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I will keep it brief; I know that there are weak field, "moderate" field, and strong field ligands and that these might contribute to a complex being high or low spin overall. My question is, does a complex having high spin indicate a weak field complex, regardless of ligand "strength" (or vice versa)?

For example, $\ce{[Fe(H2O)6]^3+}$ has a moderate strength ligand ($\ce{H2O}$) but is overall a high spin complex, so would $\ce{[Fe(H2O)6]^3+}$ be considered a weak field complex, despite the ligand strength not being weak? If not, is it only appropriate to call it a high spin complex? Thank you.

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    $\begingroup$ @NisargBhavsar Sorry, that's not right. The complex $\ce{[Co(CO)5]}$ does not exist. $\endgroup$
    – S R Maiti
    Apr 23, 2021 at 8:26
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    $\begingroup$ Also, @ajent what do you mean by "weak-field complex"? Ligands can be weak or strong-field and complexes can be low or high-spin. I have never seen complexes referred to as weak or strong-field. $\endgroup$
    – S R Maiti
    Apr 23, 2021 at 8:30
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    $\begingroup$ There is no such thing as a weak-field complex. || @NisargBhavsar There is no such thing as [Co(CO)6] either. Cobalt forms a dinuclear carbonyl: Co2(CO)8. en.wikipedia.org/wiki/Dicobalt_octacarbonyl $\endgroup$
    – orthocresol
    Apr 23, 2021 at 11:12
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    $\begingroup$ @ShoubhikRMaiti sorry for the late response, I have been busy. I'm trying to understand a question on my homework for this class; the way it's worded implies that the complex itself be labeled as weak-field or strong-field, rather than merely the ligand. I assume this is just a grammatical problem if it is inappropriate to call the complex itself weak-field or strong-field. $\endgroup$
    – ajent
    Apr 25, 2021 at 23:58

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The spectrochemical series only gives info on the relative strength of the ligand field. Whether or not a metal complex is high or low spin (assuming there is an appropriate number of d electrons that both are possible) is a function of both the ligand and the metal center.

Thus, it is possible for "weak field" ligand to be in a low spin complex if the metal center favors low spin. For example, almost all complexes of second and third series transition metals are low spin, independent of ligand identities.

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  • $\begingroup$ Giving this the answer so that it doesn't remain unanswered for eternity. I think my confusion stems from the problem I am working with being worded unclearly. Thanks for your response though! $\endgroup$
    – ajent
    Apr 26, 2021 at 0:02

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