Acceptor ligand

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

Comparison of the photoelectron spectra and electronic structures of M-NS and M-NO complexes, e.g., [CpCr(CO)2(NX)] (X = S, O), indicates that NS is a better a-donor and a stronger r-acceptor ligand than NO. This conclusion is supported by " N and Mo NMR data, and by the UV-visible spectra of molybdenum complexes. 

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

Phosphino-oxazoline)copper complex 28 was found by Helmchen et al. to be an excellent Diels-Alder catalyst [37] (Scheme 1.47, Table 1.20). The nitrogen atom acts as an electron-donating ligand, whereas phosphorus is a cr-donor-Tt-acceptor ligand. The copper complex of this phosphino-oxazoline ligand is therefore expected to have... 

The structures of several adducts can be rationalized on the basis [128] that in a 5-coordinate low-spin d8 tbp system, the acceptor ligands prefer to occupy an equatorial site (IrCl(CO)2(PPh3)2) whereas a 7r-donor prefers an axial site. In a square pyramidal situation, it is weakly bonded acceptors that prefer the apical position, e.g. (IrCl(S02)(C0)(PPh3)2. 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

There is an interesting paradox in transition-metal chemistry which we have mentioned earlier - namely, that low and high oxidation state complexes both tend towards a covalency in the metal-ligand bonding. Low oxidation state complexes are stabilized by r-acceptor ligands which remove electron density from the electron rich metal center. High oxidation state complexes are stabilized by r-donor ligands which donate additional electron density towards the electron deficient metal centre. 

Nearly all the presently known compounds contain one or more w-acceptor ligands (e.g., CO, Cp, RjP) on the transition metal. These ligands may, as has classically been assumed for alkyls (72), dissipate some of the negative charge density on the central metal. However, it will be stressed later (Section IV) that such stabilizing ligands are unnecessary, and their role may in any case be more complex. 

Transition metal alkyls are often relatively unstable earlier views had attributed this either to an inherently weak M—C bond and/or to the ready homolysis of this bond to produce free radicals. Furthermore, the presence of stabilizing ir-acceptor ligands such as Cp , CO, or RjP was regarded as almost obligatory. However, (1) the M—C bond is not particularly weak compared say to the M—N bond, and (2) the presence of the new type of ligand on the metal could make the complex kinetically stable thus, even isoleptic complexes, i.e., compounds of the form MR , might be accessible 78, 239). These predictions have largely been borne out (see Table VII). 

Perhaps the most important chemical property of these complexes is their potential as catalysts, particularly of the early transition metal isoleptic compounds for a-olefin polymerization. This arises because unlike the methyls, they are sufficiently stable to be used at temperatures where polymerization rates are adequate. Some data are summarized in Table VIII ( 9) TT-acceptor ligands are clearly disadvantageous. It will be seen that some of the systems are more active than Ziegler types, although stereoselectivity is poorer. 

Additional combinatorial variation sites allow the heterocyclic self-assembly units. Thus, it has been shown that heterocycles 11 and 14-17 can serve as A-analogous donor-acceptor ligands self-assembling with the T-analogous acceptor-donor ligands isoquinolone 12 and 7-azaindole 18 (Scheme 30) [92]. All combinations form the heterobidentate ligands exclusively upon simple mixing in the presence of a transition metal salt (proven by X-ray, NMR). 

The first report of a heart-imaging agent in animals involved a 99mTc(IH) complex with 0-phenylenebis(dimethyl arsine) [30], Several other bidentate /[-acceptor ligands were investigated and early studies focused on a prototype... 

A T structure with the strongest ct-donor D trans to the empty site (I in Scheme 1) is preferred in the case of three pure cr-donor ligands. The presence of a ir-acceptor ligand also makes the T structure more stable. When one of the ligands is a tt-donor, X, a Y structure of type II (Scheme 1) is observed. This structure permits the formation of a w bond between the empty metal d orbital and the lone pair of X. No such tt bond is present in the T structure since all symmetry adapted d orbitals are filled. This partial M—X multiple bond stabilizes Y over T. 

In the mixed case—one ir-donor and a it-acceptor ligand, as in OsHCl(CO) (P1 Pr3)2—the structure III of Scheme 1 (in which the -ir-donor X ligand and the tt-acceptor A ligand are trans to each other) is more stable.22 In a T structure, which... 

An important contribution of the resonance form b requires the donation of electron density form the metal to the dienyl ligand [M(dM) -> C(pn-) contribution], The presence of a carbonyl group (a strong TT-acceptor ligand) trans to the dienyl reduces the M(dM) - C(ptt) contribution and, therefore, the nucleo-philicity of the unsaturated ii -carbon ligand. Then the nucleophilic center of the molecule is not the alkenyl ligand but the metallic center, and the protonation at the metal leads to the olefin via reductive elimination from a hydride-dienyl intermediate.24... 

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