Reaction of (h5-C5Me5)(CO)2FeP(SiMe3)2 with carbon disulfide afforded the unstable metallophosphaalkene (h5-C5Me5)(CO)2FeP:C[SSiMe3]2 (5a), which decompd. to a doubly metalated 1,3,4-thiadiphosphole 8a by extrusion of bis(trimethylsilyl) sulfide. The transient metallophosphaalkene 5a was intercepted as the isolable [(CO)5Cr]-adduct by treatment with [(Z)-cyclooctene]Cr(CO)5. Similarly, the metallodisilylarsanes (h5-C5Me5)(CO)2M-As(SiMe3)2 (M = Fe, Ru) were converted to stable metalloarsaalkenes by exposure to CS2. Pentacarbonylchromium complexes were accessible by reaction with [(Z)-cyclooctene]Cr(CO)5. The X-ray structure anal. of (h5-C5Me5)(CO)2FeAs(SiMe3)2Cr(CO)5 revealed the mol. as the first h1-arsaalkene complex.