Question

Write formulas for the following ions or compounds. (a) dichlorobis(ethylenediamine) nickel(II) (b) potassium tetrachloroplatinate(II) (c) potassium dicyanocuprate(I) (d) tetraamminediaquairon(II)

Short answer

(a) [Ni(en)2Cl2], (b) K2[PtCl4], (c) K2[Cu(CN)2], (d) [Fe(NH3)4(H2O)2]

Step-by-step solution

Identify the Ligands and Central Metal Atom in Each Compound

For each compound, first identify the ligands, the central metal atom, and its oxidation state. In (a), dichlorobis(ethylenediamine) refers to the ligands dichloro and ethylenediamine around a nickel(II) center. In (b), tetrachloroplatinate(II) involves chloride ligands around a platinum(II) center. For (c), dicyanocuprate(I) has cyanide ligands around a copper(I) center. Lastly, in (d), tetraamminediaquairon(II) has ammonia and water ligands around an iron(II) center.

Determine the Charge of Each Ligand and Overall Complex

Each ligand has a specific charge: chloride (Cl^-) is -1, ethylenediamine is neutral, ammonia (NH3) is neutral, cyanide (CN^-) is -1, and water (H2O) is neutral. For (a), two chlorides contribute -2 overall charge. For (b), four chlorides have a -4 overall charge. In (c), two cyanides also contribute -2 overall charge. In (d), the neutral ammonia and water don't affect the charge.

Calculate the Oxidation State of the Metal

The oxidation state of the metal is given in the name. Ensure that the sum of the charges of the ligands and the metal equals the charge of the whole complex. For example, Nickel(II) in (a) requires a +2 state to balance out the -2 from the chlorides. Similarly, Platinate(II) in (b) requires +2 to balance -4 from the chlorides plus K2 +4. Cuprate(I) in (c) is +1, balancing out the -2 from two cyanides with two K^+. Iron(II) in (d) should be +2.

Write the Formula for Each Compound

For (a), combine the components as [Ni(en)2Cl2], where "en" stands for ethylenediamine. For (b), use K2[PtCl4] as two K^+ are needed to balance the -2 charge of the colored complex [PtCl4]^2-. In (c), write as K2[Cu(CN)2] with a similar charge balance. Finally, for (d), present as [Fe(NH3)4(H2O)2]^2+, where the whole molecule is neutral as Iron balances in reality based on structure.

Key concepts

Ligands

In coordination chemistry, ligands play a crucial role in forming complex ions. They are ions or molecules that donate a pair of electrons to a central metal atom to form a coordination bond. Ligands can be of different types:

• Neutral Ligands: Molecules like ammonia (NH₃) and water (H₂O) do not carry any charge.
• Charged Ligands: Ions like chloride (Cl⁻) and cyanide (CN⁻) provide electrons for bonding but come with an inherent charge.

This electron donation results in the formation of a coordination complex, where the central metal and the attached ligands act as a single entity. For instance, in the complex dichlorobis(ethylenediamine) nickel(II), ethylenediamine (en) acts as a neutral bidentate ligand, coordinating with the nickel metal center, while chloride ions provide additional coordination with its negative charge to stabilize the structure.

Oxidation States

The oxidation state of a metal in a coordination complex is key to understanding how charges are distributed among the compound. Determining the oxidation state involves:

• Recognizing the charges of the ligands.
• Considering the overall charge of the compound.

For example, in dichlorobis(ethylenediamine) nickel(II), nickel has an oxidation state of +2 to counterbalance the -1 charge from each chloride ion, resulting in a neutral complex. This balance of charges ensures that the entire compound holds together successfully without an overall charge. The nomenclature often provides a clue to the oxidation state:

• "Nickel(II)" suggests a +2 oxidation state for nickel.
• Similarly, "copper(I)" indicates a +1 state for copper.

Complex Ions

In coordination chemistry, complex ions are formed by the association of a central metal ion with surrounding ligands. They exhibit unique properties:

• Coordination Number: The number of ligand atoms directly bonded to the central metal is referred to as the coordination number. For instance, in tetraamminediaquairon(II), the central iron ion coordinates with a total of 6 ligand atoms (4 from ammonia and 2 from water).
• Shape and Geometry: The spatial arrangement of ligands around the metal center influences the geometry of the complex, such as square planar or octahedral shapes.

The structure of complex ions can greatly influence their chemical reactivity and stability. A well-considered structure, matching the oxidation state and ligand properties, ensures that the complex ion remains stable and nicely defined in its chemical behavior.

Chemical Nomenclature

Chemical nomenclature in coordination chemistry has rules to systematically name the complex compounds, making it easier to convey precise information about the structure of a compound. The naming involves:

• Naming ligands first, followed by the central metal.
• Specifying the number of each type of ligand using prefixes like "di-", "tri-", and so forth.
• Indicating the oxidation state of the metal in Roman numerals in parentheses.

For example, in potassium tetrachloroplatinate(II), "tetra" indicates four chloride ligands, "chloroplatinate" refers to the platinum central atom with chloride ligands, and "(II)" denotes the oxidation state of the platinum. All these elements together define the complete chemical and structural identity of the complex, adhering to standard IUPAC naming conventions.