Question

Determine the electron configuration and whether each of the following is paramagnetic. Also, determine if the complex undergoes a Jahn-Teller distortion.

1. [Fe(CN)6]4-

2. [Fe(CN)6]3-

3. [Co(H2O)6]3+

4. [CoF6]3-

5. [Ni(CO)6]2+

Answer 1

Basically, paramagnetic and diamagnetic refer to the way a chemical species interacts with a magnetic field. More specifically, it refers to whether or not a chemical species has any unpaired electrons or not.

A diamagnetic species has no unpaired electrons, while a paramagnetic species has one or more unpaired electrons.

1.

[Fe(CN)6]4-​

In this Fe is in +2 oxidation state.
Electronic configuration of Fe²⁺ is 4s⁰ 3d⁶. As CN^- is a strong field ligand, it causes the paring of the unpaired 3d electrons.

Since there are six ligands around the central metal ion, the most feasible hybridization is d²sp³. d₂sp³ hybridized orbitals of Fe²⁺, 6 electron pairs from CN^- ions occupy the six hybrid d2sp3 orbitals.

Since CN^- is strong field ligand it cause pairing. Hence it is diamagnetic.

2.

[Fe(CN)6]^3-​

In this Fe is in +3 oxidation state.
Electronic configuration of Fe²⁺ is 4s⁰ 3d⁵. As CN^- is a strong field ligand, it causes the paring of the unpaired 3d electrons. But one electron left unpaired.

Hence it is paramagnetic.

3.

[Co(H2O)6]3+

In this Co is in +3 oxidation state.
Electronic configuration of Co³⁺ is 4s⁰ 3d⁶. As H₂O is a weak field ligand, so it will not cause pairing of electrons and will have 6 unpaired electrons so it is paramagnetic.

4.

[CoF₆]^3-

In this Co is in +3 oxidation state.
Electronic configuration of Co³⁺ is 4s⁰ 3d⁶. As F^- is a weak field ligand, so it will not cause pairing of electrons and will have 6 unpaired electrons so it is paramagnetic.

For Jahn-Teller effects to occur in transition metals there must be degeneracy in either the t_2g or e_g orbitals. The electronic states of octahedral complexes are classified as either low spin or high spin. The spin of the system is dictated by the chemical environment. This includes the characteristics of the metal center and the types of ligands.​

Low spin complexes with d³, d⁵, d⁸, and d¹⁰ electrons that do no exhibit Jahn-Teller distortions. Degenerate electronic states occupying the e_g orbital set tend to show stronger Jahn-Teller effects. This is primarily caused by the occupation of these high energy orbitals. Since the system is more stable with a lower energy configuration, the degeneracy of the e_g set is broken, the symmetry is reduced, and occupations at lower energy orbitals occur.

1. [Fe(CN)₆]^4- - d⁶ - low spin complex (diamagnetic) - No Jahn-Teller distortion
2. [Fe(CN)₆]^3- - d⁶ - low spin complex (paramagnetic) - Jahn-Teller distortion
3. [Co(H₂O)₆]³⁺ - d⁶ -high spin complex - Jahn-Teller distortion
4. [CoF₆]^3- - d⁶ - high spin complex - Jahn-Teller distortion