Naming

[itzeelpadillaDis1A]

How do we know when to put (H2O) or (OH2)?

[404995677]

I think we put (OH2) or (H2O) when we are writing the ligand's structure to reference which element is attached to the transition metal.

[Sabah Islam 1G]

When pairing a ligand with a transition metal cation, you write the ligand based on which atom in the ligand is exactly donating electrons to the cation (whichever atom is electron rich). Therefore, you either write it forwards or backwards to emphasize which atom is donating electrons. You can think of it as, drawing the line for the coordinate covalent bond to whichever atom is donating.
https://upload.wikimedia.org/wikipedia/ ... cation.png