Orbit around an object

I want to create a camera moment that mimics the unity editor camera when you press "Alt"

my code is as follows: if (Input.GetKey (KeyCode.LeftAlt)) { if (Input.GetMouseButton (0)) {

                 axis = transform.TransformDirection(new Vector3 (-Input.GetAxis ("Mouse Y"), Input.GetAxis ("Mouse X"), 0));
 
                 transform.LookAt(center);
                 transform.RotateAround (center.position, axis, rotationSpeed * Time.deltaTime);
             
                 
             }
         }

This almost does the trick but it doesn't allow you to rotate the camera all the way around in the Y detection.

I am probably over looking something simple but I just cant figure it out .

Thank you for the help!