Chemistry · Ch 5 — Coordination Chemistry
Coordination Number
Coordination Number
The coordination number of a metal in a complex is the number of ligand donor atoms bonded to the central metal ion -- equivalently, the number of sigma bonds formed between the ligands and the central atom. It is not simply the number of ligands: a bidentate ligand contributes two donor atoms (and hence two sigma bonds) even though it is only one ligand. For example, Fe²⁺ in K₄[Fe(CN)₆] has coordination number 6 (six monodentate CN⁻ ligands, one donor atom each). In [Ni(en)₃]Cl₂, the ligand 'en' (ethane-1,2-diamine, H₂N-CH₂-CH₂-NH₂) is bidentate, offering two nitrogen donor atoms per molecule; with three en ligands, that gives 3 × 2 = 6 coord …
Worked out. Example 1: in [Fe(CN)₆]⁴⁻, let the oxidation number of iron be x. The net charge equation is -4 = x + 6(-1), which solves to x = +2. Example 2: in [Co(NH₃)₅Cl]²⁺, let the oxidation number of cobalt be x. The net charge equation is +2 = x + 5(0) + 1(-1), which solves to x = +3. Both examples apply the general relation: net charge on the complex = (oxidation state of the central metal) + [(number of each kind of ligand) × (charge on that ligand)], summed o …
Worked out. A self-check problem asking the student to identify, for the complex [Pt(NO₂)(H₂O)(NH₃)₂]Br: (i) the central metal atom/ion -- platinum; (ii) the ligand(s) and their types -- one anionic NO₂⁻ (nitro/nitrito), one neutral H₂O (aqua) and two neutral NH₃ (ammine), all monodentate; (iii) the coordination entity -- [Pt(NO₂)(H₂O)(NH₃)₂]⁺; (iv) the oxidation number of the central metal -- solving x + (-1) + 0 + 2(0) = +1 gives platinum oxidation state +2; and (v) the coordination number -- four donor atoms (N of NO₂, O of H₂O, and two N of the t …