Coordination Compounds
Coordination Compounds tests nomenclature, isomerism, coordination number, bonding models, colour and magnetic behaviour. Charge counting is the foundation for every problem.
Nomenclature and isomerism
In naming, ligands are alphabetical and the metal oxidation state is shown in Roman numerals. Ambidentate ligands such as NO2- and SCN- can bind through different atoms; bidentate ligands such as oxalate form chelates. Isomerism includes geometrical (cis/trans), optical, ionisation, hydrate, linkage and coordination isomerism. Tetrahedral complexes do not show geometrical isomerism.
Bonding and magnetism
Coordination number 6 is octahedral; 4 is tetrahedral or square planar. [Ni(CN)4]2- is square planar and diamagnetic (d8, strong field). [NiCl4]2- is tetrahedral and paramagnetic. In crystal-field terms, strong-field ligands such as CN- and CO cause a large splitting and favour low-spin configurations; weak-field ligands such as Cl- favour high spin.
Worked example
For [Co(NH3)5Cl]Cl2, let the cobalt oxidation state be x. NH3 is neutral, Cl is -1, and the complex ion has charge +2, so x - 1 = +2 and x = +3. Co3+ is d6. With NH3 as a strong-field ligand in an octahedral complex it is low spin, with t2g6 and no unpaired electrons, so the complex is diamagnetic.
Traps
Forgetting the charge on the complex ion, mixing up CN- and Cl-, and assigning tetrahedral geometry to [Ni(CN)4]2- are common errors.