municipal center vs metropolis

municipal center and metropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
municipal center	Yes	No	No	No
metropolis	Yes	No	No	No

municipal center (noun)	metropolis (noun)
the center of a city	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts