Bonding with Other Unsaturated Ligands

The series of complexes of unestablished structure, but containing presumably 7C-bonded ligands has been prepared by the cryochemical method.The interaction of 2,4-dimethylbutadiene-l,3 with lanthanoid atoms (La,Er) under conditions of cocondensation at 77 K leads to a brown solid product [25]. The product may be divided into three fractions by the successive extraction in pentane, toluene and THF. The elemental composition of the main fraction (soluble in THF) corresponds to the formula (Me2C4H4)2Ln. Butadiene-1,3 gives products of the composition (C4H )3Ln (Ln = Nd, Sm, Er) under the same conditions. Butene-2 or 2,3-dimethylbutene-2 is the main volatile product of the (R2C4H4)2Ln (R = H, Me) hydrolysis. Analogous hydrolysis products have been observed for complexes with other unsaturated ligands [26]. 

In 1978, Ashworth and the present author recognized that molecules containing either unsaturated carbon-metal or unsaturated metal-metal bonds should complex with other metal-ligand systems, as do alkenes or alkynes, provided the metal centers were electron rich yet had a vacant coordination site (4). This concept is illustrated in Scheme 1, with platinum (d10) as the paradigm metal center. 

These different ways of assigning electrons are simply models. Since all bonds between dissimilar elements have at least some irniic and some covalent character, each model reflects a facet of the truth. The covalent model is probably more appropriate for the majority of low-valent transition metal complexes, especially with Ae unsaturated ligands we will be studying. On the other hand, the ionic model is more appropriate for high-valent complexes with N, O, or Cl ligands, such as are found in coordination chemistry or in the organometallic chemistry described in Chapter IS. In classical coordination chemistry, the oxidation state... 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

The attack of nucleophiles on unsaturated ligands or functional groups bonded to metallic centers, exemplified in Scheme 9 (reaction of metallic carbenes with phosphines or pyridines) or in Scheme 15 (Wittig reaction) can be extended to a wide variety of reagents. Two main groups of reactions can be considered (1) those in which the nucleophile is an ylide and (2) those in which the nucleophile is a phosphine (and less commonly other nucleophiles). Usually these reactions give metallated ylides (type III), that is, species in which the ylide substituents are metallic centers. 

The structure of 11a has been established by X-ray diffraction. The compounds 11 are 30-cluster valence-electron (CVE) dimetal species, and are therefore electronically unsaturated. The electron distribution within the W(fi-C)fth rings may be represented by various canonical forms. However, that shown, which implies that the W=C bond in the ( 5-C2B9H9R 2)(OC)2-W=CC6H4Me-4 moiety formally contributes three electrons to the rhodium center (16-electron valence shell), accords with other results from our laboratory involving electronically unsaturated dimetal compounds with bridging alkylidyne ligands 18). 

---