Problem 79 - Chemistry An Atoms First Approach

Question

Give formulas for the following. a. hexakis(pyridine) cobalt(III) chloride b. pentaammineiodochromium(III) iodide c. tris(ethylenediamine)nickel(II) bromide d. potassium tetracyanonickelate(II) e. tetraamminedichloroplatinum(IV) tetrachloroplatinate(II)

Step-by-Step Solution

Answer

The formulas for the given compounds are: a. \([Co(pyridine)_6]Cl_3\) b. \([Cr(NH_3)_5I]I_2\) c. \([Ni(ethylenediamine)_3]Br_2\) d. \( K_2[Ni(CN)_4]\) e. \([Pt(NH_3)_4Cl_2][PtCl_4]\)

Step 1: Identify the ligands and metal ion

We have hexakis(pyridine) as the ligand and Cobalt(III) as the central metal ion.

Step 2: Determine the charges of the components

Cobalt(III) refers to a 3+ charge on the Cobalt ion. The pyridine ligand is neutral.

Step 3: Write the formula

With 6 pyridine ligands and one Cobalt(III) ion, the complex cation will be [Co(pyridine)_6]^3+. To balance the charge, we need 3 chloride ions: Cl^-. The final formula is [Co(pyridine)_6]Cl_3. b. pentaammineiodochromium(III) iodide

Step 4: Identify the ligands and metal ion

We have pentaammine and iodo as the ligands and Chromium(III) as the central metal ion.

Step 5: Determine the charges of the components

Chromium(III) refers to a 3+ charge on the Chromium ion. The ammine ligands are neutral, and the iodo ligand has a 1- charge.

Step 6: Write the formula

The complex cation will be [Cr(NH_3)_5I]^2+. To balance the charge, we need 2 iodide ions: I^-. The final formula is [Cr(NH_3)_5I]I_2. c. tris(ethylenediamine)nickel(II) bromide

Step 7: Identify the ligands and metal ion

We have tris(ethylenediamine) as the ligand and Nickel(II) as the central metal ion.

Step 8: Determine the charges of the components

Nickel(II) refers to a 2+ charge on the Nickel ion. The ethylenediamine ligand is neutral.

Step 9: Write the formula

The complex cation will be [Ni(ethylenediamine)_3]^2+. To balance the charge, we need 2 bromide ions: Br^-. The final formula is [Ni(ethylenediamine)_3]Br_2. d. potassium tetracyanonickelate(II)

Step 10: Identify the ligands and metal ion

We have tetracyano as the ligand and Nickel(II) as the central metal ion, and potassium as the counterion.

Step 11: Determine the charges of the components

Nickel(II) refers to a 2+ charge on the Nickel ion. The cyanide ligand has a 1- charge. The potassium ion has a 1+ charge.

Step 12: Write the formula

The complex anion will be [Ni(CN)_4]^2-. To balance the charge, we need 2 potassium ions: K^+. The final formula is K_2[Ni(CN)_4]. e. tetraamminedichloroplatinum(IV) tetrachloroplatinate(II)

Step 13: Identify the ligands and metal ion

We have tetraammine and dichloro as the ligands, and Platinum(IV) as the first central metal ion. For the second part, we have tetrachloro as the ligand and Platinum(II) as the second central metal ion.

Step 14: Determine the charges of the components

Platinum(IV) refers to a 4+ charge on the first Platinum ion. Platinum(II) refers to a 2+ charge on the second Platinum ion. The ammine ligands are neutral, while the chloro ligands have a 1- charge.

Step 15: Write the formula

The complex cation will be [Pt(NH_3)_4Cl_2]^2+. The complex anion will be [PtCl_4]^2-. Combining the two, the final formula is [Pt(NH_3)_4Cl_2][PtCl_4].

Key Concepts

Ligands

In coordination chemistry, ligands are ions or molecules that can donate a pair of electrons to a metal atom to form a coordination complex. They act as "attachments" to the central metal and hold the complex together. Ligands come in many varieties, each impacting the properties of the coordination complex in different ways. For example, in hexakis(pyridine) cobalt(III) chloride, pyridine acts as a ligand. It's neutral and binds to the cobalt ion through its lone pair of electrons. Ligands can vary in charge and number of donor atoms. Some common types include:

• Neutral ligands like water (H₂O) and ammonia (NH₃).
• Negatively charged ligands like chloride (Cl^-) or cyanide (CN^-).
• Polydentate ligands, which have multiple "teeth" or binding sites, such as ethylenediamine in tris(ethylenediamine)nickel(II) bromide.

The nature and arrangement of ligands significantly affect the color, reactivity, and stability of the coordination compound.

Complex Ions

Complex ions are charged species consisting of a metal ion at the center surrounded by molecules or anions, known as ligands. These ions form when ligands donate their electron pairs to the central metal ion, creating a coordinate covalent bond. For instance, pentaammineiodochromium(III) iodide forms a complex ion where the chromium ion is surrounded by five ammonia ligands and one iodide ligand. Complex ions can be categorized based on their charge:

• Cations: Positive charge; example: [Co(pyridine)₆]³⁺.
• Anions: Negative charge; example: [Ni(CN)₄]^2-.

The specific arrangement and the number of ligands around the central ion dictate the complex's geometry, such as octahedral, square planar, or tetrahedral shapes, influencing its chemical behavior.

Oxidation State

The oxidation state (or oxidation number) of an element in a compound helps us understand how electrons are distributed among the atoms in a molecule. In coordination chemistry, the oxidation state of the central metal ion is crucial for determining the formula and charge balance of the complex. For example, in hexaamminecobalt(III) chloride, the cobalt has an oxidation state of +3, indicated by the "III". Understanding oxidation states involves evaluating:

• The charge of the metal ion itself.
• The charges from bonded ligands (neutral or anionic).
• The overall charge of the complex.

Calculating the oxidation state is essential for writing correct chemical formulas and understanding the electron transfer processes in redox reactions. Often, the oxidation state is indicated in parentheses after the metal's name in the complex nomenclature.

Chemical Formulas

Chemical formulas in coordination compounds provide a systematic way to represent the composition and structure of the complex. Writing these formulas involves identifying the central metal ion, the type and number of ligands, and balancing the charges.To derive the chemical formula:

• Identify the central metal and its oxidation state.
• Note the ligands' names, types, and charges.
• Balance the overall charge using additional anions or cations if needed.

For example, with potasssium tetracyanonickelate(II), the formula is K₂[Ni(CN)₄]. Here, the complex is an anion \([Ni(CN)_4]^{2-}\) and is charge-balanced by 2 K⁺ cations, forming a neutral compound.Writing correct chemical formulas is a fundamental aspect of mastering coordination chemistry, allowing chemists to communicate the exact nature of compounds clearly and efficiently.