Dimeric (pentamethylcyclopentadienyl)rhodium and -iridium complexes. 5. Structural studies on the homogeneous hydrogenation catalysts [(.eta.5-C5Me5)IrBr]2(.mu.-Br)2 and [(.eta.5-C5Me5)IrI]2(.mu.-I)2 and systematics in the series [(.eta.5-C5Me5)IrX]2(.mu.-X)2 (X = chlorine, bromine, iodine)