complex vs difficult

complex is a noun but difficult is not a noun.

Word	Noun	Adjective	Verb	Adverb
complex	Yes	Yes	No	No
difficult	No	Yes	No	No

complex (noun)	difficult (noun)
a whole structure (as a building) made up of interconnected or related structures
a conceptual whole made up of complicated and related parts
(psychoanalysis) a combination of emotions and impulses that have been rejected from awareness but still influence a person's behavior
a compound described in terms of the central atom to which other atoms are bound or coordinated

complex (adjective)	difficult (adjective)
complicated in structure; consisting of interconnected parts	not easy; requiring great physical or mental effort to accomplish or comprehend or endure
difficult to analyze or understand	hard to control