Frankfurt on the Main vs metropolis

Frankfurt on the Main and metropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
Frankfurt on the Main	Yes	No	No	No
metropolis	Yes	No	No	No

Frankfurt on the Main (noun)	metropolis (noun)
a German city; an industrial and commercial and financial center	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts