alex1114
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Hi! So i´m preparing for the Chemistry A2 exams and i have a doubt: If OH- and NH3 act as bases and deprotonate hydrated d block ions, why isn´t OH- considered a ligand as well? Why the reaction with excess ammonia with these hydrated d block ions results sometimes in what is called ligand exchange? So, when do i name a reaction as ligand exchange and when as acid base? Please i am quite confused, i would be really grateful for your help.
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Pigster
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It is a ligand.

The O has a lone pair, with which it can form coordinate bonds.

If OH- deprotonates a water, it leaves behind an OH- attached to the central ion. It might not be actually substitution, but the end result is much the same.
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VeeOMGlob
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You could get asked to show when NH3 acts like a bronsted lowry base, this is when it accepts a proton.
Co(H2O)6]2 + 2(NH3) = Co(H2O)4(OH)2 + 2(NH4+)

or when it acts like a Lewis base, when it donates an electron pair-This is when NH3 acts like a ligand, as it donates its lone pair to form a coordinate bond with the metal ion.
Co(H2O)6]2 + 6(NH3) = Co(NH3)6 + 6(H2O)

you just have to learn that NH3 only acts as a lewis base and does a ligand exchange with the 3 metal ions (Cu, Co, Cr) and that Cu doesnt loose all of its water ligands.
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