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.