Problem 31 - Chemistry and Chemical Reactivity

Question

Give the name or formula for each ion or compound, as appropriate. (a) pentaaquahydroxoiron(III) ion (b) \(\mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\) (c) \(\mathrm{K}\left[\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]\) (d) ammonium tetrachloroplatinate(II)

Step-by-Step Solution

Answer

(a) \([\text{Fe}( ext{H}_2 ext{O})_5( ext{OH})]^{2+}\); (b) potassium tetracyanonickelate(II); (c) potassium diaquabis(oxalato)chromate(III); (d) \((\text{NH}_4)_2[\text{PtCl}_4]\).

Step 1: Determine the formula for (a)

The name given is "pentaaquahydroxoiron(III) ion." To break it down:- "penta" indicates 5.- "aqua" refers to water molecules, indicating 5 water ligands.- "hydroxo" is an OH group.- "iron(III)" indicates the iron is in the +3 oxidation state.Thus, the formula is \([\text{Fe}( ext{H}_2 ext{O})_5( ext{OH})]^{2+}\).

Step 2: Identify (b) chemical name

The compound is given as \(\text{K}_2[\text{Ni}( ext{CN})_4]\):- \(\text{Ni}\) stands for nickel.- \(\text{CN}\) is the cyanide ion.- \([\text{Ni}( ext{CN})_4]^{2-}\) indicates a complex ion with 4 cyanide ligands.- The potassium ions \(\text{K}^+\) balance the charge to 0.The name is potassium tetracyanonickelate(II).

Step 3: Identify (c) chemical name

The compound is \(\text{K}[\text{Cr}(\text{C}_2 ext{O}_4)_2(\text{H}_2 ext{O})_2]\):- \(\text{Cr}\) is chromium.- \(\text{C}_2 ext{O}_4\) is the oxalate ion, and having \(\text{C}_2 ext{O}_4)_2\) means two oxalate ligands.- \(\text{H}_2 ext{O}\) are aquo ligands.- The presence of \(\text{K}^+\) balances the charge.The name is potassium diaquabis(oxalato)chromate(III).

Step 4: Determine the formula for (d)

The compound name is "ammonium tetrachloroplatinate(II)":- "ammonium" suggests the presence of \(\text{NH}_4^+\) ions.- "tetrachloro" means 4 chlorine ligands, \(\text{Cl}^-\).- "platinum(II)" indicates a \(\text{Pt}^{2+}\) center.The formula is \((\text{NH}_4)_2[\text{PtCl}_4]\).

Key Concepts

Complex Ions

A complex ion is an ion composed of a central metal atom or ion bonded to one or more molecules or ions, known as ligands. These ligands donate electrons to the metal, forming coordinate covalent bonds. Complex ions are typically found in aqueous solutions and possess unique chemical behaviors due to their structure.
For example, in the ion \([ ext{Fe}(\text{H}_2\text{O})_5(\text{OH})]^{2+}\), iron serves as the central metal, surrounded by five water molecules and one hydroxide ion as ligands.

Understanding complex ions is important due to their role in:

• Chemical Reactivity - influencing how compounds react in solutions.
• Biological Systems - many enzymes and bio-molecules function by forming complex ions.
• Industrial Applications - such as in catalysis and coloration processes.

These ions can possess either positive or negative charges, depending on the number and nature of the ligands attached.

Oxidation States

Oxidation state refers to the degree of oxidation of an atom in a chemical compound. It is an indicator of the loss or gain of electrons by an atom. Determining the oxidation state is crucial in understanding the reactivity and chemical behavior of ions and molecules.
In coordination chemistry, the oxidation state of a metal indicates how many electrons have been removed from or added to the neutral element.

In the exercise, the iron in the "pentaaquahydroxoiron(III) ion" has an oxidation state of +3. This is denoted by the Roman numeral III in its name, implying that iron loses three electrons. Such details help in identifying:

• The charge of the complex ion.
• The need for charge balance with accompanying counter-ions, like \(\text{K}^+\) in certain structures.

Students often find mastering oxidation states helpful to predict chemical reactions and bonding patterns.

Ligands

Ligands are molecules or ions that attach to a central metal atom within a complex ion. They have at least one atom with a pair of electrons available to form a coordinate bond with the central metal.
Ligands are categorized based on their denticity, which is the number of donor atoms they contain:

• Monodentate: Single donor atom, such as \(\text{Cl}^-\) or \(\text{H}_2\text{O}\).
• Bidentate: Two donor atoms, such as in oxalate \(\text{C}_2\text{O}_4^{2-}\).
• Polydentate: Multiple donor atoms, forming more than two bonds.

In coordination compounds like "potassium tetracyanonickelate(II)," cyanide ions function as ligands, binding to nickel through their lone pairs of electrons.

The nature of the ligands influences:

• Stability - Complexes with more ligand bonds are typically more stable.
• Color - Transition metal complexes often exhibit distinct colors due to ligand field effects.

Recognizing the type and role of ligands can greatly enrich understanding of chemical nomenclature and reactivity.

Chemical Nomenclature

Chemical nomenclature refers to the systematic naming of chemical compounds, ensuring clear communication and identification among scientists. In coordination chemistry, naming follows specific conventions to describe the structure and composition of the compound.
The key rules in coordination compound nomenclature include:

• The name of the cation is stated before the anion.
• Ligands are named before the metal; for example, in "pentaaquahydroxoiron(III) ion," the ligands water and hydroxide are named before iron.
• Anionic ligands typically end in "-o," such as hydroxido for \(\text{OH}^-\).
• For metals with variable oxidation states, the oxidation number is given in Roman numerals within parentheses.

For instance, in "ammonium tetrachloroplatinate(II)," "tetra" indicates four chloride ligands, and "platinate(II)" denotes the platinum ion with a +2 charge.

Mastering the rules of chemical nomenclature equips students to decipher complex chemical formulations and understand their makeup.