To explain the observations using the Valence Bond Theory (VBT), we need to consider the bonding in the two complexes.

In the complex [Ni(CN)4]2-, the nickel ion (Ni2+) forms coordination bonds with four cyanide ligands (CN-). Each cyanide ligand donates a lone pair of electrons to form a coordinate covalent bond with the nickel ion. In the VBT, this bonding is described as the overlap of the nickel d orbitals with the ligand orbitals.

In [Ni(CN)4]2-, the nickel ion has a d8 electron configuration, meaning it has eight electrons in its d orbitals. The cyanide ligands are strong-field ligands, which means they cause a large splitting of the d orbitals. As a result, the d orbitals split into two sets: a lower energy set (t2g) and a higher energy set (eg).

In this complex, all the d orbitals are fully occupied by electrons from the nickel ion, resulting in a low-spin configuration. Since all the electrons are paired, the complex is diamagnetic. The geometry of [Ni(CN)4]2- is tetrahedral, as the four cyanide ligands are arranged around the nickel ion in a tetrahedral shape.

On the other hand, in the complex [NiCl4]2-, the nickel ion forms coordination bonds with four chloride ligands (Cl-). Similar to the previous complex, each chloride ligand donates a lone pair of electrons to form a coordinate covalent bond with the nickel ion.

In [NiCl4]2-, the nickel ion also has a d8 electron configuration. However, chloride ligands are weak-field ligands, causing a smaller splitting of the d orbitals compared to cyanide ligands. As a result, the d orbitals split into two sets: a slightly lower energy set (t2g) and a slightly higher energy set (eg).

In this complex, only two of the d orbitals are fully occupied by electrons from the nickel ion, while the other two d orbitals remain unpaired. This results in a high-spin configuration with two unpaired electrons, making the complex paramagnetic. The geometry of [NiCl4]2- is square planar, as the four chloride ligands are arranged around the nickel ion in a square planar shape.

In summary, the complex [Ni(CN)4]2- is diamagnetic because all the d orbitals are fully occupied by paired electrons, while [NiCl4]2- is paramagnetic due to the presence of two unpaired electrons. The geometries of the two complexes are tetrahedral for [Ni(CN)4]2- and square planar for [NiCl4]2-.

Question

To explain the observations using the Valence Bond Theory (VBT), we need to consider the bonding in the two complexes.

In the complex [Ni(CN)4]2-, the nickel ion (Ni2+) forms coordination bonds with four cyanide ligands (CN-). Each cyanide ligand donates a lone pair of electrons to form a coordinate covalent bond with the nickel ion. In the VBT, this bonding is described as the overlap of the nickel d orbitals with the ligand orbitals.

In [Ni(CN)4]2-, the nickel ion has a d8 electron configuration, meaning it has eight electrons in its d orbitals. The cyanide ligands are strong-field ligands, which means they cause a large splitting of the d orbitals. As a result, the d orbitals split into two sets: a lower energy set (t2g) and a higher energy set (eg).

In this complex, all the d orbitals are fully occupied by electrons from the nickel ion, resulting in a low-spin configuration. Since all the electrons are paired, the complex is diamagnetic. The geometry of [Ni(CN)4]2- is tetrahedral, as the four cyanide ligands are arranged around the nickel ion in a tetrahedral shape.

On the other hand, in the complex [NiCl4]2-, the nickel ion forms coordination bonds with four chloride ligands (Cl-). Similar to the previous complex, each chloride ligand donates a lone pair of electrons to form a coordinate covalent bond with the nickel ion.

In [NiCl4]2-, the nickel ion also has a d8 electron configuration. However, chloride ligands are weak-field ligands, causing a smaller splitting of the d orbitals compared to cyanide ligands. As a result, the d orbitals split into two sets: a slightly lower energy set (t2g) and a slightly higher energy set (eg).

In this complex, only two of the d orbitals are fully occupied by electrons from the nickel ion, while the other two d orbitals remain unpaired. This results in a high-spin configuration with two unpaired electrons, making the complex paramagnetic. The geometry of [NiCl4]2- is square planar, as the four chloride ligands are arranged around the nickel ion in a square planar shape.

In summary, the complex [Ni(CN)4]2- is diamagnetic because all the d orbitals are fully occupied by paired electrons, while [NiCl4]2- is paramagnetic due to the presence of two unpaired electrons. The geometries of the two complexes are tetrahedral for [Ni(CN)4]2- and square planar for [NiCl4]2-.

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