Question

One of the more famous species in coordination chemistry is the Creutz-Taube complex: It is named for the two scientists who discovered it and initially studied its properties. The central ligand is pyrazine, a planar six-membered ring with nitrogens at opposite sides. (a) How can you account for the fact that the complex, which has only neutral ligands, has an odd overall charge? (b) The metal is in a low-spin configuration in both cases. Assuming octahedral coordination, draw the d-orbital energy-level diagram for each metal. (c) In many experiments the two metal ions appear to be in exactly equivalent states. Can you think of a reason that this might appear to be so, recognizing that electrons move very rapidly compared to nuclei?

Short answer

(a) The odd overall charge of the Creutz-Taube complex is due to the metal ions having oxidation states with a net positive charge that leads to an overall odd charge when combined. (b) In an octahedral configuration and a low-spin configuration, the energy-level diagrams for each metal ion would both have the \(t_{2g}\) orbitals filled with three electrons and the \(e_g\) orbitals empty. (c) The equivalent states of the metal ions result from rapid electron movement compared to nuclei, allowing the electron configurations to average out over time, and the strong metal-to-metal interactions promoted by the planar pyrazine ligand.

Step-by-step solution

a) Explaining the odd overall charge of the complex

Coordination complexes consist of a central metal ion surrounded by ligands. In the Creutz-Taube complex, the central ligand is pyrazine, which is neutral. However, the complex has an odd overall charge, implying that the metal ions in the complex exhibit oxidation states with a net positive charge that, combined, leads to an overall odd charge. This is due to the fact that the metal ions lose electrons during the formation of the coordination complex, leading to an imbalance of charge.

b) Drawing the d-orbital energy-level diagram for each metal ion

To draw the d-orbital energy-level diagrams, we assume an octahedral coordination (i.e., the six ligands are arranged symmetrically around the metal ion). In addition, we know that the metal ion in the Creutz-Taube complex is in a low-spin configuration. In an octahedral configuration, the d-orbitals split into two energy levels: the lower \(t_{2g}\) and the higher \(e_g\). The lower-energy \(t_{2g}\) orbitals consist of \(d_{xy}\), \(d_{xz}\), and \(d_{yz}\), while the higher-energy \(e_g\) orbitals include \(d_{x^2 - y^2}\) and \(d_{z^2}\). For a low-spin configuration, the electrons fill the lower-energy orbitals before pairing up in the higher-energy orbitals. Therefore, the energy-level diagrams for the two metal ions would look like this: Metal Ion 1: - \(t_{2g}\): Filled with three electrons (\(d_{xy}\), \(d_{xz}\), \(d_{yz}\)) - \(e_g\): Empty Metal Ion 2: - \(t_{2g}\): Filled with three electrons (\(d_{xy}\), \(d_{xz}\), \(d_{yz}\)) - \(e_g\): Empty

c) Explaining the equivalent states of the metal ions

In many experiments, the two metal ions in the Creutz-Taube complex appear to be in equivalent states. One possible reason for this observation is that the electrons in the complex's d-orbitals move rapidly compared to the nuclei of the metal ions. This rapid motion allows the electron configuration of each metal ion to effectively "average out" over time, making them appear identical. Furthermore, the planar pyrazine ligand promotes strong metal-to-metal interactions, which may lead to rapid electron transfer between the two metal ions. This can also contribute to the effective "blurring" of the metal ions' individual states, as the electrons are constantly moving between them and undergoing exchange. As a result, the metal ions might appear to be in equivalent states during experimental observations.

Key concepts

Coordination Chemistry

Coordination chemistry delves into the study of compounds formed by the coordination of one or more ligands to a central metal atom. This central metal atom, often a transition metal, can exhibit various oxidation states, leading to a plethora of possible coordination complexes.

In the case of the Creutz-Taube complex, we encounter a fascinating example: a complex featuring neutral ligands yet carrying an odd overall charge. This oddity is accounted for by the oxidation states of the metal ions—they've lost electrons, resulting in a net positive charge which offsets the neutrality of the ligands like pyrazine. The electropositive nature of ions leads to a tendency to attract ligands, forming a coordination sphere.

The delicate interplay between the ligands and the metal ions in coordination complexes greatly influences the physical and chemical properties of the compound. These properties come into play in various applications such as catalysts, biological systems, and materials science.

d-Orbital Energy-Level Diagram

Understanding the concept of d-orbital energy-level diagrams is essential when analyzing transition metal complexes like the Creutz-Taube complex. These diagrams aid in predicting and explaining the electronic structure of the metal ions within the complex.

In an octahedral field, energy levels of d-orbitals split into two groups due to electrostatic interactions between the electrons on the metal ion and the electric field created by the surrounding ligands. The three lower-energy orbitals known as the t_2g include the d_xy, d_xz, and d_yz orbitals. Meanwhile, the two higher-energy orbitals termed e_g consist of the d_{x² - y²} and d_z² orbitals.

A key aspect to consider is the spin configuration of the complex. The Creutz-Taube complex is a low-spin complex, meaning that the electrons fill the lower-energy t_2g orbitals maximally before they start pairing-up in the higher e_g orbitals. This configuration is crucial for understanding the magnetic properties and the reactivity of the complex.

Electron Configuration

The electron configuration of a coordination complex can reveal significant insights into the complex's chemical behavior and reactivity. For transition metals, the arrangement of electrons within the d-orbitals is particularly noteworthy.

In coordination chemistry, the term 'low-spin' indicates that the complex will fill with as many unpaired electrons as possible before pairing begins, resulting in fewer unpaired electrons. This is a product of the crystal field stabilization energy that arises due to the specific arrangement of ligands around the central metal ion.

For the Creutz-Taube complex, both metal ions display a low-spin d⁵ electron configuration within an octahedral field. This means that all five d-electrons are distributed across the three lower-energy t_2g orbitals. The visibly identical states of the two metal ions, as described in experiments, can be attributed to electron delocalization and rapid electron transfer facilitated by the metal-to-metal interactions through the bridging pyrazine ligand. This results in a perceived symmetry that contributes to the complex's unique chemical properties.