The Substitution Kinetics and Electronic Absorption Spectroscopy of Some Molybdenum and Tungsten Bis-Dinitrogen Complexes

The electronic absorption spectra of ttans-[M(N2)2(dpe)2], trans-[Mo(N2)(RCN)(dpe)2], and cis-[M(N2)2(PMe2Ph)4] (M = Mo, W; dpe = (C6H5)2PCH2CH2P(C6H5)2; PMe2Ph = P(CH3)2C6H5) were measured and assigned. All of the charge transfer (CT) transitions were metal-to-ligand CT transitions. The lowest energy CT transition in each spectrum was assigned as a M → Pdπ CT transition, and the low energy shoulders on this transition were assigned as ligand field transitions. The highest energy CT transition was assigned as a M → N2π* CT transition in each case. The kinetics and mechanisms for the substitution of nitriles for dinitrogen in trans-[M(N2)2(dpe)2] and cis-[M(N2)2(PMe2Ph)4] were studied. In the reaction of trans-[Mo(N2)2(dpe)2] with nitriles, one dinitrogen was replaced by a nitrile. All of the kinetic data support an SN1 dissociative mechanism. The analogous reaction with trans-[WN2)2(dpe)2] was found to proceed only photochemically, with the loss of one dinitrogen induced by population of the 1,3Eg state. In the reactions of cis- [M(N2)2(PMe2Ph)4] with nitriles, both dinitrogen ligands were replaced by nitriles. The reaction mechanism for both compounds consists of two consecutive SN1 dissociative steps. The reactions of cis-[W(N2)2(C6H5CN)(PMe2Ph)4], cis- [W(C6H5CN)2(PMe2Ph)4], and trans-[Mo(N2)(C6H5CN)(dpe)2] with anhydrous HCl were also studied. The reaction of the latter compound with HCl to yield [Mo(Cl)(N2H2)(dpe)2] was particularly noteworthy.