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The photochemical reaction of Mn(CO)5Br with PCy3 in toluene proceeds with the fast evolution of CO. The complex formed, MnBr(CO)3(PCy3)2, 1, readily reacts with NaA (A = B[C6H3(3,5-CF3)2]4) to form the dark green salt [Mn(CO)3(PCy3)2][A], 2, which has an agostic interaction of a cyclohexyl C−H with manganese. The light yellow η2-H2 complex, [Mn(H2)(CO)3(PCy3)2][A], 3, forms at room temperature by placing solutions of 2 under 1 atm of H2. The H2 ligand is labile and readily dissociates when the H2 atmosphere is removed in vacuo. 31P NMR clearly shows that at 25 °C under 1 atm of H2, 3 exists in equilibrium with 2. At −78 °C and under <1 atm of H2, only 3 is observed. The synthesis of the new tricarbonyl complex [Mn(CO)3(PCy3)2]+ provides an excellent system of comparison for the binding of small molecules to similar known 16e fragments such as M(CO)3(PCy3)2 (M = Cr, Mo, W, Re+) and M(CO)(dppe)2 (M = Mo, Mn+). The cationic manganese complex 2 appears to give somewhat more stable binding of H2 than the isoelectronic neutral chromium congener, which would not have been expected on the basis of relative back-bonding ability of the metal centers. Thus it is clear that increased σ-donation more than compensates for decreased back-bonding in the relative metal−H2 bonding energies. Surprisingly, binding of N2, ethylene, or silanes to 2 was not observed in solution even at low temperature and SO2 binds only weakly. By comparison with other similar systems, it is clear that H2 becomes an increasingly better ligand than N2 as the electrophilicity of the metal increases. Thus nonclassical H2 is a more versatile ligand than most classical ligands in its ability to adjust to a larger range of electronic situations.

A number of general reactions, based on molybdenum-coordinated sulphido or polysulphido ligands, can account for the synthesis of many members of the [MO: (S),(S&_,]'-, [Mob,,_,]'-and [Mo3l(C?),(S),(S,),_Jl"-homologous series. These reactions include : (a) sulphur addition and sulphur abstraction reactions ; (b) oxidative removal of the Sz-ligands and their replacement by substitutionally labile ligands ; (c) oxidation of coodinated RS-ligands by S," fragments, and in situ generation of S,'-ligands ; (d) oxidative dimerization reactions of molybdenum(IV) complexes ; and (e) reductive elimination of S," fragments. The synthesis of the new [(C~)MO~(O)(~-S)~MO~(O)S~]-and [(Cp)MoV(0)(p-S),MoV(0)S2]-anions has alowed the use of 'H NMR spectroscopy to monitor the solution behaviour and reactivity of the MoV(0)S2 group. The existence of the MoV(0)S2 + S2 F? MoV(0)S4 equilibri? has been unequivocally established. The [(Cp)Mo' (O)(p-S),Mov(0)S2]-complex gives a vinyl disulphide derivative by insertion of dicarbomethoxyacetylene, DMA, into the MO-q2-S2 group. Upon prolonged heating to 70°C the vinyl disulphide ligand is converted quantitatively to a dithiolene ligand. The same conversion takes place swiftly at ambient temperatures when catalytic amounts of Ss are present in the reaction medium. Reaction pathways for both the thermal and catalytic conversions have been suggested by 'H NMR studies. Preliminary studies of the utility of the (J%N)[Mo2W&U, ~i(~~~)61~02~2~~~~1~, (EWdMa@.&l and (Phd%[Mo&l complexes as catalysts for the hydrodesulphurization of thiophene are reported. The Ni/Mo catalyst obtained from ~i(DMF)6][Mo202S,Cl]2 is at least 10 times more effective than a commercial CO/MO catalyst.

The syntheses, structures, and reactivities of *2 2(µ-8)(µ-82)(µ-88•8 2)•8 2, 1•2802, and Cp*2Cr2^-S)^-S2)(M-SS03)'(CD30D)iattice, 2, are reported. Reversible addition of S02 to Cp*2Cr2S5,1, to give 1•2802 was found to occur at room temperature. 1 possesses a novel µ-1-82 ligand, and X-ray crystallography of 1•2802 showed that S02 binds as a Lewis acid to the terminal sulfur of this ligand with an acute S-S-S angle (94°). The S-S02 distance of 2.41 A is about 0.2 A shorter than in the previously known *2 2(µ-8)(µ-82)(µ-8•802). Consistent with this greater activation, 1•2802 further reacts in the presence of excess S02 to give the thiosulfate 2 (isomorphous to the Mo analogue) at a much faster rate than in the Mo system. Significantly, the crystal structure of 1•2802 shows the presence of associated S02 molecules unsymmetrically bridging (via sulfur) the oxygen atoms of each sulfidebound S02 molecule to give a cyclic aggregate of four S02 molecules (intermolecular S-O distances of 2.78 and 2.95 A). Infrared studies demonstrate that these interactions further activate (weaken) the 8=0 bonds of the sulfide-bound S02. Thus, it is proposed that the mechanism of S02 disproportionation in solution involves baseassisted oxygen atom transfer from the sulfide-bound S02 molecule to the associated S02 to produce S03. The S03 then displaces S20 (which decomposes to Sg and S02) from the bridging sulfide ligand to give the observed thiosulfate product. The reactivities of 1•2802 and Cp*2Cr2S,cS02 vs € *2 284•802 and of 2 vs Cp*2Mo2S4'S03 are discussed.A mechanism for the oxygen atom transfer reaction is proposed on the basis of the crystal structure of 1•2802 and 34S-labeling studies. Facile reaction of Cp*2Cr2S4 with Sg was found to give 1, and all five of the core sulfurs in the latter incorporated 34S if excess 34S8 was used. 1•2802 and 2 crystallize in the space groups C2/c and P\, respectively. The cell dimensions are the following: 1•2802, a = 16.087 (4) A, b = 15.926 (3) A, c = 21.650 (3) A, ß = 105.29 (2)°, and Z = 8; 2, a = 10.336 (2) A, b = 10.891 (2) A, c = 13.219 (3) A, a = 85.15 (3)°, ß = 78.85 (3)°, y = 62.39 (3)°, and Z = 2. R = 4.85% for 1-2S02, and R = 4.84% for 2.Cp*2Cr2^-S)(M-S2)(g-SS-S02)-S02,1-2S02, and the "S03 adduct" Cp*2Cr2S4-S03, 2, which importantly now contains one less sulfur and is isostructural with the Mo species discussed above. The crystal structure of 1-2S02 as well as 34S-labeling studies suggest a possible oxygen-transfer mechanism for the conversion of 1*2S02 to 2 in which self-aggregation of SO¡ via weak Lewis acid-base interactions induced by electron donation from sulfide plays a critical role. Experimental SectionReactions were carried out under inert atmosphere using Schlenk techniques. Reagent grade solvents and S02 were usually used as received, but for some reactions solvents were dried by distillation from appropriate drying agents. 34S was obtained from the Mound Facility of Monsanto Research Corp., Miamisburg, OH. Samples of Cp*2Cr2S4, Cp*2Cr2Ss, and Cp'2W2S4 (Cp' = -BuCp) were g...