A formula is valid , nothing but a tautalogy.
If we negate a tautalogy , we will get a CONTRADICTION (all are False).... hence not SATISFIABLE.
A formula is satisfiable, iff there is at least one TRUTH value , so even a tautalogy is also satisfiable.
lets take $P \Lambda Q$
its SATISFIEABLE for P=T and Q=T. if we negate $P \Lambda Q$ then we will get (F,T,T,T)..its not a tautalogy.
In the same if you consider any tautalogy, its valid and satisfiable, but after complimenting its not a valid and satisfiable.