The Beech 200 medevac aircraft operated by PAL Aerospace Ltd, was conducting flight 810 from Halifax to Sydney, Canada, with 2 pilots, 2 patients and 2 paramedics on board. On approach into Sydney, when the landing gear was selected down, the crew heard a loud noise and the green down and locked light for the right hand main gear did not illuminate. After some troubleshooting, the crew elected to return to Halifax and completed the QRH. The crew declared an emergency and completed a fly-by for an inspection by the tower before landing on runway 14. Upon landing, the right main landing gear collapsed as the nose wheel touched down. The aircraft stopped on the runway. There were no injuries. The right flap, wing and gear incurred damage. Maintenance discovered that a taper pin had become dislodged from the gear torque tube, restricting the gear from achieving the down and locked position.