Treatment of (silox)(3)MCl (M = Mo, 1-CI; W, 2-CI; silox = (Bu3SiO)-Bu-t) with PMe3 and Na/Hg led to formation of monomeric, d(3) phosphine adducts, (silox)(3)MPMe3 (M = MO, 1-PMe3; W, 2-PMe3) via (silox)(3)CIMPMe3 (M = Mo, 1-CIPMe3; W, 2-CIPMe3). Structural studies show 1-PMe3 and 2-PMe3 to be highly distorted; calculations on full chemical models corroborate experimentally determined S = 1/2 ground states and their structural features. The compounds contain a bent M-P bond that is characteristic of significant sigma/pi-mixing. PMe3 may be thermally removed from 1-PMe3 in vacuo to produce (4)A(2)' (silox)(3)MO (1), which was derivatized with CO, NO, and 1/4 P-4 to form (SHOX)NO (1-CO), (silox)(3)MoNO (1-NO), and (silox)(3)MoP (1-P), respectively. Calculations revealed (silox)(3)W (2') to have an S = 1/2 ground state, which may render it too reactive to be isolated. Treatment of 2-PMe3 with CO, NO, and 1/4 P-4 formed (silox)(3)WCO (2-CO), (silox)(3)WNO (2-NO), and (silox)(3)WP (2-P), respectively. 2-CO and 2-NO are more conveniently prepared from Na/Hg reductions of 2-Cl in the presence of CO and NO, respectively. Calculations reveal subtle effects of ndz(2)/(n+1)s mixing in differentiating the chemistry of Mo and W and in rationalizing the generation of mononuclear species.