(a) The hydrogenation of propene is catalysed by RhCl(PPh₃)₃ or HRh(CO)(PPh₃)₃. Outline the mechanisms by which these reactions occur, indicating clearly what the active catalyst is in each case.

(b) HRuCl(PPh₃)₃ is a very active catalyst for the hydrogenation of alkenes. However, at high catalyst concentrations and in the absence of sufficient H₂, orthometallation of the catalyst may accompany alkene hydrogenation. Write a reaction scheme to illustrate this process, and comment on its effect on the activity of the catalyst.