The alkyl-for-iodide metathesis reaction that occurs when CpCr(NO)(PPh(3))I (1) is treated with 2 equiv of Me(3)SiCH(2)MgCl in THF to form CpCr(NO)(PPh(3))(CH(2)SiMe(3)) (6) has been investigated in some detail. The conversion is initiated by loss of the phosphine ligand from the chromium atom’s coordination sphere, the most compelling evidence for this step being that addition of excess phosphine (e.g. 4 equiv) to the initial reaction mixture completely inhibits the reaction. Four intermediate complexes which are formed sequentially on the reaction path from 1 to 6 have been detected by IR and ESR spectroscopy. These complexes have been identified as CpCr(NO)(THF)I (2), CpCr-(NO-->Mg{CH(2)SiMe(3)})Cl)(THF)I (3), CpCr(NO-->Mg{CH(2)SiMe(3)}Cl)(THF)(CH(2)SiMe(3)) (4), and CpCr(NO) (THF)(CH(2)SiMe(3)) (5). Complexes 3 and 4 have also been detected spectroscopically during the reaction of CpCr(NO)(THF)I (2) with Me(3)SiCH(2)MgCl which produces CpCr(NO)(THF)(CH(2)SiMe(3)) (5). This understanding of the mechanistic pathway has resulted in the development of a general synthetic route to previously inaccessible 17-valence-electron CpCr(NO)(L)R complexes (L = C5H11N Or NH(2)CMe(3), R = CH(2)SiMe(3); L = C5H11N, R = CH(2)Ph).