That-clause in "it seems that"
It seemed [that he was correct].
The subject "it" is semantically empty here, so it's not possible for the bracketed declarative content clause to function as PC, though it is an internal complement of "seemed".
The construction bears some resemblance to an extraposition construction, but differs in that the subordinate clause cannot occupy subject position: "*That he was correct seemed" is of course totally ungrammatical.