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Treatment of M(CO) 6 (M = Cr, W) with (Ph 2 PMe 2 Si) 3 CH in toluene at elevated temperatures resulted in the isolation of pale green [Cr(CO) 3 {(Ph 2 PMe 2 Si) 3 CH}] and pale brown [W(CO) 3 {(Ph 2 PMe 2 Si) 3 CH}] in high yield. These complexes have been characterized by elemental analysis, conductivity measurements, TGA, mass spectrometry, and IR, electronic, 1 H and 31 P{ 1 H} NMR spectroscopy. The complexes have pseudo octahedral geometry around the metal atom with tridentate coordination of the multidentate ligand. The six-membered MPSiCSiP metallacycles are shown to have high fluxionality at room temperature on the NMR time scale. [ABSTRACT FROM AUTHOR]