Investigation into the direct synthesis of Mo(,4)Cl(,8)P((,2)H(,5))(,3)(,4) from Mo(,2)OAc(,4) led to a synthetic procedure that produces yields greater than 80%. The single crystal structure disclosed a planar rectangular cluster of molybdenum atoms. Metal-metal bond distances suggest that the long edges of the rectangular cluster should be considered to be single bonds and the short metal-metal bonds to be triple bonds. This view is reinforced by an extended Huckel calculation;Attempts to add a metal atom to Mo(,4)Cl(,8)PR(,3)(,4) to form Mo(,5)Cl(,10)PR(,3)(,3) led instead to a compound with the composition Mo(,8)Cl(,16)PR(,3)(,4). Solution and reflectance uv-visible spectra and x-ray photoelectron spectra suggest that tetranuclear molybdenum units are present. The facile reaction between Mo(,8)Cl(,16)PR(,3)(,4) and PR(,3) imply that the linkage between tetrameric units is weak;Spectroscopic measurements of Mo(,8)Cl(,16)PR(,3)(,4) revealed similarities to (beta)-MoCl(,2). this observation led to the low temperature synthesis of (beta)-MoCl(,2) from Mo(,2)OAc(,4) and AlCl(,3). (beta)-MoCl(,2) formed by this route is much more reactive than that produced by previous methods. Isolation of Mo(,4)Cl(,8)PEt(,3)(,4) from a room temperature reaction of (beta)-MoCl(,2) with PEt(,3) suggests that (beta)-MoCl(,2) contains tetrameric units;Cyclic voltammetric measurements and chemical redox reactions^in acetonitrile have established that the Mo(,5)Cl(,13)('n-) cluster anion^may be obtained with n = 1, 2, or 3. The Mo(,5)Cl(,13)('3-) cluster is stable^for greater than 1 day in the absence of air, but the Mo(,5)Cl(,13)('1-) anion^quickly reverts to Mo(,5)Cl(,13)('2-). Electronic spectra of solutions in^acetonitrile show related bands at 725-790, 550-580, and 455-490^nm for the three anions. Magnetic susceptibility measurements over^the range 100-295(DEGREES)K for Bu(,4)N(,2)Mo(,5)Cl(,13) yield (mu) = 1.67. The^presence of a dynamic Jahn-Teller effect in the ESR spectrum ofBu(,4)N(,2)Mo(,5)Cl(,13) indicates that the HOMO is doubly degenerate.The crystal structure of C(,6)H(,5)CH(,2)N(CH(,3))(,3)(,2)Mo(,5)Cl(,13) illustrates astatic distortion of the cluster. Susceptibility measurements ofBu(,4)N(,3)Mo(,5)Cl(,13) indicate that it is diamagnetic, from which a singletHOMO is deduced. The Cl 2p photoelectron spectrum forBu(,4)N(,2)Mo(,5)Cl(,13) was resolved into components giving binding;energies for the terminal, edge-bridging, and face-bridging chlorine atoms in the cluster unit; ('1)DOE Report IS-T-1054. This work was performed under ContractW-7405-eng-82 with the Department of Energy.