Coordination Compounds & Complex Ions

In the complex ion [Fe(CN)₆]⁴⁻, the cyanide ion (CN⁻) has a charge of -1. Since there are six cyanide ions, their total contribution to the charge is -6. To find the oxidation state of iron (Fe), we set up the equation: x + (-6) = -4, where x is the oxidation state of iron. Solving for x gives x = +2. Thus, the oxidation state of iron in this complex is +2.

In the complex ion [Co(NH₃)₆]³⁺, cobalt (Co) is surrounded by six ammonia (NH₃) ligands. The coordination number refers to the number of ligand donor atoms bonded to the central metal ion. Since each NH₃ donates one lone pair of electrons to form a coordinate bond with cobalt, the presence of six NH₃ ligands results in a coordination number of 6. This octahedral arrangement is typical for complexes with six ligands, leading to the stability and characteristic geometry of the complex.

In the complex ion [Fe(H₂O)₆]²⁺, the overall charge is +2. Water (H₂O) is a neutral ligand, contributing no charge. Therefore, the oxidation state of iron must balance the overall charge of the complex. Since there are six neutral water molecules, the oxidation state of iron must be +2 to ensure the total charge sums to +2. Thus, iron in this coordination complex has an oxidation state of +2.

In the complex ion [Co(NO₂)₆]³⁻, the overall charge is -3. Each nitrite ion (NO₂) has a charge of -1, and with six nitrite ions, the total contribution is -6. To balance this and achieve the overall charge of -3, cobalt must have an oxidation state of +3. This can be calculated by setting up the equation: x (oxidation state of Co) + (-6) = -3, leading to x = +3. Thus, cobalt's oxidation state in this complex is +3.

The complex [Ni(NH₃)₆]²⁺ consists of a nickel ion (Ni) surrounded by six ammonia (NH₃) ligands. Ammonia is a neutral ligand, meaning it does not contribute any charge to the complex. The notation "²⁺" indicates that the overall charge of the complex is +2, which comes from the nickel ion itself, which has a +2 oxidation state in this complex. Therefore, the total charge of the complex is +2.

The geometry of [Zn(NH₃)₄]²⁺ is tetrahedral due to its coordination number of four. In coordination chemistry, when a central metal atom is surrounded by four ligands, the spatial arrangement that minimizes repulsion between the ligands is tetrahedral. Zinc, being a d-block metal, forms stable complexes with ammonia, and the four ammonia molecules coordinate to the zinc ion in a tetrahedral shape, resulting in a bond angle of approximately 109.5 degrees. This arrangement allows for optimal spatial distribution of the ligands around the zinc ion.

In the complex ion [NiCl₄]²⁻, the overall charge is -2. Each chloride ion (Cl⁻) contributes a -1 charge, and there are four chloride ions, totaling -4. To balance this and achieve the overall charge of -2, the oxidation state of nickel (Ni) must be +2. Therefore, the equation can be set up as follows: oxidation state of Ni + (4 × -1) = -2. Solving this gives Ni an oxidation state of +2.

In the complex [Ni(CO)₄], nickel is surrounded by four carbon monoxide (CO) ligands. CO is a neutral ligand, meaning it does not contribute any charge to the overall oxidation state of the complex. Since the complex is neutral and the ligands do not affect the charge, the oxidation state of nickel must be zero to balance the overall charge of the complex. Thus, nickel in [Ni(CO)₄] has an oxidation state of 0.

In the complex ion [Cu(NH₃)₄]²⁺, ammonia (NH₃) acts as a neutral ligand, contributing no charge. The overall charge of the complex is +2. Let the oxidation state of copper be x. Since there are four NH₃ ligands, the equation can be set up as x + 0 = +2. Solving for x gives x = +2. Thus, the oxidation state of copper in this complex is +2.

[Ni(CO)₄] contains nickel in the zero oxidation state because carbon monoxide (CO) is a strong field ligand that does not affect the oxidation state of nickel. In this complex, nickel does not lose any electrons, maintaining its oxidation state at zero. In contrast, the other complexes involve metals in higher oxidation states due to the presence of ligands that either donate electrons or stabilize the metal in a charged form. Thus, [Ni(CO)₄] is unique in having nickel in the zero oxidation state.