   Chapter 21, Problem 6RQ

Chapter
Section
Textbook Problem

# What is the major focus of the crystal field model? Why are the d orbitals split into two sets for an octahedral complex? What are the two sets of orbitals?Define each of the following.a. weak-field ligandb. strong-field ligandc. low-spin complexd. high-spin complexWhy is Co(NH3) 63+  diamagnetic whereas CoF63-is paramagnetic? Some octahedral complex ions have the same d-orbital splitting diagrams whether they are high-spin or low-spin. For which of the following is this true?a. V3+b. Ni2+c. Ru2+

Interpretation Introduction

Interpretation: The major focus of the crystal field model, the splitting of d orbitals into two sets for an octahedral complex and the two sets of orbitals are to be stated. Also the definition of weak-field ligand, strong-field ligand, low-spin complex and high-spin complex are to be defined. The magnetic behavior of the given complexes and whether they have the same d-orbital splitting diagram are to be explained.

Concept introduction: Crystal field theory states the breaking of degenerate orbitals (d or f orbitals) because of the presence of static electric field which is produced by the charge distribution of the neighboring anions. The octahedral complexes are those in which six atoms or ligands are arranged symmetrically around a central metal atom. Paramagnetic complexes are attracted towards the magnetic field and contain unpaired electron whereas diamagnetic complexes have all the electrons paired and hence, repelled from the magnetic field.

Explanation

Explanation

(I)

To determine: The major focus of the crystal field model, the splitting of d orbitals into two sets for an octahedral complex and the two sets of orbitals.

Step: 1

The crystal field model focuses on the energies of the d orbitals. The splitting of five degenerate orbitals is explained below.

The properties of complex ions are not fully illustrated by the localized electron model that is it gives no information about how the energies of d orbitals are affected by the complex ion formation. The crystal field model mainly focuses on the energies of the d orbitals and assumes that the ligands are approximated by the negative point charges and the metal ligand bonding is entirely ionic in nature.

The octahedral complexes are those having the coordination number of 6. The metal ion is present at the centre and ligands occupy the six corners of the octahedron. The splitting of five degenerate orbitals of the metal ion into sets of orbitals having different energies is called as crystal field splitting. In octahedral complex all the five orbitals are not of equal energy. The two orbitals dx2-y2 and dz2 are oriented along the axes while the remaining three orbitals dxy,dyz and dzx are oriented in between the axes. The two orbitals dx2-y2 and dz2 are designated as eg orbitals and the remaining three orbitals are designated as t2g orbitals.

Therefore the two sets of orbitals are,

• dx2-y2 and dz2.

dxy,dyz and dzx

(II)

(a)

To determine: The definition of weak-field ligand.

The definition of weak field ligand is given below.

Explanation

The ligand which produces the small crystal field splitting energy (Δ) like F,Cland Br is known as weak-field ligand.

(b)

To determine: The definition of strong-field ligand.

The definition of strong field ligand is given below.

Explanation

The ligand which produces the large crystal field splitting energy (Δ) like NO2and CN is known as strong-field ligand.

(c)

To determine: The definition of low-spin complex.

The definition of low-spin complex is given below.

Explanation

The complex in which maximum pairing of electrons occurs is called as low-spin complex. The maximum pairing of electrons results from the strong-field ligand. For example; the complex [Co(CN)6]3 is a low-spin complex because of the presence of strong field ligand CN and it has no unpaired electrons.

The crystal field diagram of [Co(CN)6]3 is shown in Figure 1.

Figure 1

The central metal atom is Co3+ has six valence electrons which are present in t2g orbital. All electrons are paired up by the influence of strong-field ligand.

(d)

To determine: The definition of high-spin complex.

The definition of high-spin complex is given below.

Explanation

The complex in which maximum number of unpaired electrons is present is called as high-spin complex. The maximum of number of unpaired electrons results from the weak-field ligand

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