Question

In which of the following pairs, the EAN of the central metal atom is not the same? (a) \(\left[\mathrm{FeF}_{6}\right]^{3+}\) and \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) (b) \(\left[\mathrm{Fe}\left(\mathrm{CN}_{6}\right)\right]^{3}\) and \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) (c) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) and \(\left[\mathrm{Cr}(\mathrm{CN})_{6}\right]^{3-}\) (d) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) and \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2}\)

Short answer

(a) Different EANs: 29 and 35.

Step-by-step solution

Understanding EAN

The Effective Atomic Number (EAN) is the total number of electrons surrounding the central metal atom after forming the complex. It's given by the sum of the metal atom's electrons and the electrons it gains from the ligands. If the EAN of two complexes is different, then they do not have the same EAN.

Calculating EAN for Pair (a)

For \([\text{FeF}_6]^{3+}\), Fe is in +3 oxidation state. Fe^3+ has 23 electrons (26 from Fe - 3 for the oxidation state). Six F^- ligands each donate 1 electron. EAN = 23 (Fe) + 6 (from F^- ligands) = 29. For \([\text{Fe(CN)}_6]^{3-}\), Fe is in +3 oxidation state as well. Thus, Fe^3+ again has 23 electrons. Each of the 6 CN^- ligands donates 2 electrons to Fe, hence the EAN = 23 (Fe) + 12 (from CN^- ligands) = 35.

Calculating EAN for Pair (b)

For \([\text{Fe(CN)}_6]^{3-}\), as calculated before, the EAN is 35. For \([\text{Fe(CN)}_6]^{4-}\), Fe is in +2 oxidation state. Fe^2+ has 24 electrons (26 from Fe - 2). Each CN^- donates 2 electrons, making the EAN = 24 (Fe) + 12 (from CN^- ligands) = 36.

Calculating EAN for Pair (c)

For \([\text{Cr(NH}_3)_6]^{3+}\), Cr is in +3 oxidation state. Cr^3+ has 21 electrons (24 from Cr - 3). Each NH_3 donates 2 electrons, thus EAN = 21 (Cr) + 12 (from NH_3 ligands) = 33.For \([\text{Cr(CN)}_6]^{3-}\), Cr is in +3 oxidation state. Cr^3+ therefore has 21 electrons. Each of the 6 CN^- ligands donates 2 electrons, making the EAN = 21 (Cr) + 12 (from CN^- ligands) = 33.

Calculating EAN for Pair (d)

For \([\text{Ni(CO)}_4]\), Ni is in 0 oxidation state, having 28 electrons. Each CO donates 2 electrons, so EAN = 28 (Ni) + 8 (from CO ligands) = 36.For \([\text{Ni(CN)}_4]^{2-}\), Ni is in +2 oxidation state. Ni^2+ has 26 electrons (28 from Ni - 2). Each CN^- donates 2 electrons, resulting in EAN = 26 (Ni) + 8 (from CN^- ligands) = 34.

Key concepts

coordination compounds

Coordination compounds are fascinating chemical entities that consist of a central metal atom or ion surrounded by molecules or ions known as ligands. These compounds offer a wide array of structural possibilities depending on the nature and number of ligands attached.
They often showcase distinctive colors and magnetic properties, making them crucial in both industrial and biological processes. The core concept here is the coordination number, which tells us how many ligand atoms are attached to the central metal atom.
The coordination sphere is determined by the geometry of these ligands around the metal. Examples include octahedral, tetrahedral, and square planar configurations. Understanding coordination compounds is key to exploiting their applications in catalysis, medicine, and environmental chemistry.

oxidation state

The oxidation state of a central metal atom in coordination compounds is a vital concept that indicates the charge it would have if all the ligands were removed along with the electron pairs shared with the central atom.
It's expressed as either a positive or negative integer. To determine the correct oxidation state, identify the charge of the ligands and the total charge of the compound.
Metal atoms can exhibit different oxidation states, influencing their reactivity and the properties of the entire compound. For example, in the complex \([ ext{Fe(CN)}_6]^{3-}\), iron exhibits an oxidation state of +3, while in \([ ext{Ni(CO)}_4]\), nickel has an oxidation state of 0. Accurate calculation of the oxidation state is essential for predicting the behavior of coordination compounds in various reactions.

electron count

Electron count in coordination compounds is the sum of the electrons present in the metal and those donated by the ligands. It provides a snapshot of the number of electrons surrounding the metal center, which influences the compound's stability.
The "18-electron rule" is a simple tool used to predict stability; compounds with 18 valence electrons around the metal center are often stable.
To find the electron count:

• Start with the valence electrons of the metal.
• Adjust for the metal's oxidation state.
• Add electrons from the ligands.

This rule is particularly helpful in simplifying electronic configurations and anticipating physical and chemical properties, including the compound's magnetic nature and reactivity.

ligands

Ligands are crucial components of coordination compounds. They are atoms, ions, or molecules capable of donating electron pairs to a metal atom or ion to form a coordination bond.
Ligands can be categorized by the number of donor atoms they contain.

• Monodentate ligands, like Cl^- and CN^-, with a single donor atom.
• Bidentate ligands, like ethylenediamine, with two donor atoms.
• Polydentate ligands, commonly referred to as chelating agents, which form multiple bonds.

The nature of the ligand affects the geometry, reactivity, and electronic properties of a coordination compound. Ligands contribute to the Effective Atomic Number (EAN) by donating electrons to the metal center, thus playing a significant role in determining the compound's stability and overall behavior.