Heating a methylcyclohexane soln. of (h5-C5Me5)(CO)2FeP(SiMe3)P:C(SiMe3)2 (1a) gives rise to the insertion of the P:C moiety into the CH bond of a ring Me group with formation of the chelate complex 2 (shown as I, L = null), which is converted into the pentacarbonylchromium deriv. 3 [shown as I, L = Cr(CO)5].. In contrast, similar treatment of (h5-C5Me5)(CO)2FeP(SiMe3)P:C(Ph)(SiMe3) (1b) furnishes the diphosphirane 5 (shown as II, L = SiMe3). By chromatog. of its (CO)5Cr adduct 6, the P-Si bond is hydrolyzed to give the (diphosphirane)chromium complex 8 [shown as II, L = Cr(CO)5]. The mol. structures of 3 and 8 were detd. by single-crystal x-ray anal.