Question

What is the coordination number of the metal in the following complexes? (a) \(\left[\mathrm{Mn}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{OH})_{\mathrm{A}}\right]^{2+}\) (b) \(\left[\mathrm{CuCl}_{4}\right]^{3-}\) (c) \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{ox})_{2}\right]^{2-}\) (d) \(\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{+}\)

Short answer

Question: Determine the coordination number of the metal in the following complexes: (a) [Mn(NH3)2(OH)A]²⁺ (b) [CuCl4]³⁻ (c) [Cr(H2O)2(ox)2]²⁻ (d) [Au(CN)2]⁺ Answer: (a) Coordination number of Mn: 2 + A (b) Coordination number of Cu: 4 (c) Coordination number of Cr: 6 (d) Coordination number of Au: 2

Step-by-step solution

(a) Coordination number of Mn in \(\left[\mathrm{Mn}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{OH})_{\mathrm{A}}\right]^{2+}\)

In the given complex, Mn is the central metal ion, with ligands NH3 and OH. There are 2 NH3 ligands (indicated by the subscript 2) and A OH ligands. Thus, the coordination number of Mn in this complex is 2 + A.

(b) Coordination number of Cu in \(\left[\mathrm{CuCl}_{4}\right]^{3-}\)

In the given complex, Cu is the central metal ion with 4 Cl ligands (indicated by the subscript 4). Therefore, the coordination number of Cu in this complex is 4.

(c) Coordination number of Cr in $\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}(\mathrm{ox})_{2}\right]^{2-}$

In the given complex, Cr is the central metal ion with H2O and ox ligands. There are 2 H2O ligands (indicated by the subscript 2) and 2 ox ligands (indicated by the subscript 2). Note that ox ligand (oxalate ion, C2O4^2-) is a bidentate ligand, which means it can donate two electron pairs to the central metal ion. Thus, the coordination number of Cr in this complex is 2 (from H2O ligands) + 2 * 2 (from 2 ox ligands) = 6.

(d) Coordination number of Au in \(\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{+}\)

In the given complex, Au is the central metal ion with 2 CN ligands (indicated by the subscript 2). Therefore, the coordination number of Au in this complex is 2.

Key concepts

Complex Ion

A complex ion is a molecule consisting of a central metal ion that is bonded to a set of surrounding molecules or ions, known as ligands. These structures carry an overall charge and are crucial in coordination chemistry. Complex ions can exhibit different geometries and properties based on the central metal ion and the nature of the ligands attached.
For example, in the complex \[\left[\mathrm{Mn}\left(\mathrm{NH}_{3}\right)_{2}(\mathrm{OH})_{A}\right]^{2+}\], manganese (Mn) acts as the central metal ion, and it is surrounded by ammonia (NH extsubscript{3}) and hydroxide (OH) ligands. Each of these ligands can form coordinate covalent bonds with Mn, resulting in the overall charged complex ion. The subscript numbers next to each ligand indicate how many ligands of that type are attached.

• Central metal ions are generally transition metals.
• Ligands can vary widely, including neutral molecules like water or ammonia, or anions such as chloride.
• The charge of a complex ion depends on the charges of the central metal and all surrounding ligands.

Central Metal Ion

The central metal ion forms the core of a complex ion and serves as the primary site for ligand attachment. Typically, metal ions that can form multiple bonds with ligands are transition metals, known for their ability to exhibit multiple oxidation states and coordination numbers.
For instance, in \[\left[\mathrm{CuCl}_{4}\right]^{3-}\], Copper (Cu) is the central metal ion bonded to four chloride (Cl) ligands. Transition metals like Cu possess partially filled d orbitals, allowing them to accommodate several ligands. Transition metals are often capable of forming colorful complexes due to electronic transitions absorbing light in the visible spectrum.
Key points about central metal ions include:

• The central metal ion defines the complex's coordination geometry, such as octahedral, square planar, or tetrahedral.
• The central metal ion's properties, such as size and charge, influence its ability to attract ligands.
• Different metals produce complexes with distinct chemical and physical properties.

Coordination Chemistry

Coordination chemistry is the study of complex structures formed between metal ions and ligands. It involves understanding how metal ions interact with molecules or ions to create complex ions with unique geometries and properties. This field is fundamental in describing how metals bind in biological systems, industrial catalysts, and medicinal applications.
For example, in the Cr complex \[\left[\mathrm{Cr}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2}(\mathrm{ox})_{2}\right]^{2-}\], coordination chemistry principles explain how chromium (Cr) binds to water (H extsubscript{2}O) and oxalate (ox) ligands. The oxalate ligands are "bidentate," meaning they can attach at two sites, allowing Cr to achieve a coordination number of 6.

• Coordination number refers to the number of ligand atoms bonded directly to the central metal ion.
• Coordination complexes often exhibit specific spatial arrangements, leading to diverse chemical behaviors.
• Understanding these complexes is crucial for innovations in biochemistry and material science.

Ligands

Ligands are ions or molecules that donate a pair of electrons to the central metal ion to form a stable complex. They can be neutral or charged and are classified based on the number of donor atoms they contain. Different ligands can lead to vastly different properties in the metal complexes they form.
Considering the complex \[\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{+}\], where gold (Au) serves as the central metal ion, the cyanide ions (CN) function as ligands providing a pair of electrons to the gold. In this case, CN is a "monodentate" ligand because it bonds through a single donor atom.
Key ligand features include:

• Monodentate ligands bind through one atom, like CN, or water (H2O).
• Bidentate ligands bind through two atoms, like oxalate (C2O4\(^{2-}\)).
• Polydentate ligands can use several atoms for bonding, creating more stable complexes.
• The nature of the ligand affects the electronic properties and color of the complex.