Synthesis, structure, and sup 31 P NMR spectra of Mo sub 2 W sub 2 Cl sub 8 (PR sub 3 ) sub 4 rectangular clusters

The new compounds Mo{sub 2}W{sub 2}Cl{sub 8}(PMe{sub 3}){sub 4}{center dot}{sub 4}H{sub 8}O (I) and Mo{sub 2}W{sub 2}Cl{sub 8} (P(n-Bu){sub 3}){sub 4} (II) have been prepared by cycloaddition reactions of MoW(O{sub 2}CCMe{sub 3}){sub 4}. Yellow-green crystals of I provided crystallographic data for a complete structure determination: tetragonal, space group P4{sub 2}2{sub 1}2, a = 12.6443 (8) {angstrom}, c = 11.2854 (9) {angstrom}, V = 1804.3 {angstrom}{sup 3} Z = 2, R = 0.034, R{sub w} = 0.045. Disordering of the Mo and W atoms on the metal atom sites permits only averaged M-M, M-Cl, and M-P distances for the rectangular cluster units. Of principal interest were the M{triple bond}M and M-M distances of 2.275 (1) and 2.843 (1) {angstrom}, respectively, for the short and long edges of the Mo{sub 2}W{sub 2} rectangular cluster units. A study of the {sup 31}P({sup 1}H) NMR spectrum for a solution of II showed that coupling of the Mo-{sup 4}W dinuclear units provided two isomers, IIa and IIb, in a ratio of ca. 1.5:1.0. Isomer IIa was assigned as arising from head-to-tail coupling, and IIb as arising from head-to-head coupling of the two MoW units. Electronic spectral data for II are also reported. 16 refs., 6 figs., 4 tabs.