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Ch.21 - Transition Elements and Coordination Chemistry
All textbooksMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblem 21-113c
Chapter 21, Problem 21-113c

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons.
(c) [Co(NCS)4]2- (tetrahedral)

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Identify the oxidation state of the metal ion in the complex. For [Co(NCS)4]2-, cobalt is bonded to four thiocyanate ions. Since each NCS ion typically acts as a -1 ligand, the overall charge of the complex is -2, leading to an oxidation state of +2 for cobalt (Co^2+).
Determine the electron configuration of the metal ion. Cobalt in its +2 oxidation state (Co^2+) has an electron configuration of [Ar] 3d^7.
Consider the geometry of the complex to determine the splitting of the d-orbitals. In a tetrahedral complex, the d-orbitals split into two sets: e (higher energy, containing the dxy, dxz, dyz orbitals) and t2 (lower energy, containing the dx^2-y^2, dz^2 orbitals).
Assign the electrons of Co^2+ to the split d-orbitals in the tetrahedral field. Start filling the lower energy t2 orbitals first (three orbitals) with up to six electrons, then fill the higher energy e orbitals (two orbitals) with the remaining electron(s).
Count the number of unpaired electrons. Electrons will fill each orbital singly before pairing according to Hund's rule. Calculate the total number of unpaired electrons to predict the magnetic properties of the complex.

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Key Concepts

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

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 tetrahedral complexes, the d-orbitals split into two sets: the lower energy e orbitals and the higher energy t2 orbitals. This splitting is crucial for determining the electronic configuration and the magnetic properties of the complex.
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The study of ligand-metal interactions helped to form Ligand Field Theory which combines CFT with MO Theory.

Electron Configuration

The electron configuration of an atom or ion describes the distribution of electrons among the available atomic orbitals. For transition metals like cobalt, the electron configuration can change based on oxidation state and ligand field strength. In the case of [Co(NCS)4]2-, understanding the electron configuration helps in predicting how many electrons occupy the d-orbitals and whether they are paired or unpaired.
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Electron Configuration Example

Unpaired Electrons and Magnetism

Unpaired electrons in an atom or ion contribute to its magnetic properties. A complex with unpaired electrons exhibits paramagnetism, while one with all paired electrons is diamagnetic. By analyzing the electron distribution in the crystal field energy-level diagram, one can determine the number of unpaired electrons in [Co(NCS)4]2-, which is essential for predicting its magnetic behavior.
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Related Practice
Textbook Question

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons. 

(d) [Cu(en)2]2+ (square planar) 

Textbook Question

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons.

(a) [Pt(NH3)4]2+ (square planar)

Textbook Question

For each of the following complexes, draw a crystal field energy-level diagram, assign the electrons to orbitals, and predict the number of unpaired electrons.

(b) [MnCl4]2- (tetrahedral)

Textbook Question

Which of the following complexes are paramagnetic?

(a) [Mn(CN)6]3-

(b) [Zn(NH3)4]2+ (tetrahedral)

(c) [Fe(CN)6]4-

(d) [FeF6]4-

Textbook Question

Which of the following complexes are diamagnetic?

(a) [Ni(H2O)6]2+

(b) [Co(CN)6]3-

(c) [HgI4]2- (tetrahedral) 

(d) [Cu(NH3)4]2+ (square planar) 

Textbook Question

Although Cl- is a weak-field ligand and CN- is a strong field ligand, [CrCl6]3- and [Cr(CN)6]3- exhibit approximately the same amount of paramagnetism. Explain.