How do you find oxidation numbers in complex compounds?

To determine oxidation numbers (also known as oxidation states) in complex compounds, you can follow these general rules and guidelines:

Assign oxidation numbers to known elements: Start by assigning oxidation numbers to the elements you're familiar with based on the following guidelines:

The oxidation number of an atom in its elemental form is always 0. For example, O2, N2, H2, etc., all have oxidation numbers of 0.
The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of Na+ is +1, and the oxidation number of Cl- is -1.
Assign hydrogen and oxygen oxidation numbers: In most cases, hydrogen is assigned an oxidation number of +1, and oxygen is assigned an oxidation number of -2. However, there are exceptions to these rules, which should be considered based on the compound's overall charge and structure.

Consider the charge of the complex: If the complex has a net charge, assign oxidation numbers while taking this charge into account. For example, in a complex ion like [Fe(CN)6]4-, the overall charge is -4, which means the sum of the oxidation numbers of all atoms in the complex should equal -4.

Use algebraic equations: For complex compounds, you may need to set up algebraic equations to find the oxidation numbers of certain elements. These equations should be based on the overall charge of the complex and the known oxidation numbers of other elements. For example, in the complex [Fe(H2O)6]3+, you can set up an equation to solve for the oxidation state of iron (Fe) using the known oxidation states of hydrogen and oxygen.

Be aware of ligands: In complex compounds, ligands (such as water, ammonia, cyanide, etc.) can coordinate with a central metal atom. The oxidation state of the metal atom can be affected by the charge and nature of the ligands. You may need to consider the charge of the ligands and their typical oxidation states when assigning oxidation numbers to the metal atom.

Check for redox reactions: When dealing with complex compounds in a chemical reaction, it's essential to track changes in oxidation states. Elements that change their oxidation states during a reaction are undergoing redox reactions.

Refer to tables and databases: Some complex ions and compounds may have established oxidation states based on chemical conventions and empirical data. You can consult reference tables or databases for help in assigning oxidation numbers to complex compounds.

In practice, the process of assigning oxidation numbers to complex compounds can be complex, and it may require some trial and error. Additionally, it's essential to have a good understanding of the chemical structure and the behavior of the specific elements and ligands involved in the complex.