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I am not sure how accurate I can predict the magnetic properties of actinide diamagnetic complexes, due to the fact that they share the F-block electron as Lanthanides, which are already tricky, combined with the fact that the high atomic mass of actinides starts affecting its electron shell in ways that lower mass elements do not achieve. If there are, could some examples be given or perhaps an accessible page?

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    $\begingroup$ Actinium 3+ complexes would be diamagnetic for example, because Actinium 3+ does not have any f-electrons. $\endgroup$
    – S R Maiti
    Jun 20 at 9:07
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Shoubhik's comment mentions actinium, but thorium offers a more common example. The relativistic effects referred to in the OP actually have relatively little chemical impact in the early groups of the Periodic Table, including lanthanides/actinides, and thorium can be ionized all the way to its radon core ($\ce{Th^{4+}}$). Such a core would then be diamagnetic as shown in the negative magnetic susceptibility reported for $\ce{ThO2}$.

Among complexes, Wikipedia mentions halide complexes ans specifically gives $\ce{ThF6^{2-}}$. From the WP article on the element thorium:

Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.[1] For example, when treated with potassium fluoride and hydrofluoric acid, $\ce{Th^{4+}}$ forms the complex anion $\ce{ThF6^{2-}}$, which precipitates as an insoluble salt, $\ce{K2ThF6}$.[2]

Cited References

  1. Wickleder, M. S.; Fourest, B.; Dorhout, P. K. (2006). "Thorium". In Morss, L. R.; Edelstein, N. M.; Fuger, J. (eds.). The Chemistry of the Actinide and Transactinide Elements (PDF). 3 (3rd ed.). Springer-Verlag. pp. 78-94. doi:10.1007/1-4020-3598-5_3. ISBN 978-1-4020-3598-2. Archived from the original (PDF) on 14 December 2017.

  2. Hyde, E. K. (1960). The radiochemistry of thorium (PDF). National Academy of Sciences. Retrieved 29 September 2017.

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I still do not have enough points to comment but I would like to add something to Oscar’s comment on relativistic effects not being present on such early elements in the Periodic Table. I am afraid that that is not the case. You are right that it is not enough to affect the magnetic properties but yes for other physical properties.

https://www.chemistryworld.com/news/relativity-behind-mercurys-liquidity/6297.article

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  • $\begingroup$ They're present, but they have relatively little effect on Chemistry early in the Periodic Table. The greater chemical effects come in later groups. $\endgroup$ Jun 20 at 20:19

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