The pilot stated the airplane was topped off with fuel before departing, and he did not have any problems in opening or closing the fuel caps during the preflight inspection. He switched the fuel tanks every 30 minutes while en-route and did not notice any siphoning of fuel. Air traffic control cleared the flight to descend about 30 to 45 miles from the destination airport. The pilot elected to remain on the right main fuel tank since the fuel gauges indicated it had the most amount of fuel. The controller cleared the flight for a visual approach to runway 6 and the engine stopped about one half mile from the airport. The pilot immediately switched the fuel tanks, and attempted an engine restart, which was unsuccessful. He flared the airplane high over the runway. The airplane landed hard, bounced, and landed hard again. Examination of the airplane revealed structural damage to the right wing. A mechanic who examined the airplane stated that a lanyard holding the fuel cap down interfered with the latch and that the cap may not have been seated properly, leading to a fuel leak. Blue fluid staining was present in the vicinity of the right main fuel cap and aft section of the right wing. The right fuel tank was de-fueled and about one quart of fuel was present. The left main fuel tank was de-fueled and about 18 gallons of fuel was present.