The total number of unidentate ligand atoms directly bonded to the central metal atom in a coordination complex is called its:

**Coordination number** (CN) counts the donor atoms attached directly to the metal — i.e. the number of bonds it forms with the ligands. For a bidentate ligand each ligand contributes *two* to the CN; for unidentate, one each. Common CNs are 2 (linear), 4 (tetrahedral or square planar) and 6 (octahedral). - Option A (oxidation state) is the formal charge on the metal — Werner called this the *primary valency*. - Option C is Werner's term for oxidation state, so it overlaps with A but is not the CN.