Quadruply Bonded Dimers

Irradiation (254 nm) of a solution of Mo2(S04) in 5 M H2SO4 results in the photooxidation to the one-electron oxidized complex Mo2(S04)4 and the simultaneous reduction of a proton into hydrogen (Ref. 115)  

Similarly, irradiation (2 335 nm) of Mo2(HP04)4 in 2 M H3PO4 results in the rapid photooxidation to the two-electron oxidized complex Mo2(S04)4 and hydrogen (Ref. 116)  

If it is necessary to carry out photoreactions of the quadruply bonded M02 core in nonaqueous solvent, the dialkyl or diaryl phosphate complexes Mo2(02P(OR)2)4 (R = Ph, Et, Bu) can be used. The diphenyl derivative, for example, luminesces (A = 560 nm) from the a n 58 excited state in both the solid and the solution state. The excited state lifetime is 68 ns. Irradiation (A 530 nm) of a solution of Mo2(C 2P(OPh)2)4 in 1,2-dichloroethane solvent leads to the formation of Mo2(02P(OPh)2)4Cl and ethylene  

Such a photoprocess could involve the reaction of the excited state Mo2(02P(OPh)2)4 as a diradical, whereby it abstracts a chlorine atom from 1,2-dichloroethane. Alternately, the excited state Mo2(C 2P(OPh)2)4 could act as a reducing agent to give chloride ion and the alkyl radical. The quantum yield of the reaction is decreased if the photoreaction is carried out in a coordinating solvent such as THE or acetonitrile because such compounds coordinate into the axial coordination position of the excited state dimer, which is the site of reactivity. 

The quadruply metal-metal bonded complexes M2X4(L-L)2 (M = Mo, W, X = halide L-L = bridging tertiary phosphine) also show photochemical reactions with added substrates. When the complex W2CU(dppm)2 is photolyzed (A 436 nm) in the presence of methyl iodide, the product is W2Cl4l(Me)(dppm)2. This product complex results from the oxidative addition across the tungsten-tungsten quadruple bond  

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The newly-developed capability to observe metal-metal vibrational fine structure in the valence ionizations of quadruply bonded dimers is illustrated for the delta-bond ionization of Mo2(02CCH3)if. Observation of this structure provides direct information on the bonding influence of an electron in a delta-bonding orbital by showing the significant changes in metal-metal force constant and bond distance that occur when that electron is removed. 

The reaction of [Mo2(02CMe)4] with a warm concentrated solution of HC1 or HBr affords the [Mo2X8H]3- anions. These anions contain the Mo(fi-H)( U-Cl)2Mo bridging unit,278 and possess structures similar to that of [W2C19]3 . This type of oxidative addition is also possible with the quadruply bonded dimer [MoW(02CCMe3)4], This molecule when treated in the same way279 affords [MoWC18H]3 with a Mo—W distance of 2.495 A. 

Addition of alcohols (R OH) to l,2-[Mo2R2(NMe2)4] compounds yields either 1,2-[Mo2R2(OR )4] or [Mo2R(OR )5] (Scheme 1). The latter reaction involves the elimination of the alkane RH and production of the quadruply bonded dimer which then undergoes oxidative... 

The work reported here concerns the addition of quadruply bonded dimers of the type Mo2Cl L according to eqn. 1,... 

Such a process was considered likely because in the quadruply bonded dimers the metal atoms have vacant coordination sites to accept the ligand atoms of X and Y, and the X-Y bond energy plus the weak 6-bond of the dimer are compensated by formation of M-X and M-Y bonds. On the further assumption that unfavorable steric crowding of ligands about the metal atoms account for the lack of addition chemistry previously observed for these dimers we sought to prepare compounds having weakly bound ligands. Dissociation in solution as in eqn. 2... 

Reactions 5 and 6 are particularly convenient since the most common starting material for synthesis of quadruply bonded dimers of molybdenum, viz. Ik CC CCH ), is converted directly into the rectangular clusters in high yield. The products of reactions 7, 8 and 9 are formulated as tetramers and assumed to consist of rectangular cluster units because their yellow color and electronic absorption maxima are quite similar to Mo,Cl (PBu ), and each is easily converted into Mo Cl (PEt ) or Mc CI PEE upon reaction at 25° with the stoichiometric amount of PEt. In the presence of excess PEt all of these tetramers are converted back into dimers ... 

An exception to this metal-carbonyl photochemistry is that irradiation of the quadruply bonded dimer, [RejClg] ", in CH3CN gives monomeric trans-[ReCl -(CH3CN)2J . Cleavage of the quadruple bond by one photon is not likely given the photoinertness of triple bonds (see 13.2.4.2) the process likely includes a large component of solvent assistance. 

Ryan TR, McCarley RE (1982) Synthesis and characterization of rectangular tetranuclear cluster complexes of molybdenum(II) by condensation of quadruply bonded dimers. Inorg Chem 21 2072-2079... 

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