Question

Write equations for the stepwise formation of each of the following complex ions. a. \(\mathrm{CoF}_{6}^{3-}\) b. \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\)

Short answer

The stepwise formation of the complex ions can be summarized as follows: a. Formation of \(\mathrm{CoF}_{6}^{3-}\) complex ion: 1. \(\mathrm{Co^{3+}} + \mathrm{F^{-}} \rightarrow \mathrm{CoF^{2+}}\) 2. \(\mathrm{CoF^{2+}} + 2\,\mathrm{F^{-}} \rightarrow \mathrm{CoF_3^{+}}\) 3. \(\mathrm{CoF_3^{+}} + 3\,\mathrm{F^{-}} \rightarrow \mathrm{CoF_{6}^{3-}}\) b. Formation of \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\) complex ion: 1. \(\mathrm{Zn^{2+}} + \mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)^{2+}}\) 2. \(\mathrm{Zn(NH_3)^{2+}} + 2\,\mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)_3^{2+}}\) 3. \(\mathrm{Zn(NH_3)_3^{2+}} + \mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)_4^{2+}}\)

Step-by-step solution

Formation of first coordination site

Add one \(\mathrm{F^{-}}\) ion to the \(\mathrm{Co^{3+}}\) ion to form a monodentate complex: \[ \mathrm{Co^{3+}} + \mathrm{F^{-}} \rightarrow \mathrm{CoF^{2+}} \]

Formation of second and third coordination sites

Add two more \(\mathrm{F^{-}}\) ions to the complex formed in step 1: \[ \mathrm{CoF^{2+}} + 2\,\mathrm{F^{-}} \rightarrow \mathrm{CoF_3^{+}} \]

Formation of final complex ion

Finally, add the remaining three \(\mathrm{F^{-}}\) ions to the complex formed in step 2: \[ \mathrm{CoF_3^{+}} + 3\,\mathrm{F^{-}} \rightarrow \mathrm{CoF_{6}^{3-}} \] b. Formation of \(\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\) complex ion:

Formation of first coordination site

Add one \(\mathrm{NH_3}\) molecule to the \(\mathrm{Zn^{2+}}\) ion to form a monodentate complex: \[ \mathrm{Zn^{2+}} + \mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)^{2+}} \]

Formation of second and third coordination sites

Add two more \(\mathrm{NH_3}\) molecules to the complex formed in step 1: \[ \mathrm{Zn(NH_3)^{2+}} + 2\,\mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)_3^{2+}} \]

Formation of final complex ion

Finally, add the last \(\mathrm{NH_3}\) molecule to the complex formed in step 2: \[ \mathrm{Zn(NH_3)_3^{2+}} + \mathrm{NH_3} \rightarrow \mathrm{Zn(NH_3)_4^{2+}} \]

Key concepts

Coordination Chemistry

In the fascinating world of chemistry, coordination chemistry is a branch that focuses on the properties and behaviors of complex compounds. These complexes are formed when central metal atoms or ions bond with surrounding molecules or ions, known as ligands. A central topic in chemistry, coordination compounds have diverse applications in areas like catalysis, medicine, and material science.

The formation of these complexes involves coordination sites, where the central metal ion interacts with ligands. These interactions are mostly through coordinate covalent bonds, where ligands donate electron pairs to the metal ion. This connection grants the metal ion its unique characteristics, directly influencing the color, magnetic properties, and reactivity of the complex.

Coordination chemistry also deals with the geometry around the central atom. The number of coordination sites and the nature of ligands can lead to different configurations like square planar or octahedral, significantly affecting the chemical behavior of the complex.

Ligand Exchange Reactions

Ligand exchange reactions are an integral part of coordination chemistry. These reactions involve the replacement of one or more ligands in a coordination complex with new ligands. These exchanges can occur due to changes in conditions such as temperature, pressure, or concentration, significantly altering the properties of the complex.

There are various types of ligand exchange reactions, categorized based on the mechanism by which the exchange occurs:

• Associative Mechanism: A new ligand attaches to the metal center before the old ligand departs.
• Dissociative Mechanism: The existing ligand first leaves the metal center, creating a vacant site for the new ligand to attach.
• Interchange Mechanism: This involves simultaneous departure and arrival of ligands without forming an intermediate.

These mechanisms help predict and explain the dynamic behavior of complexes in various chemical contexts.

Complex Ions

Complex ions are a hallmark of coordination chemistry, consisting of a metal center bound to one or more ligands, each of which has donor atoms that provide lone pairs of electrons. These ions can possess positive, negative, or neutral charges depending on the nature of the central metal and ligands.

These ions are vital in many chemical processes and reactions, like in catalysis where they can lower activation energies, or in biochemical systems, where they play roles in transporting ions and molecules across membranes.

The charge of a complex ion is determined by the metal’s oxidation state and the charges of the ligands attached to it. For instance, in the complex ion \(\text{CoF}_6^{3-}\), the cobalt ion has an oxidation state of +3, while fluoride ligands each having a charge of -1 contribute to the overall charge. Understanding the composition and structure of complex ions is crucial in predicting and rationalizing their chemical behavior and reactivity.

Transition Metal Complexes

Transition metal complexes are unique due to the involvement of d-orbitals in bonding, lending them distinct properties like variable oxidation states, colorful appearance, and the ability to form intricate, stable structures. These complexes are important in industrial and biological systems, offering significant functionality and versatility.

Transition metals like cobalt and zinc frequently form complexes with a variety of ligands, ranging from simple ions like Cl^- or F^- to larger organic molecules.

The properties of transition metal complexes are influenced by factors such as:

• Electronic configuration of the metal ion.
• Nature and arrangement of ligands.
• Overall oxidation state and coordination number.

For example, the cobalt complex \(\text{CoF}_6^{3-}\) displays its specific properties due to the cobalt ion's electron arrangement and its interaction with fluoride ions. Similarly, zinc complexes like \(\mathrm{Zn(NH_3)_4^{2+}}\) exhibit distinctive characteristics useful in numerous applications, such as sensors and enzyme mimics.