My understanding is that, for flight stability, the center of mass must be ahead of the aerodynamic center.
There have been some rather big jets with 2 or 3 jet engines in the tail. Tu-154 and Boeing 727 have 3 in the tail. The DC-9 has 2, and there seem to be countless small private jets with twin engines in/near the tail, like the Learjet 45.
Another example is the Concorde and Tu-144 which had a whopping 4 engines very near the tail. If I'm not mistaken those were some extra-heavy engines too since they had to be supersonic.
So if we're putting the heavy engines in the tail, what is done to compensate? Do they just use ballast in the nose? How is the center of mass kept under control?
A few more details: Engines at the tail requires stronger (heavier) empennage, which also adds more weight in a direction we do not want our CG moving. Wings still hold the fuel, so moving the large wing/fuel mass backwards just to move the aerodynamic center backwards doesn't seem like enough.
I noticed that those tailed trijets and large tailed twinjets seem rather old. There don't seem to be any new designs of large commercial aircraft with tail jets. This makes me suspicious that the configuration is inefficient. However, small private jets seem to always be tailed twinjets, whether new or old designs.