5. Let the complex number $\alpha=a+b i(a, b$ be real numbers, and $b>0) \frac{\alpha^{2}}{1+\alpha}$ and $\frac{\alpha}{1+\alpha^{2}}$ are both real numbers, then $\alpha=$
5. $-\frac{1}{2}+\frac{\sqrt{3}}{2} i$