its element name; in an anionic complex it ends in -ate (cobaltate), with Latin roots for some metals (ferrate for Fe, argentate for Ag).

Worked names: [Cr(NH₃)₃(H₂O)₃]Cl₃ is triamminetriaquachromium(III) chloride; [Co(en)₃]₂(SO₄)₃ is tris(ethane-1,2-diamine)cobalt(III) sulphate; [Ag(NH₃)₂][Ag(CN)₂] is diamminesilver(I) dicyanidoargentate(I).

Finding the oxidation state: charge on the entity = oxidation state + sum of ligand charges. In [Cr(NH₃)₃(H₂O)₃]³⁺ all ligands are neutral, so Cr is +3; in K₃[Fe(CN)₆], −3 = x + 6(−1) gives Fe = +3.

Name endings show binding atoms for ambidentate ligands: nitrito-N (through N) and nitrito-O (through O); thiocyanato-S binds through S, as in Hg[Co(SCN)₄], mercury(I) tetrathiocyanato-S-cobaltate(III).