The reaction of W(,2)(mhp)(,4) (mhp = anion of 2-methyl-6-hydroxypyridine) with ClSiMe(,3) in refluxing pyridine gives the oxidative addition product W(,2)((mu)-H)((mu)-Cl)Cl(,4)(pyridine)(,4). The spectral characterization of this compound as well as the characterization and crystal structure of its 4-ethylpyridine analog have been accomplished. The structure determination of the 4-ethylpyridine complex implies that a tungsten-tungsten double bond is present in the dimer;Reaction of MoW(O(,2)CCMe(,3))(,4) with ClSiMe(,3) in the presence of excess trimethylphosphine affords the metal-metal quadruply bonded dimer MoWCl(,4)(PMe(,3))(,4). The crystalline heteronuclear dimer is isostructural with its dimolybdenum and ditungsten homonuclear analogs and possesses intermediate structural features. Its ('31)P NMR consists of two equal-intensity resonances each split into a 1:2:1 triplet by a P-P' coupling of 24,4 Hz across the metal-metal quadruple bond;A preparative reaction similar to that used for formation of thedimer, but using a reduced quantity of PMe(,3), yields the hetero-nuclear tetramer Mo(,2)W(,2)Cl(,8)(PMe(,3))(,4). The tetramer possesses arectangular arrangement of metal atoms and can be viewed as acycloaddition product of two dimer units. It is structurally akin torectangular homonuclear tetramers of molybdenum and tungsten,but exhibits certain anomalous bonding features compared tothe homonuclear analogs. From a ('31)P NMR spectrum ofMo(,2)W(,2)Cl(,8)(PBu(,3)('n))(,4), it is possible to distinguish two tetrameric;isomers differing only by the arrangement of the metal atoms around the rectangle;('1)DOE Report IS-T-1044. This work was performed under ContractW-7405-eng-82 with the Department of Energy.