I was watching an old [Explosions and Fire video](https://youtu.be/a60UewomiCg) where he talks about BCNP. He mentions it being a complex of a transition metal, but doesn’t really explain what that means.
WIkipedia says that complexes are something that happens when atoms or groups of atoms join to another molecule, but it doesn’t explain how that’s different from a normal compound. It even says that complexes form via ionic and covalent bonds just like compounds do.
Later in the video, Ex&F says that similar compounds based on different metals can take different numbers of ligands. What determines how many ligands a compound can take? Is there something like electron valency happening here, or is it some other property?