For each of the following, (i) give the systematic name of the compound and specify the oxidation state of the transition metal, (ii) draw a crystal field energy-level diagram and assign the d electrons to orbitals, (iii) indicate whether the complex is high-spin or low-spin (for d4 - d7 complexes), and (iv) specify the number of unpaired electrons. (d) K4[Os(CN)6]
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Identify the components of the complex K4[Os(CN)6]. The complex consists of potassium ions (K+) and the coordination complex [Os(CN)6]4-. The ligand CN- is a strong field ligand.
Determine the oxidation state of osmium (Os) in the complex. The overall charge of the complex ion [Os(CN)6]4- is -4. Each CN- ligand has a charge of -1, contributing a total of -6 from the six ligands. Therefore, the oxidation state of Os is +2, calculated as: x + 6(-1) = -4, where x is the oxidation state of Os.
Draw the crystal field energy-level diagram for the d orbitals of Os in the [Os(CN)6]4- complex. Since CN- is a strong field ligand, it will cause a large splitting of the d orbitals. The t2g orbitals will be lower in energy than the eg orbitals.
Assign the d electrons to the orbitals. Osmium in the +2 oxidation state has 6 d electrons (Os is in group 8, so it has 8 - 2 = 6 d electrons). Place these electrons in the t2g and eg orbitals according to the crystal field splitting.
Determine if the complex is high-spin or low-spin and specify the number of unpaired electrons. Since CN- is a strong field ligand, the complex is likely to be low-spin, meaning the electrons will pair up in the lower energy t2g orbitals before occupying the higher energy eg orbitals. Count the number of unpaired electrons based on this configuration.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Oxidation States
The oxidation state of an element in a compound indicates the degree of oxidation of that element, reflecting the number of electrons lost or gained. In transition metal complexes, the oxidation state is crucial for determining the metal's charge and its behavior in chemical reactions. For example, in K4[Os(CN)6], the oxidation state of osmium can be calculated based on the charges of the ligands and the overall charge of the complex.
Crystal Field Theory (CFT) explains the electronic structure of transition metal complexes by considering the effect of ligands on the d-orbitals of the metal ion. According to CFT, the presence of ligands causes the degenerate d-orbitals to split into different energy levels, which influences the arrangement of electrons. This theory is essential for drawing energy-level diagrams and determining the electronic configuration of the complex.
The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.
High-Spin vs. Low-Spin Complexes
High-spin and low-spin designations refer to the arrangement of electrons in the d-orbitals of transition metal complexes, influenced by the strength of the ligands. High-spin complexes have unpaired electrons in higher energy orbitals, resulting from weak field ligands, while low-spin complexes have paired electrons in lower energy orbitals due to strong field ligands. This distinction is critical for predicting magnetic properties and the number of unpaired electrons in the complex.
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