Preparatory it; not possible for complements

I was reading Practical English Usage, by Michael Swan and got into something that has got me deeply confused. It basically says that preparatory it can be used as a preparatory subject or object, but not as a preparatory complement.
Firstly, I didn’t even know that English made a difference between complement and object, nor had I ever found a context in which I would really care. I searched online and I think I understand the difference now, but the issue with "it" still makes me want to cry.
The examples the book uses are:
-This chair is comfortable to sit on. (not It is comfortable to sit on this chair).
(to sit on is the complement of comfortable.)
They both sound fine to me, although I think I get that that "it" is making comfortable a subject. Here I ask: what about This chair is to sit on/ It is to sit on, this chair. I know they sound weird, but I use them as an example, as I want to understand the grammar behind it.
-The impression was given that travel expenses would be paid. Here expenses is the complement of the impression).

As you see, both these examples contain complement objects, not complements that affect directly the subject (which are far more common and easy to grasp, I believe).

So, how does this work? Could you give me some more examples that make the issue clearer?
What about, say… "The chair is wonderfully beautiful"; "It is a wonderfully beautiful chair"; "It is wonderfully beautiful, the chair".

PS: I am confused/ it confuses me/ it is Confucius.

Thanks in advance!