Question

Draw a reasonable structure for (a) \(\left[\operatorname{CoBr}\left(\mathrm{NH}_{3}\right)_{3}\right]^{2+}\) and (b) dichloroethylenediamineplatinum(II).

Short answer

The structure of \(\left[\operatorname{CoBr}\left(\mathrm{NH}_{3}\right)_{3}\right]^{2+}\) is tetrahedral with Co at the center, one Br, and three NH3 ligands. The structure of dichloroethylenediamineplatinum(II) is square planar with Pt at the center, two Cl, and one bidentate ethylenediamine ligand.

Step-by-step solution

Analyzing the Cobalt Complex

To draw the structure of the cobalt complex \(\left[\operatorname{CoBr}\left(\mathrm{NH}_{3}\right)_{3}\right]^{2+}\), identify that cobalt (Co) is at the center of the complex, bromine (Br) is likely to be a ligand, and there are three ammonia (\(\mathrm{NH}_3\)) ligands. This makes a total of four ligands, suggesting a tetrahedral or square planar geometry. However, considering cobalt's common coordination numbers and preference, a tetrahedral geometry is most reasonable for a coordination number of four.

Drawing the Cobalt Complex

Place the cobalt ion in the center and arrange the three ammonia ligands and one bromine ligand around it in a tetrahedral shape. Each ligand is connected to the central metal by a single bond. Draw the bracket around the structure and indicate the charge of +2 on the outside of the bracket.

Analyzing the Platinum Complex

For the dichloroethylenediamineplatinum(II) complex, it is known that platinum usually forms square planar complexes with a coordination number of four. The complex has two chloride (Cl) ligands and the ethylenediamine ligand, which is bidentate (can attach at two sites). This takes up the remaining two coordination sites.

Drawing the Platinum Complex

Draw the platinum ion in the center with two chloride ligands on opposite sides to maintain a square planar geometry. Add the ethylenediamine ligand with its two nitrogen atoms each forming a bond with the platinum ion, filling the remaining two coordination sites. Since the overall charge is not indicated and typically ethylenediamine and chloride are neutral ligands, the platinum in oxidation state II means the complex does not carry a net charge.

Key concepts

Cobalt Complexes

Cobalt complexes in coordination chemistry are intriguing entities with cobalt as the central metal ion surrounded by molecules or ions known as ligands. These ligands donate electron pairs to cobalt, forming coordination bonds. A conspicuous example is the complex \(\left[\operatorname{CoBr}(\mathrm{NH}_{3})_{3}\right]^{2+}\), where the central cobalt ion is bonded to one bromine (Br) and three molecules of ammonia (\(\mathrm{NH}_3\)). The geometry of the complex is influenced by the number of ligands and their spatial arrangement. Here, with four ligands, a tetrahedral geometry is favored, depicting a 3-dimensional structure where the ligands are equally spaced around the cobalt ion. This structure significantly affects the complex's properties, including its reactivity and magnetic behavior.

In the cobalt complex given in the exercise, we deduce that it has a 2+ charge and tetrahedral geometry, thanks to the versatile coordination chemistry of cobalt which allows it to form stable complexes with different geometries and oxidation states.

Platinum Complexes

Platinum complexes are cornerstone compounds in coordination chemistry, especially known for their roles in catalysis and medicinal chemistry. In platinum complexes, the platinum center often adopts a square planar molecular geometry, which is crucial for the complex's stability and reactivity. An iconic example provided in the exercise is the dichloroethylenediamineplatinum(II), where platinum (II) is central and coordinated by two chloride ions and one ethylenediamine ligand, which donates two electron pairs, making it a bidentate ligand. The square planar geometry results from the spatial requirements of the ligands and the d-electron configuration of the platinum ion.

The overall charge of the complex is zero when platinum is in the +2 oxidation state and is surrounded by neutral ligands such as chloride and ethylenediamine. The synthesis and analysis of such complexes contribute significantly to understanding chemical bonding in transition metals.

Molecular Geometry

Molecular geometry refers to the spatial arrangement of atoms within a molecule. In coordination compounds, the geometry around the central metal ion is determined by the type and number of coordinating ligands. Common geometries include tetrahedral, where four ligands are symmetrically arranged around the central atom, and square planar, characteristic of eight-electron d-metals like platinum. The significance of molecular geometry extends beyond a molecule's shape; it influences properties such as color, magnetism, and biological activity of compounds.

Understanding the molecular geometry in complexes such as those with cobalt and platinum is vital in predicting interactions and reactivity, which is essential for applications in materials science and pharmaceuticals.

Chemical Bonding

Chemical bonding in coordination complexes is pivotal in holding the central metal ion and ligands together. This bond formation involves the sharing or donation of electron pairs from the ligands to the empty orbitals of the metal ion, leading to either covalent or ionic character in the bonds. Coordination compounds frequently exhibit complex bonding where electron pairs from ligands, like \(\mathrm{NH}_3\) or chloride ions, facilitate the formation of coordination covalent bonds. The number of bonds and their strength determine the stability and structure of the complex.

When drawing structures of coordination complexes, the chemical bonding denotes how ligands are connected to the central metal, such as in the \(\left[\operatorname{CoBr}(\mathrm{NH}_{3})_{3}\right]^{2+}\) and dichloroethylenediamineplatinum(II) complexes we discussed here.

Ligand Field Theory

Ligand Field Theory (LFT) extends beyond the basic concepts of chemical bonding, providing insights into how ligands affect the energy levels of d-orbitals in the central metal ion. LFT explains the splitting of d-orbitals into different energy levels upon bonding with ligands, which is fundamental in understanding the color, magnetic properties, and overall stability of coordination complexes.

The splitting pattern and the extent of splitting depend on the geometry of the complex and the nature of the ligands. A tetrahedral complex such as \(\left[\operatorname{CoBr}(\mathrm{NH}_{3})_{3}\right]^{2+}\) causes a different splitting pattern compared to the square planar complex, like the dichloroethylenediamineplatinum(II). For students exploring coordination chemistry, LFT serves as a tool to rationalize the electronic structure and properties of metal complexes.