Problem 18 - Chemistry and Chemical Reactivity

Question

Give the name or formula for each ion or compound, as appropriate. (a) tetraaquadichlorochromium(III) chloride (b) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{SO}_{4}\right] \mathrm{Cl}\) (c) sodium tetrachlorocobaltate( 11 ) (d) \(\left[\mathrm{Fe}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]^{3-}\)

Step-by-Step Solution

Answer

(a) [Cr(H2O)4Cl2]Cl, (b) [Cr(NH3)5SO4]Cl, (c) Na2[CoCl4], (d) [Fe(C2O4)3]3-

Step 1: Understanding the Compound Name

For compound (a), 'tetraaquadichlorochromium(III) chloride', identify the ligands and the metal center. The name gives chromium as the metal, with 'tetra-aqua' indicating four water molecules ( H_2O ) and 'dichlorine' referring to two chlorine ions ( Cl^- ). The metal ion is chromium(III), indicating a charge of +3.

Step 2: Determining Overall Composition

For the given name, write the complex: [Cr(H_2O)_4Cl_2] and note that it's paired with a chloride ion outside the complex, as indicated by 'chloride' in the name. Hence, the full formula is [Cr(H_2O)_4Cl_2]Cl .

Step 3: Formula Decomposition and Explanation

For compound (b), [Cr(NH_3)_5SO_4]Cl is given. Identify Cr as the central metal ion, coordinated with five ammonia ( NH_3 ) ligands and one sulfate ( SO_4 ext{^2-} ) inside the bracket, balanced by a single chloride ion ( Cl^- ) outside the bracket.

Step 4: Understanding Charged Complexes

Compound (c) 'sodium tetrachlorocobaltate(II)' implies a cobalt complex ion charged due to its interaction with sodium outside the complex. Sodium indicates Na^+ balances the charge of [CoCl_4] ext{^2-} , resulting in the formula Na_2[CoCl_4] .

Step 5: Identifying the Ion Composition

For compound (d), recognize [Fe(C_2O_4)_3]^{3-} as a complex ion containing iron with oxidation state determined from the ligands' charges. Oxalate ( C_2O_4^{2-} ) presents in three occurrences, making the total charge 3- from this tridentate ligand configuration.

Key Concepts

Ligands

In coordination compounds, ligands play a crucial role. They are molecules or ions that bind to the metal center to form a complex ion. Ligands can be neutral molecules like water \(H_2O\) or charged species like chloride \(Cl^-\). Their role is to donate electrons to the metal center, creating a coordinate covalent bond. This process stabilizes the overall structure of the coordination complex.
Types of ligands include:

• **Monodentate Ligands:** These ligands have only one donor atom which bonds to the metal center. Examples include ammonia \(NH_3\) and chloride \(Cl^-\).
• **Polydentate Ligands:** These have multiple donor atoms that can bind to the metal. For example, oxalate \(C_2O_4^{2-}\) binds through two oxygen atoms, therefore, called a bidentate ligand.

Understanding the type and number of ligands is crucial for determining the geometry and properties of the complex ion.

Metal Center

At the heart of every coordination compound lies the metal center. The metal center is typically a transition metal, like chromium, cobalt, or iron. These metals have empty orbitals that can accept electrons from ligands. The interaction between the metal and ligands forms the backbone of the complex.
The characteristics of the metal center include:

• **Electron Configuration:** Transition metals have d-orbitals where electron pairing and interactions occur. Their unique configurations allow them to form various colored complexes.
• **Coordination Number:** This term represents the total number of ligand donor atoms bonded to the metal. For instance, in \([Cr(H_2O)_4Cl_2]\), the coordination number is 6.

The nature of the metal, such as its size and charge, influences the overall geometry of the complex, as well as its reactivity and magnetic properties.

Oxidation State

The oxidation state of the metal in a coordination compound is a key factor in understanding its chemical behavior. It indicates the charge the metal would possess if all the bonds were completely ionic. In essence, it's a way of keeping track of electrons.
To determine the oxidation state:

• Adjust for each ligand's contribution to charge. Neutral ligands like water do not alter the oxidation state by charge, whereas charged ligands do.
• Consider any additional anions or cations outside the complex. For example, the oxidation state of chromium in \(\text{tetraaquadichlorochromium(III) chloride}\) is +3, matched by the dichlorine and leftover external chloride balancing.

Knowing the oxidation state helps in understanding the reactivity, color, and type of chemical reactions the complex can undergo.

Complex Ion

Complex ions are charged species composed of a central metal atom bonded to one or more ligands. The charge of a complex ion depends on the oxidation state of the metal and the charges of the ligands.
Characteristics of complex ions include:

• **Stability:** Dictated by the metal and ligand type. Chelate ligands often enhance stability through the chelate effect.
• **Charge:** Complex ions can be anionic or cationic, depending on the net charge. For instance, \([Fe(C_2O_4)_3]^{3-}\) is an anionic complex.

The understanding of complex ions is crucial because it affects properties such as solubility and reactivity in biological and industrial contexts.