已知\(a∈R\)，函数\(f(x)= \begin{cases} {a+ \dfrac {1}{x},x > 0} \\ {e^{-x},x < 0}\end{cases}\)，若存在三个互不相等的实数\(x _{1}\)，\(x _{2}\)，\(x _{3}\)，使得\( \dfrac {f(x_{1})}{x_{1}} = \dfrac {f(x_{2})}{x_{2}} = \dfrac {f(x_{3})}{x_{3}} =-e\)成立，则\(a\)的取值范围是______．