Question

Sketch the structure of the complex in each of the following compounds and give the full compound name: (a) \(c i s-\left[\operatorname{PtBr} \mathrm{Cl}\left(\mathrm{NO}_{2}\right)_{2}\right]^{2-}\) (b) \(\left[\mathrm{Mn}(\mathrm{CO})_{3}\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\right]^{+}\) (c) \(\left.c i s-\left[\mathrm{Cr} \mathrm{Cl}_{4}\right)\left(\mathrm{OH}_{2}\right)_{2}\right]^{-}\) (d) trans-[Co(OH)(en) \(\left._{2} \mathrm{Cl}\right]^{+}\)

Short answer

(a) The structure of cis-\([PtBrCl(NO2)_2]^{2-}\) is square planar, with Br and Cl cis to each other, and the two NO2 groups cis to each other as well. The full name is cis-dichlorodibromidoplatinum(II). (b) The structure of \([Mn(CO)_3(C_6H_6)]^{+}\) is tetrahedral, with three carbonyl ligands and one benzene ligand surrounding the manganese center. The full name is manganese(I) tricarbonylbenzene. (c) The structure of cis-\([CrCl_4(OH_2)_2]^{-}\) is octahedral, with two aqua ligands cis to each other and two of the four chlorine ligands cis to each other as well. The full name is cis-tetrachlorodihydroxochromium(III). (d) The structure of trans-\([Co(OH)(en)_2Cl]^{+}\) is octahedral, with hydroxide and chlorine ligands trans to each other. The full name is trans-chloridohydroxobis(ethylenediamine)cobalt(III).

Step-by-step solution

Identify the central atom

The central atom in this complex is Pt (platinum).

Identify the ligands

The ligands in the complex are Br (bromine), Cl (chlorine), and NO2 (nitrite).

Determine the geometry of the complex

The complex has four ligands bonded to the center. Thus, it most likely has a square planar geometry.

Sketch the structure and name the complex

In a cis configuration, similar ligands are positioned next to each other. In this case, Br and Cl are cis to each other, as are the two NO2 groups. The full name of this complex is cis-dichlorodibromidoplatinum(II). (b) Sketching structure and naming \([Mn(CO)_3(C_6H_6)]^{+}\)

Identify the central atom

The central atom in this complex is Mn (manganese).

Identify the ligands

The ligands in the complex are CO (carbonyl) and C6H6 (benzene).

Determine the geometry of the complex

The complex has four ligands bonded to the center. Thus, it most likely has a tetrahedral geometry.

Sketch the structure and name the complex

The structure can be sketched to show three carbonyl ligands and one benzene ligand around the manganese center. The full name of this complex is manganese(I) tricarbonylbenzene. (c) Sketching structure and naming cis-\([CrCl_4(OH_2)_2]^{-}\)

Identify the central atom

The central atom in this complex is Cr (chromium).

Identify the ligands

The ligands in the complex are Cl (chlorine) and OH2 (aqua).

Determine the geometry of the complex

The complex has six ligands bonded to the center. Thus, it most likely has an octahedral geometry.

Sketch the structure and name the complex

In a cis configuration, similar ligands are positioned next to each other. In this case, two aqua ligands are cis to each other, as are two of the four chlorine ligands. The full name of this complex is cis-tetrachlorodihydroxochromium(III). (d) Sketching structure and naming trans-\([Co(OH)(en)_2Cl]^{+}\)

Identify the central atom

The central atom in this complex is Co (cobalt).

Identify the ligands

The ligands in the complex are OH (hydroxide), en (ethylenediamine), and Cl (chlorine).

Determine the geometry of the complex

The complex has six ligands bonded to the center. Thus, it most likely has an octahedral geometry.

Sketch the structure and name the complex

In a trans configuration, opposite ligands are different. In this case, hydroxide and chlorine ligands are trans to each other. The full name of this complex is trans-chloridohydroxobis(ethylenediamine)cobalt(III).

Key concepts

Complex Compounds

Complex compounds are fascinating structures in coordination chemistry. These compounds consist of a central metal atom or ion surrounded by molecules or ions called ligands.
Various characteristics, such as electronic properties and reactivity, are influenced by the type and arrangement of ligands around the central atom.
A central feature of complex compounds is the coordination bond, where ligands donate electron pairs to the metal center, forming a stable structure.

• Stability: Complex compounds are often quite stable due to the strength of these coordination bonds.
• Variety: They can exist in several forms and arrangements, resulting in unique physical and chemical properties.

Understanding how complex compounds are structured is essential for grasping coordination chemistry, which is vital in many fields from medicine to materials science.

Ligands

Ligands are crucial components of complex compounds, acting as the "arms" that connect to the central metal atom or ion. Each ligand donates at least one pair of electrons to the metal, establishing a bond.
Ligands can be simple ions like chloride (Cl⁻) or complex organic molecules like ethylenediamine (en). Their nature can drastically affect the property of the complex, such as its color, reactivity, and overall geometry.

• Types: Ligands may be classified by their denticity, which refers to the number of bonds they can form with the metal. Examples include monodentate (single bond) and bidentate (two bonds) ligands.
• Role in geometry: The number and type of ligands determine the geometry of the coordination compound.

It’s essential to recognize the ligands in a complex, as they define multiple properties and the naming conventions of the compound.

Molecular Geometry

The geometry of a complex compound is determined primarily by the number and type of ligands attached to its central atom. These geometries influence the compound's physical properties and reactivity.
Common geometries in coordination chemistry include:

• Square planar: Common in complexes with four ligands, such as those featuring Pt in a cis configuration. Ligands are arranged in a single plane around the central atom.
• Tetrahedral: Often found in complexes with four ligands, such as those with Mn surrounded by ligands. Similar to a pyramid structure.
• Octahedral: With six ligands, this geometry features a "hexagon" of ligands around the central atom, typical in complexes with Cr or Co.

Each configuration has distinguishing features and is affected by the ligand types, compactness, and electron arrangements. Recognizing and predicting these geometries is crucial for effectively interpreting or designing compounds.

Nomenclature in Chemistry

Nomenclature, or the system of naming, is essential in chemistry for systematic communication about compounds, ensuring researchers and students can accurately identify and speak about specific chemicals.
For coordination complexes, the nomenclature is detailed and highly structured due to the complexity of these compounds. Here are some key points:

• Order: The ligands are named first in alphabetical order, followed by the metal.
• Prefix use: Prefixes like di-, tri-, for simple ligands indicate the number of each type present. Complex ligands use different prefixes like bis-, tris- to avoid confusion.
• Cis/Trans positioning: Terms like "cis" and "trans" indicate the relative spatial arrangement of ligands around the central metal in isomers, impacting compound properties.

Learning the nomenclature helps in formulating and sketching structures confidently, as it provides a clear, universally recognized format for representing complex information.