Nomenclature and isomerism of complex compounds Historical Alfred Werner, a chemist from Switzerland, said that a complex is a compound in which an atom is bounded to more groups than expected with respect to their charge and position in the periodic table. It consists out of a central atom (mostly a transition metal) and the surrounding groups are called ligands. The number of ligands is called coordination number (CN). Nomenclature Complex formulas are marked with parenthesis []. First the metal ion is written and then charged ligands follow. After that neutral ligands are written in the brackets. Sometimes the position of atoms in a well known ligand like water are changed to show which atom is bonded (e.g. [Fe(OH2)3]). The counter ions are written outside the brackets. Cations stand in front of the complex bracket and anions after it. There are some special short forms for larger ligands, which often can build chelats like en = ethylendiamine or ox = oxalato. To name a complex you begin with the ligands in alphabetic order. For an ionic ligand an “o” is added to its name like fluoro (F‐) or hydroxo (OH‐). Neutral examples are aquo = H2O (here the “o” is an exception) carbonyl = CO or ammine = NH3. After the ligands the central atom is named. For neutral and cationic complexes the “normal” element name is used, for an anionic complex “at” is added to its name (e.g. manganat, niobat) or to its Latin name (e.g. for Fe: ferrat, for Au: aurat, and for Pb: plumbat). At the end the oxidation state of the central atom follows (Latin numbers). In front of the whole name prefixes can be added to describe the structure like cis or trans. To describe that there are more than one ligand or central atom Greek numbers as prefixes are used (Di‐, tri‐, tetra...). In special cases (e.g. for the ligand dienamine) bis, tris, tetrakis… is used. For bridging ligands where two central atoms are connected a μ should be used (e.g. μ‐oxo‐bis(pentaaquochromium(III)) = [(H2O)5Cr‐
O‐Cr(H2O)5]4+) Structural isomerism and the Stereoisomerism There exist two types of isomerism, the structural isomerism and the stereoisomerism. Structural isomers have the same chemical formula but differ in their bonding situation. For the structural isomerism you can distinguish 4 different groups: 1) Ionization isomerism: ligands are exchanged with counter ions e.g. [Pt(NH3)4Cl2]Br2 and [Pt(NH3)4Br2]Cl2 2) Hydrate isomerism: ionization isomerism with water as ligand e.g. [Cr(H2O)6]Cl3 and [CrCl(H2O)5]Cl2×H2O and [CrCl2(H2O)4]Cl×2H2O 3) Linkage isomerism: a ligand is bounded to the central atom by different atoms e.g. M‐CN (cyano) M‐NC (isocyano) M‐NO2 (nitro) M‐ONO (nitrito) 4) Coordination isomerism: the central ions in a compound with more than one complex change their places e.g. [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] Stereoisomers have the same chemical formula and the bonds to the ligands are the same, but the arrangement differs. The most important property is, that they are nonsuperimposable. 2 Types of stereoisomerism can be distinguished: 1) Enantiomers: Two enantiomers behave like picture and mirror picture. The central atom is a chiral centre and there is now mirror plane or inversion centre in the molecule. Pure enantiomers are optical active. 2) Diastereomers: These are not enantiomers but they are nonsuperimposable, too. They differ in the arrangement of their ligands: a) cis/trans: If two similar ligands are arranged in opposite direction it is called trans. If they are neighbored it is a cis‐complex. b) fac/mer: These diastereomers can be observed in octahedral complexes. If three ligands of the same species are arranged in a plane with the central atom it is called mer(idional). If the three ligands occupy one face of the complex it is called fac(ial).