The reaction of (h5-C5Me5)(CO)2FeP(SiMe3)2 [MP(SiMe3)2] with RAsCl2 [I, R = 2,4,6(Me3C)3C6H2] forms the heterocycles II, III, and IV. The unstable arsaphosphenyl complex [(h5-C5Me5)(CO)2FeP:AsR] can be intercepted as its Cr(CO)5-adduct V (M = Cr). Analogously the ring compds. [(h5-C5Me5)(CO)2RuP]2AsR and (h5-C5Me5)(CO)2RuP(AsR)2 are obtained by treatment of (h5-C5Me5)(CO)2RuP(SiMe3)2 with I. Here again the primarily generated arsaphosphene has to be stabilized by coordination to Cr(CO)5 which gave E-(h5-C5Me5)(CO)2RuP[Cr(CO)5]:AsR and its Z-isomer. A comparable reaction sequence furnished the phosphaarsenyl complex (h5-C5H5)(CO)(PPh)3FeP[Cr(CO)5]AsR. The mol. structures of II and IV were elucidated by x-ray diffraction anal. The most interesting feature of II is the AsP2-triangle, in which the As-P(2) bond length [235.0(2) pm] is slightly elongated with respect to the As-P(1) distance [231.6(1) pm]. This effect is presumably due to severe steric interactions at the cis-substituted As-P(2) bond. IV displays the picture of a bended 1,2-diphospha-3,4-diarsetane (interplanar angle 137.6 Deg) with its substituents in the all trans-orientation. The As-P and P-P sepns. are normal whereas the As-As bond [249.7(4) pm] is slightly widened with respect to the calcd. value for a single bond (ca. 244 pm).