Inverting op-amp does not give expected result
The opamp is trying to invert your signal, meaning that it will try to change the sign.  If your resistors are the same value, then you have

gain = -R/R = -1


If you put 2.5V into the input of the circuit, then your opamp is trying to 

Voutput = gain * Vinput
Voutput = -1 * 2.5V
Voutput = -2.5V


Since the opamp cannot go to -2.5, it gets as close as it can at 0.75V, saturating there.  The same is true for your entire range of 0V to 5V.  The circuit will try to drive the output from 0V to -5V, but will only go as low as it can until saturation.

Based on your comments, I suspect that you are trying to implement:

Vout = 5V - Vin


In this case, you want to implement a differential amplifier with one end tied to +5V and with the gain of 1.