I'm a little fuzzy on the question, but let's just review using complex numbers to rotate the complex plane.
Multiplication by a fixed complex number creates a transformation of the plane. Explicitly we're talking about the map $x\mapsto cx$ for some fixed $c\in \mathbb{C}$. The transformation will stretch and rotate the plane. If $|c|=1$ there will be no stretching, so we'll restrict our attention to this case.
If you experiment using $c=i$, then you'll find that
$1\mapsto i$,
$i\mapsto -1$,
$-1\mapsto -i$ and
$-i\mapsto 1$.
You can play a bit and show that this is truly a 90 degree counterclockwise rotation of the complex plane.
In general, the rotation caused by multiplication with a complex number can be seen in the polar form of the complex number. Complex numbers with modulus 1 have the form $e^{i\theta}$, and $\theta$ is going to give the angle of rotation achieved. In the case of $i$, $i=e^{i \pi/2}$, and in the case of $-1$, $-1=e^{i\pi}$