Skip to main content
Chemistry LibreTexts

Homework 5: Coordination Complexes

There are select solutions to these problems here.


If \(\ce{A}\), \(\ce{B}\), \(\ce{C}\), \(\ce{D}\) are four different ligands in a tetrahedral complex of \(\ce{[CuABCD]^2+}\). Will this complex ion exhibit optical isomerism?


Explain how the crystal field theory explains the different color of transition metal compounds.


If the \(\ce{Co^2+}\) ion is linked with strong-field ligands to produce an octahedral complex, the complex has one unpaired electron, but if linked with weak-field ligands, the compound has four unpaired electrons. How does one account for this?



  1. which of the following complex ions, \(\ce{[Co(CN)6]^3-}\) and \(\ce{[MnI6]^4-}\), is diamagnetic and which is paramagnetic
  2. the number of unpaired electrons expected for the tetrahedral complex ion\(\ce{[Mn(NO2)4]^2-}\).


If the complex \(\ce{[Fe(NH3)6]^2+}\) is paramagnetic, can you tell whether it is an octahedral or tetrahedral complex? What if the complex were diamagnetic? 


Write equations for the following:

  1. When \(\ce{NaOH(aq)}\) is added to \(\ce{Co(NO3)2(aq)}\), a rose-red precipitate forms.
  2. Adding \(\ce{NH3}\) makes the precipitate redissolve. 


Which of the following would you expect to have the largest overall \(\mathrm{K_f}\) and why? \(\ce{[Co(NH3)6]^3+}\), \(\ce{[Co(en)3]^3+}\), \(\ce{[Co(H2O)6]^3+}\)


Give an explanation as to why transition metal compounds colored?  Include the crystal field theory when answering.


When \(\ce{Fe^3+}\) is linked with a strong field ligand it has a single unpaired electron.  When \(\ce{Fe^3+}\) is linked with a weak field ligand, there are five unpaired electrons.  Draw the orbital occupancy for each and explain the difference. 


  1. Which of the complex ions, \(\ce{[FeI6]^3-}\) and \(\ce{[Co(CN)6]^3-}\), is paramagnetic and which is diamagnetic?
  2. How many unpaired electrons are expected for the tetrahedral complex ion \(\ce{[FeBr6]-}\)?


Using magnetic properties, we are able to choose between a tetrahedral, octahedral, or square planar structure for complexes. Using this information, what is the estimated structure for \(\ce{[Cu(Cl)6]^2-}\)?


By mixing an aqueous solution of \(\ce{Cu^2+}\) with \(\ce{NH3}\), the color changed from light blue to deep blue. Write an equation to represent this observation.


Considering the following complex ions, \(\ce{[Fe(H2O)4(en)]^2+}\), \(\ce{[Fe(H2O)3(NH3)3]^2+}\), \(\ce{[Fe(en)3]^2+}\), \(\ce{[Fe(NH3)6]^2+}\), which would you believe to have the largest overall \(\mathrm{K_f}\)? Explain your reasoning.


  1. How many unpaired e- would you find in the octahedral complex \(\ce{[ZnCl6]^4-}\)?
  2. How many unpaired electrons would you expect to find in the tetrahedral complex \(\ce{[CuI4]^2-}\)? Would you expect more unpaired electrons in the octahedral complex \(\ce{[Mn(NH3)6]^3+}\)?


  1. Use crystal field theory to draw the electron structure of \(\ce{[CoF6]^3-}\) in a weak field.
  2. Determine whether \(\ce{[Cu(H2O)4]^2+}\) is paramagnetic or diamagnetic.


  1. The color of \(\ce{[Fe(H2O)6]^3+}\) is violet, and \(\ce{[Fe(NH3)6]^3+}\) is yellow in color. Explain why each metal complex has a different color.
  2. One of the following nickel complexes ( \(\ce{[Ni(H2O)6]^2+}\) and \(\ce{[Ni(CN)4]^2-}\)) is green and the other is yellow. Indicate the color of each, and explain how you came to this conclusion.  

Additional Problems without Solutions


State the types of isomerism that may be exhibited by the following complexes, and draw structures of the isomers.

  1. \([Co(en)_2 (ox)]^+\)
  2. \( [Co(en)(NH_3)_2Cl_2]^{2+}\) 


The complexes \([NiCl_4]^{2–}\) and \([Ni(CN)_4]^{2–}\) are paramagnetic and diamagnetic, respectively. What does this tell you about their structures?


Consider a pair of isomeric cationic complexes having the molecular formula \([Co(en)_2Br_2]ClO_4\). One is optically active but the other is not.

  1. Write structural formulas showing each isomeric complex but only one of the enantiomers. en = \(H_2NCH_2CH_2NH_2\).
  2. What is the oxidation state of cobalt in these complexes? How many \(d\) electrons are found in each complex?


When the d orbitals of the central metal ion are split in energy in an octahedral ligand field, which orbitals are raised least in energy?

  1. \(d_{xy}\) and \(d_{x^2-y^2}\)
  2. \(d_{xy}\), \(d_{xz}\) and \(d_{yz}\)
  3. \(d_{xz}\) and \(d_{yz}\)
  4. \(d_{xz}\), \(d_{yz}\) and \(d_{z^2}\)
  5. \(d_{x^2-y^2}\) and \(d_{z^2}\)


Which one of the following statements is FALSE?

  1. In an octahedral crystal field, the d electrons on a metal ion occupy the \(e_g\) set of orbitals before they occupy the \(t_{2g}\) set of orbitals.
  2. Diamagnetic metal ions cannot have an odd number of electrons.
  3. Low spin complexes can be paramagnetic.
  4. In high spin octahedral complexes, \(\Delta_{o}\) is less than the electron pairing energy, and is relatively very small.
  5. Low spin complexes contain strong field ligands.


Consider the complex ion \([Mn(OH_2)_6]^{2+}\) with 5 unpaired electrons. Which response includes all the following statements that are true, and no false statements?

  1. It is diamagnetic.
  2. It is a low spin complex.
  3. The metal ion is a d5 ion.
  4. The ligands are weak field ligands.
  5. It is octahedral.


  1. I, II
  2. III, IV, V
  3. I, IV
  4. II, V
  5. III, IV


Consider the violet-colored compound, \([Cr(OH_2)_6]Cl_3\) and the yellow compound, \([Cr(NH_3)_6]Cl_3\). Which of the following statements is false?

  1. Both chromium metal ions are paramagnetic with 3 unpaired electrons.
  2. \(\Delta_{o}\) for \([Cr(NH_3)_6]^{3+}\) is calculated directly from the energy of yellow light.
  3. \(\Delta_{o}\) for \([Cr(OH_2)_6]^{3+}\) is less than \(\Delta_{o}\) for \([Cr(NH_3)_6]^{3+}\).
  4. A solution of \([Cr(OH_2)_6]Cl_3\) transmits light with an approximate wavelength range of 4000 - 4200 angstroms.
  5. The two complexes absorb their complementary colors.


Strong field ligands such as CN-:

  1. usually produce high spin complexes and small crystal field splittings.
  2. usually produce low spin complexes and small crystal field splittings.
  3. usually produce low spin complexes and high crystal field splittings.
  4. usually produce high spin complexes and high crystal field splittings.
  5. cannot form low spin complexes.