Problem 8 - General Chemistry: Principles and Modern Applications

Question

Draw plausible structures of the following chelate complexes. (a) \(\left[\operatorname{Pt}(\text { ox })_{2}\right]^{2-}\) (b) \(\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-}\) (c) \([\mathrm{Fe}(\text { EDTA })]^{2-}\)

Step-by-Step Solution

Answer

Each ligand forms a different number of rings or cycles with the metal ion: two for oxalate with Platinum, three for oxalate with Chromium, and one for EDTA with Iron. The number and shape of these structures depend on the number of bonds the ligand can make, and the ratio of ligands to metal ions in the complex. The charges on the complex ions depend on the charge of the metal ion and the aggregate charges of the ligands bonding with it.

Step 1: Draw the structure of platinum(II) Oxalate

Oxalate (ox) is a bidentate ligand. Platinum makes two bonds with oxygen atoms of two oxalate ions. It means platinum is at the center and two oxalate ions are bonded through oxygen atoms. The negative charges from the oxalate ions balance the 4+ charge on platinum; the resulting complex is \( \left[\operatorname{Pt}(\text { ox })_{2}\right]^{2-}\) and has a square planar geometry.

Step 2: Draw the structure of chromium(III) Oxalate

Chromium in this complex makes six bonds with three oxalate ions, forming a 6-membered ring structure. Each oxalate ion provides two oxygen atoms for bonding, making this a bidentate ligand. The resulting negative charge on the complex ion is 3-, balancing the 3+ overall charge on chromium. The resulting complex is \( \left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-}\) and has an octahedral geometry.

Step 3: Draw the structure of iron(III) EDTA complex

In this complex, the ligand EDTA forms six bonds with iron(III) making an octahedral complex. EDTA is a hexadentate ligand; it forms six membered cyclic rings with the iron ion. The overall charge on the complex ion is 2- to balance the 3+ overall charge on iron, and the 1- charge from the EDTA. This forms a complex ion \( [\mathrm{Fe}(\text { EDTA })]^{2-}\).

Key Concepts

Coordination Chemistry

Coordination chemistry is the area of chemistry that involves the study of coordination compounds, where central metal atoms or ions are bonded to a set of molecules or anions called ligands. A key feature of these compounds is the coordination number, which is the number of ligand atoms bonded to the central atom. These ligands can donate electrons to the metal to form coordinate covalent bonds, creating a wide variety of complex structures and geometries.

The exercise focuses on the structure of chelate complexes, where the ligands form rings that include the central metal atom. Chelate complexes tend to be more stable than complexes with monodentate ligands due to the 'chelate effect'. This increased stability arises because the formation of such ring structures decreases the probabilities of the ligands detaching from the central metal.

Ligands in Coordination Compounds

Ligands are ions or neutral molecules that surround the central metal atom in coordination compounds. They feature atoms with lone pairs of electrons which can be used to form bonds with the metal. Ligands are classified based on the number of bonding sites they have:

• Monodentate ligands have one point of attachment.
• Bidentate ligands have two points of attachment, as seen with oxalate (ox) in the given exercise.
• Polydentate ligands, or chelating agents, such as EDTA in the exercise, have multiple bonding sites.

In the provided examples, oxalate is a bidentate ligand forming two bonds with metals, while EDTA is a hexadentate ligand, forming up to six bonds. The points where the ligands attach to the metal are called donor sites. Oxygen and nitrogen atoms are common donor atoms in chelating ligands.

Geometries of Coordination Complexes

The geometry of a coordination complex refers to the arrangement of the ligands attached to the central metal ion. The geometries are largely influenced by the coordination number. Common geometries include:

• Tetrahedral and square planar geometries, typically with coordination number 4.
• Octahedral geometry with coordination number 6.

In the exercise, the platinum complex forms a square planar geometry, while the chromium and iron complexes form octahedral geometries. These shapes depend on the types of ligands and the electronic repulsion between them, aiming to minimize energy and repulsion within the complex. The octahedral complex with chromium(III) and oxalate showcases how a bidentate ligand can create a symmetrical arrangement around the central metal, leading to a well-defined shape. Meanwhile, the iron(III) EDTA complex demonstrates the intricate coordination possible with hexadentate ligands, also resulting in an octahedral geometry.