Currently reading "A Student's Introduction to English Grammar" by Geoffrey K. Pullum and Rodney Huddleston.
Consider the following contrast between the phrasal verbs ask for and come across.
"The information for which I asked."—is given as grammatical.
"The information across which I came."—is given as ungrammatical, as in this case preposition fronting is not allowed.
Are there any good dictionaries which clearly state when preposition fronting is allowed? None of the freely available ones seem to have this feature (Merriam-Webster, Macmillan, Oxfordlearnersdictionaries), not even the web version of the Oxford English Dictionary.
Edit in response to comments: In the example above, 'across' is correctly classified as a preposition. You can see that it can be a complement for a noun phrase in the example: "My travels across the ocean have been perilous."