Synthesis of Terminal Phosphido Ligand Complexes

One of the first isolated and structurally characterized phosphido complexes [(t-Bu Ph N)3Mo=P] (la) was obtained from the reaction of a molybde-num(III) complex with P4 in 79% yield (Eq. 1) [8]. The molybdenum starting material [(f-Bu Ph N)3Mo] is planar, and the three unpaired electrons are localized on the Mo atom [10], engendering an ideal environment for the reduction of white phosphorus and formation of the triple bond, la can transfer the phosphido ligand to another complex [(t-BuPhN)3Moj via a het-erocumulene intermediate to give [(t-BuPhN)3Mo=Pj (lb) [11] (Eq. 2). 

Trialkoxy complexes of tungsten with terminal phosphido ligands could not yet be isolated. They were postulated to be very reactive intermediates in different transformation reactions, e.g., during the metathesis reaction of [W2(0R)6] with phosphaalkynes [6, 14]. However, we were able to characterize the complex [(t-BuO)3W=P] (3c) by P-NMR spectroscopy by monitoring the metathesis reaction of [W2(Ot-Bu)6] with MesC=P in the tempera- 

Simultaneously with the published synthesis of la [8] Schrock et al. reported on the use of the N3N ligand for the synthesis of the phosphido complexes [(N3N)M=P] (4) (M = Mo (4a), M = W (4b)) [9]. This ligand smoothly promotes the formation of a multiple bond between the transition metal and the ligand trans to the intramolecular N donor. Thus, the reaction of two equivalents of LiP(H)Ph with [(N3N)MC1] (M = Mo, W) (Eq. 6) gives the terminal phosphido complexes 4a,b [16]. 

Structure and Bonding of Terminal Phosphido Ligand Complexes 

Stretching frequencies and force constants for the M=P bond have been derived from Raman spectroscopic data for compounds [(N3N)Mo=P] (4a) and [(N3N)W=P] (4b), as well as for related nitrido and arsenido complexes [30]. For W=E (E = N, P, As), force constants were shown to decrease with increasing M=E bond length, while for each respective pnicogenido ligand E, the force constant for the W complex was greater than that of the Mo complex, a trend also reflected in literature data for transition metal nitrido and 0X0 complexes [20]. 

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