Complete these statements for the complex ion \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{CN}\right]^{2+}\) (a) The term "en" is the abbreviation for ____ (b) The oxidation number of \(\mathrm{Co}\) is _____(c) The coordination number of \(\mathrm{Co}\) is ____ (d) _____ is a bidentate ligand.

In the fascinating world of inorganic chemistry, coordination compounds occupy an essential place both in theory and practical applications. They consist of a central metal atom or ion (usually transition metals) surrounded by molecules or ions called ligands. These ligands have lone pairs of electrons and can donate them to the metal, forming coordinate covalent bonds.

Ligands can be of various types based on the number of binding sites they have. Some ligands, like water in our exercise example, can bind through a single site (monodentate). Coordination compounds are designated by square brackets and often have distinct physical and chemical properties. For instance, the color and magnetic behavior of complexes are intensely researched for their utility in various industries, including medicine and materials science.

It's imperative to understand how ligands can change the overall charge and the oxidation states within coordination compounds. Grasping the overall structure can unravel details about reactivity patterns, stability, and mechanisms of reactions involving these compounds.

Determining the oxidation number of a central metal in a coordination compound, like cobalt in our example, is a fundamental skill in chemistry. The oxidation number is a concept that helps in understanding the transfer of electrons during a chemical reaction. It is assigned to an atom in a molecule based on arbitrary rules, which assume ionic character in what are actually covalent bonds.

To calculate the oxidation number, you first need to know the overall charge of the complex ion and then subtract the sum of charges of all the attached ligands. The oxidation number of the central metal is then the difference between that sum and the overall charge. It is critical to remember that generally, neutral ligands do not contribute to the overall charge, whereas charged ligands do.

In our exercise, 'en' (ethylenediamine) does not carry a charge, while the cyanide ion carries a negative charge. By adding up these contributions, we can deduce the oxidation state of Cobalt, which is integral to predicting the compound's reactivity and other chemical behavior.

Diving deeper into the types of ligands, a bidentate ligand has two points of attachment, meaning it can form two bonds to a central metal atom or ion. The term 'bidentate' literally means 'two teeth', symbolizing the two 'bites' it takes on the metal atom. These types of ligands are often organic compounds that can form ring structures (chelates) with the central metal, which significantly contributes to the stability of the coordination compound.

Ethylenediamine ('en') is a classic example of a bidentate ligand. It has two nitrogen atoms, each with a lone pair of electrons, available for bonding to the metal. These multiple attachments make bidentate ligands more secure and stable. This increased stability is often explained by the chelate effect, which is a central concept in coordination chemistry for understanding the preferential formations of these types of complexes.