Problem 27 - Chemistry and Chemical Reactivity

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)

Step-by-Step Solution

Answer

(a) \([Ni(en)_2Cl_2]\); (b) \(K_2[PtCl_4]\); (c) \(K[Cu(CN)_2]\); (d) \([Fe(NH_3)_4(H_2O)_2]^{2+}\).

Step 1: Understand Complex Naming Convention

In coordination compounds, the name is composed of the ligands followed by the central metal atom or ion. Prefixes like 'dichloro' indicate the number of such ligands, and Roman numerals denote the oxidation state of the central metal.

Step 2: Break Down Compound Components - Part (a)

For 'dichlorobis(ethylenediamine)nickel(II)', the ligands are 2 chloride ions (Cl-) and 2 ethylenediamine molecules (neutral ligand). Nickel has an oxidation state of +2. Thus, the formula is \([Ni(en)_2Cl_2]\), where 'en' represents ethylenediamine.

Step 3: Break Down Compound Components - Part (b)

'Potassium tetrachloroplatinate(II)' consists of potassium cations (K+) and the complex ion \([PtCl_4]^{2-}\). Platinum is in the +2 oxidation state, and there are 4 chloride ions. To balance charges, the formula is \(K_2[PtCl_4]\).

Step 4: Break Down Compound Components - Part (c)

'Potassium dicyanocuprate(I)' consists of potassium cations (K+) and the anion \([Cu(CN)_2]^{-}\), with copper in the +1 oxidation state and 2 cyanide ions. The formula is \(K[Cu(CN)_2]\).

Step 5: Break Down Compound Components - Part (d)

'Tetraamminediaquairon(II)' involves 4 ammonia molecules (neutral ligands) and 2 water molecules (also neutral) coordinating with iron in the +2 oxidation state. The formula is \([Fe(NH_3)_4(H_2O)_2]^{2+}\).

Key Concepts

Complex Ions

Complex ions are fascinating components in coordination chemistry. They consist of a central metal atom or ion bound to a group of molecules or ions called ligands. These ligands can be either neutral or charged, and they form coordinate covalent bonds with the metal center. Understanding the structure of complex ions is key:

• The central metal acts as a Lewis acid (electron-pair acceptor).
• Ligands act as Lewis bases (electron-pair donors).

Complex ions can display various geometries such as tetrahedral, octahedral, or square planar, based on the number of ligands and the specific coordination of the metal. For example, in the complex ion \([PtCl_4]^{2-}\), platinum is coordinated with four chloride ions, giving it a square planar geometry.

Oxidation States

The oxidation state is a crucial concept in chemistry, particularly in coordination compounds. It describes the degree of oxidation of an atom in a compound, indicating how many electrons it has gained or lost. In coordination chemistry:

• The oxidation state helps us determine the formula of a compound.
• It is signified by a Roman numeral in parentheses after the name of the metal.

For example, in "nickel(II)" the II denotes an oxidation state of +2 for nickel. This means nickel has lost two electrons compared to its elemental state. Understanding oxidation states allows us to balance overall charges in coordination compounds, such as maintaining electrical neutrality in \([Ni(en)_2Cl_2]\) where nickel has an oxidation state of +2.

Ligand Naming

In coordination chemistry, ligands are named as part of the full name of a coordination compound. They can be simple ions like chloride, or more complex molecules such as ethylenediamine (en). Here are some key aspects:

• The names of anionic ligands end with the suffix '-o'
• Neutral ligands are typically named as molecules, although common ones like water (aqua) or ammonia (ammine) have standard names

Prefixes are used to indicate the number of each type of ligand: 'di-' for two, 'tri-' for three, etc. Additionally, when the ligand name itself contains a number or prefix, such as ethylenediamine, alternative prefixes like 'bis-' may be used to avoid confusion.

Chemical Formulas

Creating chemical formulas for coordination compounds involves understanding the balance of ions and the arrangement of ligands around the metal center. Key steps include:

• Identifying the central metal and its oxidation state.
• Listing ligands and their charges.
• Balancing the overall charge by ensuring the sum of oxidation states and ligand charges equals the charge of the ion.

For example, in the structure \(K_2[PtCl_4]\), two potassium ions balance the 2- charge from the complex ion \([PtCl_4]^{2-}\). Understanding how these components fit together helps in constructing the correct chemical formula of any coordination compound.