Quadruply bonded

Figure 98 Quadruply bonded dinuclear dimolybdenum(II) complexes (a) Mo2(pyphos)4, (b) Mo2Pd2Cl2...

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 synthesis of the first quadruply bonded tungsten(II) car-boxylate, W2(02CCF3)4 (hereafter referred to as V TFA) ), was reported by us last year (1). The structure of its diglyme adduct, W2(TFA)4 2/3 diglyme is shown in Figure 1. This particular view shows the partial contents of two unit cells and it emphasizes the tridentate nature of the axially coordinated polyether which "stiches" W2(TFA) units together in the solid state. We now have in hand several other adducts of W2(TFA) and one of these, W2(TFA)4 2PPh3 (an axial adduct), has been structurally characterized (2). Despite many attempts, X-ray quality crystals of unsolvated W2(TFA) have not been obtained. 

In this chapter we illustrated how the CASSCF/CASPT2 method can be used to explore the nature of such chemical bonds. Classic cases are the Re-Re multiple bond in Re2Cl, and the Cr-Cr bond ranging from the quadruply bonded Cr(II)-Cr(II) moiety to the formal hextuple bond between two neutral chromium atoms. The bonding between the 3d5 electrons is weak and should be considered as an intermediate between two pairs of antiferromagnetically... 

Irradiation of [Re(CNC6Fl3-2,6-Me2)6] with UV light in CH2CI2 in the presence of halide ions yields complexes of the type [Re(CNCgH3-2,6-Me2)5X] (X = Cl, Br, or I). The quantum yield of this reaction is high and results in chemical yields between 30% and 40%. Products of the same composition have been obtained directly from the cleavage of the metal-metal bond in quadruply bonded rhenium complexes of various compositions in the presence of isocyanides. ... 

Electrochemical data for a number of multiply bonded Rc2 complexes are represented in Table 2 [42-50]. The oxidation of quadruply bonded Re2 pro-... 

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. 

The first W—W quadruply bonded complexes characterized were [W2Me Cl8 ]4- prepared by treatment of WC14 with methyllithium. Crystal structure analysis has established the short W—W distances of 2.264 and 2.263 A for the methyl and the mixed chloromethyl derivatives respectively. In addition, these compounds have the expected eclipsed stereochemistry.327,328... 

The reaction of WCU with K2C8H8 in THF gives a reasonably air-stable material that was shown to be [W2(C8Hg)3]. The W—W bond length of 2.375 A, although long, is consistent with the presence of a quadruple bond.329 However, it is recognized that this compound (and the related chromium and molybdenum) is electronically different from other quadruply bonded species. 

The complexes [W2CI4L4] undergo a reversible one-electron oxidation. A comparison of the oxidation potential of analogous molybdenum and tungsten compounds clearly shows that the tungsten compounds are considerably easier to oxidize. This ease of oxidation is believed to be responsible for the fact that it is difficult, if not impossible, to obtain quadruply bonded tungsten compounds by synthetic procedures which are known for molybdenum.339... 

The ease of oxidation of the quadruply bonded tungsten dimers is also exemplified by the reaction of [W2(mhp)4]343 or [W2(TFA)4]333 with HC1 to give [W2C19]3-, which is also readily obtained from [W2C18]4- even at -78 °C. It is believed that these reactions proceed via the hydride intermediate [W2C18H]4. ... 

Mo2R2(02CNMe2)4], in which the Mo—Mo triple bond is retained, but the other compounds react according to equation (1) to produce the quadruply bonded species [Mo OzCNMe ].66... 

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... 

Fig. 16.65 A comparison of the two chloro complexes of Motlll (a) quadruply bonded IMoXlHi - (b) singly bonded [From Cotton. F. A. Acc. Client. Res. 1969.2,...

Molybdenum complexes in this class are formed on dissolution of the Mo—-Mo quadruply-bonded Mo2Cl4L2L 2 species, which dissociate ligands and undergo [2 + 2] cycloaddition ... 

With a view to examining whether bulky substituents at the amidine nitrogen atoms can prevent axial coordination in quadruply bonded dimetal compounds, Cotton and coworkers have studied the reactions of (Bu4N)2Re2Cl8 with PhN(CMe)NHPh and MeN(CPh)NHMe.83 The structures of the crystalline products can be represented as (34) and (35) respectively. 

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