Well... I'd say you placed the main light properly, but you need a fill light. And yes... the photograph as presented here is very underexposed.
The clothing selection and setting is OK, using all deep tones of color. The posing, however is week, with her entire lower portion of her body out in front of her torso and head.
A slightly longer focal length would have helped this one too.
I wouldn't worry too much about her right toe coming to the edge of the frame. Interrupting the circles formed by the front of the chair arms is more troubling.
I hope this helps.
-Pete