Problem 17 - 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) \([[\mathrm{Fe(OH)(H_2O)_5}]^{2+})\; (b)\;\text{dipotassium tetracyanonickelate(II)};\; (c)\;\text{potassium bisoxalatoaquachromate(III)};\; (d)\; (\mathrm{NH}_4)_2[\mathrm{PtCl}_4].

Step 1: Identify Components in the Chemical Name or Formula

For (a) pentaaquahydroxoiron(III) ion, identify that 'penta' indicates five water molecules (aqua), 'hydroxo' indicates one hydroxide group (OH), and iron(III) indicates the iron is in a +3 oxidation state.

Step 2: Construct the Chemical Formula from Name

Construct the formula for pentaaquahydroxoiron(III) ion as: \[ [\mathrm{Fe(OH)(H_2O)_5}]^{2+} \] Indicating it contains five water molecules, one hydroxide ion, and iron in a +3 state, resulting in a +2 overall charge.

Step 3: Name the First Chemical Formula

For (b) \(\mathrm{K}_{2}[\mathrm{Ni}( ext{CN})_4]\), break down to find it contains 2 potassium ions \((\mathrm{K}^{+})\) and a tetracyanonickelate(II) ion, since nickel forms a complex with four cyanide ions \((\mathrm{CN}^{-})\) and is +2 oxidation state.

Step 4: Name the Second Chemical Formula

Identify (c) \(\mathrm{K}[\mathrm{Cr}(\mathrm{C}_{2}\mathrm{O}_{4})_2(\mathrm{H}_{2}\mathrm{O})_2]\) as potassium bisoxalatoaquachromate(III) ion. The complex ion consists of chromium bound to two oxalate ions \((\mathrm{C}_{2}\mathrm{O}_{4}^{2-})\) and two water molecules, with chromium in a +3 oxidation state.

Step 5: Identify Components in the Chemical Name

For (d) ammonium tetrachloroplatinate(II), recognize it involves the ammonium ion \((\mathrm{NH}_4^+)\) and tetrachloroplatinate ion where platinum is in a +2 state coordinated to four chloride ions \((\mathrm{Cl}^-)\).

Step 6: Construct the Formula from Name

The formula for ammonium tetrachloroplatinate(II) is: \[ (\mathrm{NH}_4)_2[\mathrm{PtCl}_4] \] consisting of two ammonium ions and a tetrachloroplatinate complex.

Key Concepts

Chemical Nomenclature

Chemical nomenclature is the system used for naming chemical compounds and ions. Understanding this system allows chemists to identify and differentiate between compounds accurately. There are distinct rules when it comes to naming different chemical species:

• Coordination compounds, such as the ones in the problem, are named starting with the name of ligands before the central metal atom or ion. Prefixes like 'penta', 'tetra', and 'di' indicate the number of each type of ligand present in the complex.
• Ligands are alphabetical in name. Neutral ligands like water are named as ‘aqua’, hydroxide ions as ‘hydroxo’, and cyanide ions as ‘cyano’.
• The oxidation state of the central metal ion is indicated as a Roman numeral in parentheses, helping identify the metal's charge.

The systematic approach allows anyone familiar with the rules to deduce the formula or the name of a given compound with certainty. Naming can aid in understanding the structural and reactive nature of the compounds.

Oxidation State

The oxidation state refers to the degree of oxidation of an atom within a compound, essentially an indicator of electron ownership. In coordination compounds, it is crucial for the balancing of overall charge and guiding the naming process.

• In coordination chemistry, it helps to determine both the number and type of ions or ligands that can attach to the central metal ion.
• For example, in the pentaaquahydroxoiron(III) ion, the oxidation state of iron is +3, meaning it has lost three electrons.
• Similarly, nickel in tetracyanonickelate(II) maintains an oxidation state of +2, which implies each nickel ion has lost two electrons in its neutral state in the compound.

The concept helps predict the geometry and the reactivity of complexes, as it closely ties into the electron density and affinity of the central metal atom for bonding.

Complex Ions

Complex ions are ions comprised of a central metal atom or ion surrounded by molecules or anions called ligands. The entire assembly, held together by coordinate covalent bonds, forms a stable ion usually indicated within square brackets.

• Ligands donate a pair of electrons to the central ion; these can be negatively charged or neutral, providing the necessary electrons for bonding.
• Such assemblies increase the functionality and stability of the chemical species, making them critical in various biological and industrial processes.
• In the problem, \( [\text{Ni}(\text{CN})_4]^{2-} \) and \( [\text{Cr}(\text{C}_2\text{O}_4)_2(\text{H}_2\text{O})_2]^{-} \) are examples, showcasing how different ligands form stable attachments, significantly altering the chemical properties of the nickel and chromium respectively.

Complex ions play a vital role in catalysis, color formation in compounds, and biofunctionality due to their versatile bonding and reactivity characteristics.

Transition Metals

Transition metals are elements found in the d-block of the periodic table, known for their ability to form a variety of oxidation states and complex ions. Key attributes include:

• They often exhibit high melting and boiling points and can lend multiple oxidation states due to their unpaired electrons available for bonding.
• In coordination chemistry, transition metals like iron, nickel, chromium, and platinum readily form complexes due to the availability of vacant d orbitals that can accept electron pairs from ligands.
• The variety of oxidation states not only affects the naming but also the physical, chemical, and magnetic properties of the compounds.

Their ability to transition between different oxidation states makes them valuable in redox reactions, and they are key components in numerous industrial applications, such as catalysis and electronic materials. Understanding transition metals and their behavior is essential for mastering complex chemical cycles and processes.