Naming Complex Ions

Naming Complex lons A coordination compound (or complex) contains at least one complex ion - a central metal cation bonded to molecules or ions called ligands. To maintain overall formula neutrality, the complex ion has an associated counter ion. Counter ions may be cations or anions, depending on the overall charge of the complex ion itself. [Co(NH3)6]Cl3 has the complex ion (in square brackets) of [Co(NH3)6]" where the six NH3 molecules (ligands) are bonded to Co³t central metal cation. The three Cl ions are the counter ions. All complex ions are described with a coordination number that describes the number of ligands that are bonded to the central ion. The coordination number is specific for a given metal ion in a particular oxidation state and compound. In the complex ion [Co(NH3)6]3*, the coordination number of Co³+ is 6-there are six ligand atoms (the N in NH3) attached to the Co³. The coordination number of Pt2+ ion in many of its complexes is 4, while for the Pt* ion in its complexes the coordination number is 6. Copper (II) may have a coordination number of 2, 4, or 6 in different complex ions. In general, the most common coordination number is 6, but 2 and 4 are often seen, and some higher ones are known. The geometry of the molecule will also be indicative of the coordination number of the metal cation. When the coordination number is 2, then the geometry of the complex ion will be linear. When the coordination number is 4, the geometry may be either square planar or tetrahedral most då metal ions will form a square planar complex ion, while a d¹º metal cation will form a tetrahedral complex ion. A ligand is a molecule or ion with one or more donor atoms each of these donor atoms will donate a pair of electrons to the metal ion, forming a covalent bond. The donor atoms, which must have at least one lone pair of electrons, are those elements form groups SA, 6A, or 7A. Depending on the number of donor atoms a ligand may be monodentate (one bonding donor atom), bidentate (two bonding donor atoms), or polydentate (more than two bonding donor atoms). The presence of bi- and polydentate ligands give rise to rings in complex ions and are called chelate ligands. To determine the name or formula for a coordination compound, we must first determine the charge of the metal ion. This can be done using simple arithmetic - using the known ionic charges for the other ions present in the formula: For the complex anion K2[Co(NH3)2Cl4], we know that the two K+ ions balance the charge of the complex ion, which contains two NH3 molecules and four Cl anions. The two NH3 molecules are neutral, and there is an overall charge for -4 from the four chloride ions. The entire complex ion, then, must have a -2 charge to balance the two K ions. Comlpex ion charge = Charge of metal ion + total charge of ligands Charge of metal ion = (-2)-(-4) = +2 So, this is the Co²* ion in the complex ion formula. The same approach can be used for a compound with a complex cation - like [Co(NH3)4Cl2]Cl. This complex ion is [Co(NH3)4Cl2] with the counter ion Cl. There are four neutral NH3 ligands and two Cl- ligands, for a total ligand charge of -2. The complex ion has a charge of +1, so the metal ion must be Co³+: Charge of metal ion = charge of complex ion total charge of ligands Charge of metal ion = (+1)-[(4x0) + (2x-1)] = +3 Overall, there are three rules for writing the formulas of a coordination compound: 1. The cation is written before the anion. 2. The charge of the cation(s) is balanced by the charge of the anion(s). 3. For the complex ion, neutral ligands are written before anionic ligands, and the formula of the whole ion is placed in brackets. Naming Complex Ions: The cation is named before the anion Within a complex ion the ligands are named first, in alphabetical order, and the metal ion is named last The names of the anionic ligands end with o, whereas a neutral ligand is usually called by the name of the molecule exceptions: H₂O is aquo, CO is carbonyl, and NH3 is ammine When several ligands of a particular kind are present, Greek numerical prefixes are used to state hoe many are present these prefixes play no role in alphabetizing. If the ligan itself contains a Greek prefix, we use the prefixes bis (2), tris (3), tetrakis (4) The Roman numeral for the charge of the metal cation is listed following the name of the metal If the complex ion is an anion then name ends with-ate