Problem 2 - Chemistry: The Central Science

Question

Draw the structure for \(\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\) and use it to answer the following questions: (a) What is the coordination number for platinum in this complex? (b) What is the coordination geometry? (c) What is the oxidation state of the platinum? (d) How many unpaired electrons are there? [Sections \(23.2\) and 23.6]

Step-by-Step Solution

Answer

In the complex \(\mathrm{Pt}(\mathrm{en})\mathrm{Cl}_{2}\): a) The coordination number of platinum is 4. b) The coordination geometry is square planar. c) The oxidation state of platinum is +2. d) There are 0 unpaired electrons in this complex.

Step 1: Draw the complex structure

The complex structure can be drawn as follows: The platinum atom is in the center, ethylenediamine binds to platinum with both its nitrogen atoms (forming a 5 membered ring with Pt and 2 carbons in between the nitrogen atoms), and two chloride ions are attached to the platinum. \[ \mathrm{Pt} - \mathrm{N} - \mathrm{CH_{2}} - \mathrm{CH_{2}} - \mathrm{N} - \mathrm{Pt} - \mathrm{Cl} - \mathrm{Cl} \]

Step 2: Determine the coordination number

The coordination number is the number of ligand atoms bonded to the central metal ion. In this complex, platinum is bonded to two nitrogen atoms from ethylenediamine and two chloride ions. Therefore, the coordination number of platinum in this complex is 4.

Step 3: Determine the coordination geometry

As there are four ligand atoms bonded to the central platinum ion, the coordination geometry for this complex is square planar.

Step 4: Determine the oxidation state of platinum

To determine the oxidation state of platinum, we need to consider the charges on the ligands. Ethylenediamine is a neutral ligand and doesn't contribute to the charge, while each chloride ion has a charge of -1. Since the complex is neutral, the oxidation state of platinum has to balance the charges of the ligands. Therefore, the oxidation state of platinum in this complex is +2.

Step 5: Determine the number of unpaired electrons

We know that platinum has an electron configuration of \(\mathrm{[Xe] \ 4f^{14} \ 5d^9 \ 6s^1}\). When platinum has an oxidation state of +2, it loses 2 electrons, leaving its electron configuration as \(\mathrm{[Xe] \ 4f^{14} \ 5d^8}\). All the electrons in the \(5d\) sublevel of platinum are paired in this complex. Therefore, there are 0 unpaired electrons in this complex. In conclusion: a) The coordination number of platinum in this complex is 4. b) The coordination geometry is square planar. c) The oxidation state of platinum is +2. d) There are 0 unpaired electrons in this complex.

Key Concepts

Coordination Number

Understanding the coordination number is crucial when delving into the structures of coordination compounds. The coordination number refers to the number of ligands, or molecules and ions, that are directly bonded to the central metal ion in a coordination compound. Imagine grasping the central atom with all the ligand 'hands' it can hold; that's the coordination number for you.

For instance, in the given complex \(\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\), the central metal platinum is bonded to two nitrogen atoms from one ethylenediamine (en) ligand and two chloride ions. Hence, the coordination number is 4. This concept is fundamental to understanding the rest of the properties of coordination compounds, such as their geometry, the metal's oxidation state, and magnetic properties.

Coordination Geometry

While coordination compounds come in various shapes, the coordination geometry specifically describes the three-dimensional arrangement of the ligands that are bonded to the central metal ion. This layout crucially influences the compound's color, magnetic properties, and reactivity.

For a coordination number of 4, as in our example with platinum, common geometries are square planar and tetrahedral. In this particular case, the \(\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\) complex exhibits a square planar geometry. This geometry arises because of the specific spatial arrangement that minimizes electron repulsion, satisfying both aesthetic symmetry and quantum mechanical stability.

Oxidation State

In coordination chemistry, the oxidation state of the central metal ion can be understood as the hypothetical charge that metal would have if all its ligands were removed along with the electrons they provided or shared. This number is essential as it influences the compound's stability, reaction capability, and even color.

For \(\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\), determining the oxidation state involves examining the ligand charges. With ethylenediamine being a neutral molecule and each chloride ion contributing a -1 charge, it becomes clear that platinum must be in the +2 oxidation state to balance the charges to zero, since the entire coordination compound is neutral.

Unpaired Electrons

The electron pairing in coordination compounds plays a vital role in determining their magnetic properties. Unpaired electrons contribute to paramagnetism, where the material is weakly attracted to a magnetic field. In contrast, fully paired electrons lead to diamagnetism, in which the compound is slightly repelled by a magnetic field.

With reference to the \(\mathrm{Pt}(\mathrm{en}) \mathrm{Cl}_{2}\) complex, after losing two electrons to achieve a +2 oxidation state, the electron configuration for platinum shows that all electrons in the outer shell are paired. Consequently, the complex has zero unpaired electrons, indicating that it is diamagnetic.