In octahedral complexes, the choice between high-spin and low-spin electron configurations arises only for d4 - d7 complexes. Explain.

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Understand that octahedral complexes have a central metal ion surrounded by six ligands, creating a specific arrangement of d-orbitals.

Recognize that the d-orbitals in an octahedral field split into two sets: the lower energy t2g set and the higher energy eg set.

Identify that the energy difference between these sets is called the crystal field splitting energy, denoted as Δ_oct.

Realize that for d4 to d7 electron configurations, the choice between high-spin and low-spin states depends on the relative size of Δ_oct compared to the pairing energy of electrons.

Conclude that for d4 to d7 configurations, if Δ_oct is small, electrons will occupy higher energy eg orbitals (high-spin), whereas if Δ_oct is large, electrons will pair in the lower energy t2g orbitals (low-spin).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Octahedral Complexes

Octahedral complexes are coordination compounds where a central metal ion is surrounded by six ligands arranged at the corners of an octahedron. The geometry and electronic properties of these complexes are influenced by the nature of the metal ion and the ligands, which can affect the splitting of d-orbitals and the overall stability of the complex.

Crystal Field Theory

Crystal Field Theory (CFT) explains how the arrangement of ligands around a central metal ion affects the energy levels of its d-orbitals. In octahedral complexes, the d-orbitals split into two energy levels: the lower-energy t2g and the higher-energy eg. The extent of this splitting determines whether electrons will occupy the lower or higher energy levels, leading to high-spin or low-spin configurations.

High-Spin vs. Low-Spin Configurations

High-spin and low-spin configurations refer to the arrangement of electrons in the d-orbitals of transition metal complexes. High-spin configurations occur when electrons occupy higher energy orbitals to minimize electron pairing, typically favored in weak field ligands. In contrast, low-spin configurations result from strong field ligands that cause greater splitting, leading to more electron pairing in the lower energy orbitals, particularly relevant for d4 to d7 complexes.

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Textbook Question

The reaction of the octahedral complex Co(NH3)3(NO2)3 with HCl yields a complex [Co(NH3)3(H2O)Cl2]+ in which the two chloride ligands are trans to one another.

(a) Draw the two possible stereoisomers of the starting material [Co(NH3)3(NO2)3]. (All three NO2- ligands are bonded to Co through the N atom.)

(b) Assuming that the NH3 groups remain in place, which of the two starting isomers could give rise to the observed product?

Textbook Question

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.

(a) (NH₄)[Cr(H₂O)₆](SO₄)₂

Textbook Question