Why is \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\), nearly \(10^{10}\) times more stable than \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+} ?\) (en \(=\) enthlene diamine \()\) (a) \(\mathrm{NH}_{3}\) is the weakest ligand (b) 'en' is a chelating ligand and forms thermody namically more stable complexes (c) \(\operatorname{six} \mathrm{NH}_{3}\) ligands cause steric hindrance around the \(\mathrm{Ni}^{2+}\) centre (d) \(\mathrm{NH}_{3}\) evaporates easily and causes instability to the \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) complex

When studying coordination complexes, it's essential to be familiar with the term 'ligand'. A ligand is essentially a molecule or ion that binds to a central metal atom to form a coordination complex. Among the various types of ligands, 'bidentate ligands' play a unique and significant role. Bidentate ligands, as suggested by the prefix 'bi-', have two donor atoms that can simultaneously bind to a single metal center, creating a more robust connection.

The enhylene diamine ligand, abbreviated as 'en', is a classic example of a bidentate ligand. The presence of two nitrogen atoms in ethylenediamine allows it to form two bonds with a metal ion, leading to the production of a ring structure. This ring is often described as a 'chelate ring', and the process of ring formation is called 'chelation'. This process enhances the stability of the metal complex due to the rigidity of the resultant ring structure and the multiple attachment points.

In the case of the complex \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\), each 'en' molecule forms two bonds with the nickel ion, resulting in the formation of three chelate rings. This multi-ring structure is much more stable compared to complexes with monodentate ligands, which is part of the reason why it's so much more stable than the complex with six ammonia ligands.

The concept of ligand exchange energy is crucial for understanding the stability of coordination complexes. Ligand exchange energy refers to the net energy change that occurs when ligands are replaced in a coordination complex. This process is influenced by both enthalpic and entropic factors.

In a chelated complex, like \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\), the bidentate ligand 'en' displaces more than one monodentate ligand (such as NH3) to form a more stable complex. This exchange is often energetically favorable due to what's known as 'ligand exchange enthalpy', which involves release or absorption of heat. Additionally, 'ligand exchange entropy' plays a role, typically contributing to stability when a single ligand replaces multiple ligands, as it often leads to a decrease in disorder.

The stable ring structures form with an apparent 'entropy gain' – an important concept because it defies the more intuitive scenario where forming more bonds would lead to a decrease in entropy. However, since chelating ligands like 'en' replace multiple individual ligands, they create fewer particles in solution, leading to a net increase in entropy, which contributes to the overall stability of the complex.

The stability of coordination compounds is a multifaceted topic, with influence from various physical and chemical factors. Stability can refer to several aspects, such as thermodynamic stability, which involves energy considerations, and kinetic stability, which pertains to the rate at which the complex reacts or changes.

A coordination complex with bidentate ligands, such as \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\), typically exhibits greater thermodynamic stability. This is due to the ring structures formed, which are energetically favorable and less prone to decomposition. This attribute comes from the 'chelate effect', which is a specific case of the 'macrocycle effect'. These effects explain why complexes like \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}\) are substantially more stable compared to complexes with only monodentate ligands.

In summary, the enhanced stability is not because NH3 is an exceptionally weak ligand or because it evaporates easily, but rather due to the stronger and more energetically stable chelating capability of 'en'. Understanding this concept helps to grasp why certain coordination compounds can be much more stable than others that may appear similar at first glance.