The generally accepted explanation for the existence of the asteroid belt is that constant gravitational disturbances from the nearby Jupiter would prevent any large body of mass to form.
As for 'Tiamat', I really doubt there's enough solid matter in the inner solar system to create a planet the size of Neptune.
Also, a planetary collision does not imply the creation of an asteroid field. It is widely accepted that a collision between a Mars-sized planet and Earth in the early days of planetary formation resulted in the creation of our moon, not an asteroid belt. (
http://exosci.com/news/146.html )