Four-electron donor, metal-alkyne complexe

The extent of alkyne ir donation in Mo(RC=CR)(SBu )2(CNBu% (SO) is not easily quantified as discussed in the structural and molecular orbital sections. Proton shifts for HC=CH and PhC=CH ligands in these complexes are near 10.4 ppm, above the N = 4 median value and approaching an N = 3 chemical shift. The 13C chemical shifts range from 170 to 185 ppm, also above classic four-electron donor alkyne values, presumably reflecting competition with the two adjacent equatorial thiolates for donation to the two vacant metal dir acceptor orbitals. Single bond VCH values of 215 and 211 Hz are typical of terminal alkynes bound to molybdenum(II) (133). 

Nearby elements also display chemistry reflecting alkyne 7rx donation to a vacant metal dir orbital. In Group V, Lippard s coupled carbonyl product Ta(Me3SiOC=COSiMe3)(dmpe)2Cl is a d4 Ta(I) alkyne monomer (116), similar in electron count to CpV(CO)2(RC=CR) complexes (231). The preparative route and physical properties of a series of Ta(CO)2-(RC=CR)(I)L2 d4 monomers are compatible with a four-electron donor description for the alkyne ligands (231a). The d2 configuration has also... 

Scheme 4-42) [134]. Werner s group has shown that alkynylsilanes also undergo 1,2-silyl migration in the same system via an intermediate 7i-complex [135]. Alternatively, the rearrangement may be catalyzed by base [136] or induced by sequential deprotonation/protona-tion [137]. The relative stability of the alkyne and vinylidene complexes is dependent on the electron density and the d-electron count of the metal, as illustrated by the behavior of the d -Mo complexes 84 in which the alkyne is a four-electron donor addition of CO causes the 84 85 conversion whereas tautomer 84 is favored with the phosphite ligand (Scheme 4-43)... 

Some simpler ligands can also serve as ir-donors. We saw earlier that acetylenes can act as CT-donors and ir-acceptors in the same way as olefins. However, the rr-bonding orbitals that are perpendicular to those oriented toward the metal can also serve as ir-donors, as shown in Figure 1.29. Acetylene ligands in complexes that possess less than 18 electrons without such TT-donation are sometimes considered to be "four-electron donors." This is the origin of the listing of alkynes as 2-electron or 4-electrori ligands in Table 1.1. 

Whilst solutions of complex 4 a are reasonably stable at room-temperature, the remaining species 4 imdergo a variety of transformations. Dichloromethane solutions of 4b release CO if warmed above -20 °C, cleanly affording complex 5. Treatment of 4b with PMes, at low temperature, yielded the mono-alkyne complex 6. Complexes 4b, 5 and 6 thus represent a series in which diphenylacetylene acts, respectively, as a formal two-, three- and four-electron donor to a metal centre. 

Even so, there are examples that are consistent with the structure being of the metallocyclopropane variety. One ready measure of the amount of each type of bonding is the location of the alkyne stretching fiequency in the IR, This absorption appears about 2200 cm in alkyne complexes while the band appears about 1750 cm for the metallacyclopropene complexes. Alkyne complexes can also act as four-electron donors. Back-donation into the metallic n orbitals is negligible in these cases, but sigma-donation occurs. Finthermore, alkynes form a number of din-uclear complexes where each perpendicular n system can be considered to formally bond to a separate metal atom. Thus care must be exercised in assigning the type of bonding present. 

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

As mentioned earlier, many Lewis base metallocenes have been synthesized and structurally characterized. An examination of their structures shows that they are all predictably bent, more so than their base-free congeners, and have one two-electron, two two-electron or one four-electron Lewis base donor. Ca[l, 3-(SiMe3)2C5H3]2(THF),89 shown in Fig. 17, is an example of a Lewis base monosubstituted Ca metallocene complex, whereas Sr(t-BuC5H4)2(THF)2,88 shown in Fig. 18, is an example of a disubstituted metallocene. Even weak donor base such as alkynes, Me3SiCCCCSiMe3, will bind to the alkaline-earth metal centers (Fig. 19).104 In this case, the weak donor is easily displaced by stronger bases. 

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