The pilot advised ATC that he was experiencing difficulties in obtaining normal extension of the landing gear and requested emergency services to be alerted for the landing. During the landing roll, the right main landing gear collapsed. The pilot and passengers evacuated the aircraft safely. Examination of the aircraft revealed that the right main landing gear retract/extend rod assembly was bent in two places. The rigged length of the rod was critical to the correct "down" locking of the main landing gear, with any rod misalignment resulting in the possibility of landing gear collapse when weight was placed upon it. In addition to rod damage which occurred at the time of the gear collapse, there was evidence of some pre-existing damage. Such damage could result from the inadvertent placement of weight upon the landing gear whilst not correctly "down and locked".It was not possible to determine when this particular damage occurred.