Q. 46

Using crystal field theory, draw energy level diagram, write the electronic configuration of the central metal atom/ion and determine the magnetic moment value in the following:

(i) [CoF6]3–, [Co(H2O)6]2+ , [Co(CN)6]3–

(ii) [FeF6]3–, [Fe(H2O)6]2+, [Fe(CN)6]4–

Answer :

(i)

• In [CoF6]3–, the central metal atom is cobalt Co and oxidation state of Co in hereis: x=-6 times (-1)-3=+3


Hence, Co3+has a d6 configuration (1s22s22p63s23p63d6, while losing 1 3d and 2 4s electrons.) So the orbital energy level diagram is:



And the number of unpaired electrons=4.


Therefore, the magnetic moment of Co= = 4.9 BM. (BM stands for Bohr Magneton)


• For [Co(H2O)6]2+ the central metal atom is also Co and the oxidation state of Co here is x=2+; as aqua molecules are neutral.


Hence Co2+ has a d7 configuration (1s22s22p63s23p63d7,while losing only the 2 4s electrons.) So the orbital energy level diagram is:



And the number of unpaired electrons=3.


Therefore, the magnetic moment of Co= = 3.87 BM. (BM stands for Bohr Magneton).


• For [Co(CN)6]3– the central atom is Co, the oxidation state of Co in here is x=-6 times (-1)-3=+3.


Hence, Co3+ has a d6 configuration (1s2 2s2 2p6 3s2 3p6 3d6, while losing 1 3d and 2 4s electrons.) So the orbital energy level diagram is:


2.PNG


There will be no unpaired electrons as it is diamagnetic,(as CN- is a strong field ligand, the energy gap becomes larger and all the electrons get paired) therefore no value of magnetic moment can be found.


(ii).


• In the case of [FeF6]3– the central metal atom here is Fe and the oxidation state of Fe x=-6 times (-1)-3=+3.


Hence, Fe3+ will have a d5 configuration (1s2 2s2 2p6 3s2 3p6 3d5).So the orbital energy level diagram is:



Therefore it is paramagnetic and number of unpaired electrons are =5


Therefore the value of magnetic moment for Fe3+ = = 5.92 BM.


For [Fe(H2O)6]2+the central metal atom is iron Fe and the oxidation state of Fe in here is x=2+.


Hence, it has a d6 configuration (1s2 2s2 2p6 3s2 3p6 3d6).


So the orbital energy level diagram is:



Therefore, it has 4 numbers of unpaired electrons


Hence, the magnetic moment= = 4.9 BM. (BM stands for Bohr Magneton).


For [Fe(CN)6]4– the central metal atom is Fe and its oxidation state is x=-6 times(-1)-4=+2.


Hence, it has a d6 configuration (1s2 2s2 2p6 3s2 3p6 3d6).


So the orbital energy level diagram is:



It has no unpaired electrons as CN-are strong field ligand and the energy gap becomes larger, therefore [Fe(CN)6]4– is diamagnetic.


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