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Thread: Metal complex oxidation states

  1. #1 Metal complex oxidation states 
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    Hello everyone, I am just studying inorganic chemistry as a hobby really and have a question about oxidation states of metals in a complex.

    In one example I saw there was a cobalt with 6 h20 ligands attached and a overall oxidation state of +3 but in the second example the same complex was shown with a +2 charge. How can the same complex have different charges?

    Does the metal have a +3 or +2 charge to start with before the ligands attach?

    Thanks.


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  3. #2  
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    Quote Originally Posted by Nick72 View Post
    Hello everyone, I am just studying inorganic chemistry as a hobby really and have a question about oxidation states of metals in a complex.

    In one example I saw there was a cobalt with 6 h20 ligands attached and a overall oxidation state of +3 but in the second example the same complex was shown with a +2 charge. How can the same complex have different charges?

    Does the metal have a +3 or +2 charge to start with before the ligands attach?

    Thanks.
    Cobalt has two commonly observed oxidation states, +2 and +3, and can form two cations, Co2+ and Co3+. Either of these can, I think, be present as hydrated octahedral complexes, i.e. with 6 H2O molecules coordinated to it. So they are not "the same" complex. but two different ones. In fact, from what I read, the hydrated Co3+ ion is formed by oxidation of the corresponding hydrated Co2+ ion. But in most reactions the Co3+ complex turns into something else (oxide or hydroxide etc), goes dark and precipitates.

    Maybe PhDemon can tell you more. I am very rusty on this stuff.


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  4. #3  
    Bullshit Intolerant PhDemon's Avatar
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    Yep, you're right exchemist. Cobalt can form +2 and +3 ions which are different. They will both be octahedral complexes surrounded by 6 water molecules in solution but they have different electronic structures (the +3 has one electron less than the +2 obviously) so have different chemistry. The +3 complex due to the higher charge density will be more acidic than the +2 complex and it will be easier for this to form a hydroxide precipitate than the +2 complex (although both will do this if you add a hydroxide to them).

    The OP might find this page useful: cobalt
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  5. #4  
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    Quote Originally Posted by PhDemon View Post
    Yep, you're right exchemist. Cobalt can form +2 and +3 ions which are different. They will both be octahedral complexes surrounded by 6 water molecules in solution but they have different electronic structures (the +3 has one electron less than the +2 obviously) so have different chemistry. The +3 complex due to the higher charge density will be more acidic than the +2 complex and it will be easier for this to form a hydroxide precipitate than the +2 complex (although both will do this if you add a hydroxide to them).

    The OP might find this page useful: cobalt
    Aha yes thanks for this. I had forgotten the bit about the higher acidity of the more highly charged species. So some of the water molecules lose H+, turning into OH-, thereby reducing the net charge on the complex...and then precipitate as hydroxide.

    I was never much good at transition metal chemistry - too complicated, fiddly and unsatisfyingly unpredictable. Apart from cool things like ligand field theory and the Jahn-Teller Theorem...... I remember molybdates and vanadates more or less finished me off. Maybe now, in my dotage, I should take an interest in it again.
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    Bullshit Intolerant PhDemon's Avatar
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    Ah the old Jahn-Teller Effect, that was always my stock answer to explain any "funny" behavior in inorganic chemistry

    (For completeness the others are quantum tunneling for physical chemistry and "solvent interactions" for organic )
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    Thank you exchemist and Phdemon for your replies.

    Are the metal cations formed before the water ligands attach? For the +2 charge is there a counter -2 anion present? And the same for the +3 charge? Thanks.
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  8. #7  
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    Quote Originally Posted by PhDemon View Post
    Ah the old Jahn-Teller Effect, that was always my stock answer to explain any "funny" behavior in inorganic chemistry

    (For completeness the others are quantum tunneling for physical chemistry and "solvent interactions" for organic )

    Thank you exchemist and Phdemon for your replies.

    Are the metal cations formed before the water ligands attach? For the +2 charge is there a counter -2 anion present? And the same for the +3 charge? Thanks.
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  9. #8  
    Bullshit Intolerant PhDemon's Avatar
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    Yes, the ions will be present in an ionic compound which is a solid, the complexes (with water ligands) only form when it is dissolved in water. The counterions will be whatever negative ions were present in the original salt.
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