SINGLE ENG IFR ACFT PLT TAKES EVASIVE ACTION TO AVOID IFR DEPARTING TFC IN CMH AIRSPACE.

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MY FLT WAS HANDLED IN A ROUTINE MANNER AND CMH APCH CTL ASSIGNED ME 4000 FT AND A 360 DEG HDG AS A VECTOR TO OSU. WHEN I WAS JUST E OF THE DOWNTOWN AREA OF COLUMBUS AND W OF CMH AT 4000 FT ON MY ASSIGNED 360 DEG HDG; I SAW A CITATION (SII; I BELIEVE) SUDDENLY APPEAR IN MY R WINDSHIELD TRAVELLING FROM R TO L AT MY ALT. I IMMEDIATELY BANKED R TO PASS BEHIND THE CITATION AND AS IT FLEW BY; I READ ITS TAIL NUMBER RECOGNIZING IT TO BE AN XYZ COMPANY ACFT. I ESTIMATE THE CITATION WAS LESS THAN 100 YARDS FROM MY ACFT AT MY ALT. I CAME UP ON APCH CTL FREQ AND INDICATED THAT 'I WAS ALMOST JUST RUN OVER BY A CITATION.' THE CTLR INDICATED THAT THE TWR CTLR SHOULD HAVE KEPT THE CITATION FROM MY POS. AS I UNDERSTOOD HIM; HE INDICATED THAT THERE WAS A POINTER ON MY RADAR TARGET. (I ASSUME THAT IS SOME TYPE OF INDICATION ON THE RADAR SCREEN.) I INDICATED THAT THE CONFLICT HAD BEEN WAY TOO CLOSE AND HE AGREED. I LATER LEARNED THAT THE CITATION WHICH HAD DEPARTED FROM PORT COLUMBUS (IFR) WAS CLRED TO 5000 FT; THROUGH MY ALT. THIS NMAC WAS SECONDS (AT MOST) AWAY FROM BECOMING A DISASTER WHICH WOULD HAVE TAKEN THE LIVES OF AT LEAST 3 PERSONS. IN THAT THE CITATION WAS UNANNOUNCED BY APCH; WAS UNEXPECTED AND WAS APCHING MY ACFT FROM BELOW AND THE R; IT WAS DIFFICULT TO SEE UNTIL IT FILLED MY WINDSHIELD. I DON'T KNOW IF THE CITATION EVER SAW MY ACFT BUT I DON'T KNOW HOW THEY COULD HAVE AVOIDED SEEING ME THROUGH THE CAPT'S WINDSHIELD. I FAULT ATC FOR CLRING ANOTHER IFR ACFT THROUGH MY ALT AND FOR FAILING TO NOTIFY ME OF THE VERY REAL CONFLICT WHICH COULD HAVE RESULTED IN A MIDAIR COLLISION. EVEN THOUGH THE WX WAS VFR; AT MY FORWARD SPD OF APPROX 170 KTS AND THE CITATION'S FORWARD SPD OF OVER 200 KTS BOTH OF US ON IFR CLRNCS; WITH SEPARATION ESSENTIALLY NON EXISTENT; THE POTENTIAL FOR A DISASTER WAS HUGE. HAD EITHER THE CITATION OR MY ACFT BEEN TRAVELING JUST A FEW KTS FASTER THERE WOULD HAVE BEEN A COLLISION AND A TRAGIC TAKING OF LIVES. WHILE I RECOGNIZE THE SEE AND AVOID RESPONSIBILITY OF BOTH CREWS IN VFR CONDITIONS; AS A PRACTICAL MATTER; GIVEN THESE CIRCUMSTANCES OF ONE CREW BUSY WITH DEP DUTIES AND THE OTHER WITH ARR TASKS; IFR SEPARATION SHOULD HAVE BEEN ASSURED. AS OF THIS WRITING; I HAVE CONTACTED TRACON AND THEY ARE INVESTIGATING THE INCIDENT. A FINAL THOUGHT -- WE ALL NEED TO BE EXTRA VIGILANT IN TERMINAL AREAS EVEN ON IFR FLT PLANS.