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. (f) Na2[Fe(CO)4]
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Identify the systematic name of the compound: The compound is a coordination complex with sodium (Na) as the counterion and [Fe(CO)4] as the complex ion. The systematic name is sodium tetracarbonylferrate. The oxidation state of iron (Fe) can be determined by considering the charge of the complex ion. Since CO is a neutral ligand, the charge on the complex ion is the same as the charge on the metal. Therefore, the oxidation state of Fe is -2.
Draw the crystal field energy-level diagram: In a tetrahedral complex like [Fe(CO)4], the d-orbitals split into two sets: e (lower energy) and t2 (higher energy). The e set contains two orbitals, and the t2 set contains three orbitals.
Assign the d electrons to orbitals: Iron in the -2 oxidation state has an electron configuration of [Ar] 3d^8. Distribute these 8 d electrons among the e and t2 orbitals according to Hund's rule and the Pauli exclusion principle.
Determine if the complex is high-spin or low-spin: Tetrahedral complexes are typically high-spin because the energy difference between the e and t2 orbitals is small, and electrons prefer to remain unpaired to minimize electron-electron repulsions.
Specify the number of unpaired electrons: Count the number of unpaired electrons in the d orbitals after assigning them according to the high-spin configuration.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Coordination Compounds
Coordination compounds consist of a central metal atom or ion bonded to surrounding molecules or ions called ligands. The nature of these bonds and the arrangement of ligands around the metal center significantly influence the properties of the complex, including its color, magnetism, and reactivity. Understanding the structure and bonding in coordination compounds is essential for determining their systematic names and oxidation states.
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 can be represented in a crystal field energy-level diagram. This theory helps predict the electronic configuration of the metal ion and whether the complex will be high-spin or low-spin based on the strength of the ligands.
The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.
Spin States and Unpaired Electrons
The spin state of a transition metal complex refers to the arrangement of electrons in the d orbitals, which can be either high-spin or low-spin. High-spin complexes have more unpaired electrons due to a weaker field from the ligands, while low-spin complexes have fewer unpaired electrons as a result of stronger ligand fields that promote pairing of electrons. The number of unpaired electrons is crucial for understanding the magnetic properties of the complex and is determined by the electron configuration derived from the crystal field splitting.
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