Problem 79 - General Chemistry: Principles and Modern Applications

Question

Draw structures to represent these four complex ions: (a) \(\left[\mathrm{PtCl}_{4}\right]^{2-} ;\) (b) \(\left[\mathrm{FeCl}_{4}(\mathrm{en})\right]^{-} ;\) (c) \(\operatorname{cis}-\left[\mathrm{FeCl}_{2}(\mathrm{ox})(\mathrm{en})\right]^{-}\) (d) trans- \(\left[\mathrm{CrCl}(\mathrm{OH})\left(\mathrm{NH}_{3}\right)_{4}\right]^{+}\).

Step-by-Step Solution

Answer

These complex ions have different central atoms and varying ligands. The overall structures are an indication of the geometry based on the central atom and ligands present around it, along with any cis- or trans- notation indicating the arrangement of these ligands.

Step 1: Draw \(\left[\mathrm{PtCl}_{4}\right]^{2-}\) structure

In this chemical structure, the central atom is Pt, surrounded by 4 Cl atoms. It's a square planar structure. First, draw a Pt atom in the center, then draw four Cl atoms around it, forming the corners of a square. Finally, indicate the overall charge of -2 on the structure.

Step 2: Draw \(\left[\mathrm{FeCl}_{4}(\mathrm{en})\right]^{-}\) structure

This structure consists of Fe at the center surrounded by four Cl atoms and 'en' which stands for ethylenediamine, a bidentate ligand. As the overall coordination number is 6, the geometry will be octahedral. Draw a Fe atom in the center, four Cl atoms around it and indicate the positions of the 'en' group. Note that the 'en' group occupies two positions as it's bidentate and links at two places to the Fe. The overall charge on the molecule is -1.

Step 3: Draw \(\operatorname{cis}-\left[\mathrm{FeCl}_{2}(\mathrm{ox})(\mathrm{en})\right]^{-}\) structure

The 'cis-' prefix means that both Cl atoms are on the same side of the complex and the ox (oxalate) and en ligands are opposite of each other. First, draw the central Fe atom and then draw the Cl atoms, 'en' and 'ox' groups in the correct orientation. The 'ox' ligand is a bidentate ligand (like 'en') and will have two coordinate bonds with Fe. The overall molecule carries a charge of -1.

Step 4: Draw trans-\(\left[\mathrm{CrCl}(\mathrm{OH})\left(\mathrm{NH}_{3}\right)_{4}\right]^{+}\) structure

The 'trans-' prefix indicates that the Cl and OH are opposite to each other (180°). The molecule has the octahedral geometry, as the central atom Cr is surrounded by 6 ligands. Draw the Cr atom in the center, the Cl and OH ligands on opposite sides and fill in the 4 NH3 groups around it. The overall molecule carries a charge of +1.

Key Concepts

Coordination Chemistry

Coordination chemistry is a branch of inorganic chemistry that deals with the study of compounds typified by the interaction of metal ions with other, often organic, molecules called ligands. These interactions form complex ions or coordination compounds, which have unique structural, spectroscopic, and reactivity characteristics.
Complex ions consist of a central metal atom or ion (often a transition metal) connected by coordinate covalent bonds to a set of surrounding molecules or anions, known as ligands. The number of ligands attached to the central metal is known as the coordination number, which typically ranges between two to twelve, with four and six being the most common coordination numbers, leading to tetrahedral, square planar, and octahedral geometries, among others.
Complex ions can carry a positive, negative, or neutral charge, and their shape and charge will depend on the specifics of the metal ion and ligands involved. These coordination compounds play significant roles in various areas, including biological enzyme function, catalysis, and materials chemistry.

Geometrical Isomers

Geometrical isomers are one type of stereoisomer, where molecules have the same sequence and types of bonds but differ in the three-dimensional orientation of the atoms. In the context of coordination chemistry, geometrical isomerism frequently occurs with octahedral and square planar complexes.
For octahedral complexes with a coordination number of six, 'cis' and 'trans' configurations are prevalent types of geometrical isomers. A 'cis' configuration means that two identical or related ligands are adjacent to each other while 'trans' indicates that they are opposite, typically at a 180-degree angle. An example of this is the ways in which chloride ligands can be oriented with respect to each other in an octahedral iron complex.
Geometrical isomerism greatly affects the chemical behavior and properties of a compound. For instance, 'cis-platin' is a chemotherapeutic drug whereas its 'trans' isomer is not biologically active.

Bidentate Ligands

Bidentate ligands are a type of chelating ligand that can form two coordinate covalent bonds with a single metal ion. They essentially 'bite' the metal at two separate points, thus the prefix 'bi-' for two. Because they form ring-like structures with the metal ion, bidentate ligands tend to create more stable complexes than their monodentate counterparts.
Common examples of bidentate ligands include ethylenediamine (en) and oxalate (ox). In ethylenediamine, both nitrogen atoms donate an electron pair to the central metal ion, while in oxalate, the donation comes from two oxygen atoms.
When drawing structures involving bidentate ligands, it's important to account for the fact that they occupy two coordination sites. This aspect is crucial when predicting the geometrical structure or when figuring out isomerism possibilities, as showcased in the exercises provided.