Summary
Highlights
A complex ion is formed by combining a transition metal cation with a ligand molecule or ion. For example, silver metal cation reacts with two ammonia molecules to form a diamine silver cation. The coordination number is the number of ligand molecules attached to the transition metal cation, which is two in this case. For a coordination number of two, the geometry is linear.
The video analyzes the hexacyanoferrate ion. The ligand is cyanide, a polyatomic ion with a negative charge on carbon. The coordination number is six, as there are six cyanide ions attached to the iron. The oxidation state of iron (Fe) is determined to be +3 by balancing the charges of the complex ion and the ligands.
Alfred Werner's theory states that transition metal ions have two types of valence: primary valence and secondary valence. The primary valence relates to the oxidation state, which determines ionic bonding. The secondary valence relates to the coordination number, which determines interaction with ligands. For the hexacyanoferrate ion, the primary valence of Fe is +3, and the secondary valence is 6.
If the coordination number is six, the geometry of the complex ion is octahedral. The video illustrates how to draw this structure. For a coordination number of four, there are two possible geometries: tetrahedral or square planar, and both are visually represented.
The video discusses the coordination numbers of different transition metal ions. For +1 charge ions (e.g., Cu+, Ag+), the coordination number is typically low (2 or 4). As the charge increases to +2 (e.g., Co2+, Ni2+, Fe2+, Zn2+), the coordination number can be 4 or 6. For +3 charge ions (e.g., Au3+, Cr3+, Co3+), the coordination number is often higher, usually 4 or 6. Generally, as the charge on the metal ion increases, the coordination number increases, allowing it to bind to more ligands.
Ligands are classified by how many interactions they make with a metal ion. Unidentate (or monodentate) ligands make one interaction, like water, ammonia, chloride, and cyanide. Bidentate ligands make two interactions per molecule. Examples include the oxalate ion and ethylene diamine (en), where each molecule can bind to the metal ion at two different points. Bidentate ligands are also known as chelating ligands, as they interact with the metal ion in more than one way.
A coordination compound is electrically neutral and consists of a complex ion and a counter ion. The complex ion is typically enclosed in brackets in chemical formulas. The counter ion balances the charge of the complex ion. For example, in Cr(NH3)6Cl3, the complex ion is Cr(NH3)63+ and the counter ion is Cl-. The video also shows an example where the complex ion is an anion, such as in K3Fe(CN)6, where Fe(CN)63- is the complex ion and K+ is the counter ion. The net charge of a coordination compound is always zero.