10.4.6: The Magnetochemical Series

The relative strengths of metal-ligand binding interactions distilled into the spectrochemical series depend on an inference drawn from the difference between the ground and excited states. In contrast, the magnetochemical series offers similar information based solely on observation of the ground state. These observations are possible in iron(III) porphyrin complexes because of a subtle change in spin state that occurs upon substitution of the axial position with different ligands. The result is a series analogous to the spectrochemical series that is called the magnetochemical series. This series was developed chiefly by the lab of Christopher Reed at University of Southern California and University of California, Riverside.1

There are two unusual things that happen with these porphyrin complexes that allow measurement of metal-ligand interactions in this way. A distortion that occurs in the weak field case of these square pyramidal complexes results in spin pairing in the weak field configuration rather than the high field case. Also, in these porphyrin complexes, a quantum mechanical admixture occurs in which the 5/2 and 3/2 states exist in superposition with each other. As a result, the spin state in these complexes is often intermediate between these two cases.

One additional feature makes these porphyrin complexes quite useful in measuring a magnetochemical series. Paramagnetic complexes produce dramatic shifts in nuclear magnetic resonance spectroscopy. In this case, the hydrogen atoms of the 5-membered pyrrole rings in the porphyrin system shift from about -60 ppm in the S = 3/2 case to about +80 ppm in the S = 5/2 case. In the case of a quantum admixture, the shift ranges in between these two limiting values. This NMR shift can be used to compare the field strength of the axial ligand.

An array of experiments eventually lead to a magnetochemical series. Example ligands from this series are shown in order here, from strong field to weak field:

\[\ce{NO+ = CO > R3Sn- > -CH3 > RS- > F- > -OPh > N3- = -OAc > NCS- > Cl- = -OH > Br- > I-} \nonumber \]

In addition, this method has allowed the inclusion of several very weakly bound ligands. These can be appended to the series as follows:

\[\ce{I- > ReO4- > BF4- > CF3SO3- > ClO4- > H2O > SbF6- > CB11H12-} \nonumber \]

Note the unusual position of water, which shows up as a much lower field ligand than in the Co(III) based spectrochemcial series for octahedral complexes. The difference is thought to come from the low spin Co(III) vs. the usually high spin Fe(III). Pi bonding is less favourable in the latter case and so the order in the iron porphyrin complexes is more strongly reflective of sigma donating effects. The anionic hydroxide is a better sigma donor than water because of greater electrostatic attraction to the metal.

Example \(\PageIndex{1}\)

Organometallic ligands such as \(\ce{-CH3}\) are not typically included in the spectrochemical series.

1. Characterize \(\ce{-CH3}\) in terms of ligand type (pi donor, sigma donor, pi acceptor).
2. Explain why it appears so high in the magnetochemical series compared to other anions such as F-.

Solution

1. -CH3 is a sigma donor.
2. The -CH3 anion would be extraordinarily basic. The pKa of CH4 is estimated to be approximately 50, compared to a pKa of approximately 4 for HF. The -CH3 anion is a very strong sigma donor.

References

1. Reed, C.; Guiset, F. "Reversal of H2O and OH- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes." J. Am. Chem. Soc., 2000, 122, 3281-2.