Problem 35 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write the formula for each of the following compounds, being sure to use brackets to indicate the coordination sphere: (a) triamminetriaquachromium(III) nitrate (b) dichlorobis(ethylenediamine)platinum(II) (c) pentacarbonyliron(0) (d) ammonium diaquabis(oxalato)Co(II) (e) tris(bipyridyl)cobalt(III) sulfate

Step-by-Step Solution

Answer

The formulas for the given compounds are: (a) \([Cr(NH₃)₃(H₂O)₃](NO₃)₃\) (b) \([PtCl₂(en)₂]²+\) (c) \([Fe(CO)₅]₀\) (d) \((NH₄)₂[Co(H₂O)₂(C₂O₄)₂]\) (e) \([Co(bipy)₃]₂(SO₄)₃\)

Step 1: Identify the central metal ion, ligands, and counter-ion

The central metal ion is chromium(III) (Cr³+), the ligands are three ammine groups (NH₃) and three aqua groups (H₂O), and the counter-ion is nitrate (NO₃⁻).

Step 2: Write the formula for the complex

Since there are multiple ligands, we need to indicate the coordination sphere using brackets. The formula for this complex is [Cr(NH₃)₃(H₂O)₃]³+.

Step 3: Write the formula including the counter-ion

Since the complex has a 3+ charge, and the counter-ion (NO₃⁻) has a -1 charge, we need three nitrate ions to balance the overall charge. The formula is [Cr(NH₃)₃(H₂O)₃](NO₃)₃. (b) dichlorobis(ethylenediamine)platinum(II)

Step 4: Identify the central metal ion, ligands, and counter-ion

The central metal ion is platinum(II) (Pt²+), the ligands are two chloro groups (Cl⁻) and two ethylenediamine molecules with the abbreviated name of en (C₂H₄(NH₂)₂), and there is no counter-ion indicated.

Step 5: Write the formula for the complex

The formula for this complex is [PtCl₂(en)₂]²+. (c) pentacarbonyliron(0)

Step 6: Identify the central metal ion and ligands

The central metal ion is iron(0), and the ligands are five carbonyl groups (CO).

Step 7: Write the formula for the complex

The formula for this complex is [Fe(CO)₅]₀. (d) ammonium diaquabis(oxalato)Co(II)

Step 8: Identify the central metal ion, ligands, and counter-ion

The central metal ion is cobalt(II) (Co²+), the ligands are two aqua groups (H₂O) and two oxalato groups (C₂O₄²⁻), and the counter-ion is ammonium (NH₄⁺).

Step 9: Write the formula for the complex

The formula for this complex is [Co(H₂O)₂(C₂O₄)₂]²-.

Step 10: Write the formula including the counter-ion

Since the complex has a 2- charge, and the counter-ion (NH₄⁺) has a +1 charge, we need two ammonium ions to balance the overall charge. The formula is (NH₄)₂[Co(H₂O)₂(C₂O₄)₂]. (e) tris(bipyridyl)cobalt(III) sulfate

Step 11: Identify the central metal ion, ligands, and counter-ion

The central metal ion is cobalt(III) (Co³+), the ligands are three bipyridyl molecules with the abbreviated name of bipy (C₁₀H₈N₂), and the counter-ion is sulfate (SO₄²⁻).

Step 12: Write the formula for the complex

The formula for this complex is [Co(bipy)₃]³+.

Step 13: Write the formula including the counter-ion

Since the complex has a 3+ charge, and the counter-ion (SO₄²⁻) has a -2 charge, we need two cobalt complexes and three sulfate ions to balance the overall charge. The formula is [Co(bipy)₃]₂(SO₄)₃.

Key Concepts

Complex Ions

Complex ions are the heart of coordination chemistry. They consist of a central metal atom or ion and are surrounded by molecules or ions known as ligands. This central metal atom is at the core of the structure, forming multiple bonds with the ligands.

The metal ion, often found in transition elements, displays high electronegativity and can exhibit variable oxidation states. This allows them to form stable complexes with ligands through coordinate covalent bonds. These complexes are often enclosed by brackets to indicate the coordination sphere. This is crucial when writing their chemical formulas.

• Example: In the complex [Cr(NH₃)₃(H₂O)₃]³+, chromium (Cr) is the central metal ion.
• The formula's brackets indicate that within lies the whole coordination sphere.

Overall charge balance is achieved when metal ions form complex ions, sometimes attracting counter-ions that balance out their charge.

Ligands

Ligands are ions or molecules that bind to the central metal ion in a complex. They are integral in determining the properties and reactivity of the complex ion.

Ligands can be classified based on the number of binding sites or donor atoms:

• Monodentate Ligands: These ligands have a single donor atom that binds to the metal ion, like chloride (Cl⁻) or ammonia (NH₃).
• Bidentate Ligands: These ligands bind through two donor atoms, creating a more stable chelate formation. Examples include ethylenediamine (en) and oxalate (C₂O₄²⁻).

Ligands affect the color, magnetic properties, and stability of the complex. For instance, in the compound [Co(bipy)₃]³+, bipyridyl acts as a ligand creating a stable octahedral structure around cobalt.

Understanding ligands and their interactions with metal ions is essential in predicting the behavior and application of coordination compounds.

Metal Ions

At the center of complex ions, metal ions serve as the primary bonding site for ligands. These are generally transition metals due to their ability to form various oxidation states and stable complex geometries.

The metal ion contributes significantly to the properties of the coordination compound. It dictates:

• The number of ligands the complex can accommodate, also called the coordination number.
• The oxidation state of the metal, impacting the overall charge.

For example, chromium in [Cr(NH₃)₃(H₂O)₃]³+ exerts a 3+ oxidation state. Likewise, cobalt in [Co(bipy)₃]³+ has to accommodate the 3+ charge, affecting how the complex interacts with counter-ions to maintain charge neutrality.

Metal ions are crucial in fields ranging from industrial catalysts to biological systems, where they are key in enzymes and active sites of proteins.

Counter Ions

Counter ions are essential in maintaining charge balance in coordination compounds. While they are not part of the coordination sphere, their role in charge neutrality cannot be overlooked.

In many complexes, counter ions are required to balance out the charge derived from the metal ions and the ligands in the coordination sphere:

• In [Cr(NH₃)₃(H₂O)₃](NO₃)₃, nitrate ions (NO₃⁻) serve as counter ions for the 3+ charge from the chromium complex.
• In (NH₄)₂[Co(H₂O)₂(C₂O₄)₂], ammonium ions (NH₄⁺) neutralize the 2- charge from the cobalt complex.

Counter ions are important not only for maintaining the compound's stability but also for ensuring the crystal structure when the complex solidifies.

They significantly influence solubility and conductivity of the compounds in solutions, making them key to understanding the physical attributes of coordination compounds.