Phosphaalkenes, Metallo-l,2-dihydrophosphetes, X-Ray The metallo-phosphaalkenes (/75-C5Me5)(CO)2M -P = C (N R 2)2 (4a,b, 5) result from the reac tion of the corresponding bromo compounds (/75-C5Me5)(CO)2MBr with Me3Si-P = C (N R 2)2. The complexes 4a and 5a are cleanly converted into the l-metallo-l,2-dihydrophosphetes (//5-C5Me5)(CO)2M -PCH(E)-C(E) = C -NM e2 (lO a -c) (M = Fe, Ru; E = C 0 2Me, CN) by treatment with equimolar amounts of dimethyl fumarate or fumarodinitrile, respectively. The products 4a,b, 5 and lO a -c have been characterized by elemental analysis as well as by spec troscopic data (IR, 'H, 13C, 3IP NM R, MS). The molecular structures o f 4a and 10a have been determined by single crystal X-ray analysis.