The reduction of [( (t) Bu 3SiO) 2MoCl] 2 ( 2 2) provided the cyclometalated derivative, (silox) 2HMoMo(kappa-O,C-OSi (t) Bu 2CMe 2CH 2)(silox) ( 3), and alkylation of 2 2 with MeMgBr afforded [( (t) Bu 3SiO) 2MoCH 3] 2 ( 4 2). The hydrogenation of 4 2 was ineffective, but the reduction of 2 2 under H 2 generated [( (t) Bu 3SiO) 2MoH] 2 ( 5 2), and the addition of 2-butyne to 3 gave [(silox) 2Mo] 2(mu:eta (2)eta (2)-C 2Me 2) ( 6), thereby implicating the existence of [(silox) 2Mo] 2 ( 1 2). The addition of (silox)H to Mo(NMe 2) 4 led to (silox) 2Mo(NMe 2) 2 ( 7), but further elaboration of the core proved ineffective. The silanolysis of MoCl 5 afforded (silox) 2MoCl 4 ( 8) and (silox) 3MoCl 3 ( 9) as a mixture from which pure 8 could be isolated, and the addition of THF or PMe 3 resulted in derivatives of 9 as (silox) 2Cl 3MoL (L = THF, 10; PMe 3, 11). Reductions of 11 and (silox) 2WCl 4 ( 15) in the presence of excess PMe 3 provided (silox) 2Cl 2MPMe 3 (M = Mo, 12; W, 16) or (silox) 2HW(eta (2)-CH 2PMe 2)PMe 3 ( 14). While "(silox) 2W(PMe 3) 2" was unstable with respect to W(IV) as 14, a reduction of 12 led to the stable Mo(II) diphosphine, (silox) 2Mo(PMe 3) 2 ( 17). X-ray crystal structures of 10 (pseudo- O h ), 12 (square pyramidal), and 14 and 17 (distorted T d ) are reported. Calculations address the diamagnetism of 12 and 16, and the distortion of 17 and its stability to cyclometalation in contrast to 14.