[_Espard_]

Are there any metal ions that have a higher affinity for nitrogen-based ligands over oxygen-based ligands (in aqueous solution)? Is it possible for bound oxygen ligands to be replaced at the expense of nitrogen ligands?

[kriggy]

Everything is possible?
I think you can read about HSAB theory it might help you.
For example Cu, Zn or Fe are usualy coordinated by histidine in biological systems so I assume those prefer nitrogen based ligands. Basicaly, if you synthesize complex with N-donors in water it means that the metal has more afinity to nitrogen than to oxygen because the metal ion in water is usualy in form on hexaaqua complexes (depending on pH could be hydroxo or oxo complex)