Tetranuclear complexes containing quadruply bonded dimolybdenum units joined by μ-hydride ions

Two tetranuclear complexes of the type [Mo-2(DArF)(3)](2)(mu-H)(2), DArF = N,N-diarylformamidinate, Ar = p-tolyl (1) and p-anisyl (2), have been prepared by reaction of Mo-2(DArF)(3)Cl-2 and NaHBEt3. The cores are similar and show two quadruply bonded dimolybdenum units joined by two hydrogen atoms. These are the first structurally characterized tetranuclear molecules with localized quadruple bonds and also the first molecules with two independent M-(mu-H)-M units, in which each metal atom is also bonded to another hi atom. It is remarkable that in each case the compound resists hydrolysis by water but will react with HCl. The delta --> delta* transitions, which are normally the lowest energy bands in the visible region of the electronic absorption spectra, appear as shoulders at 449 and 444 nm for 1 and 2. However, each compound also shows two more transitions at lower energy (ca. 505 and 728 nm). These bands have been assigned to transitions involving the three center-two electron Mo-H-Mo bridges.