Ligands 7T-acceptors

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). 

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). 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

D. Complexes with a-donor/7t-acceptor ligands 1. Alkenyls and alkynyls... 

Figure 7 The effect of turning on the jt interaction between a 7T-acceptor ligand and the metal. The unoccupied, and relatively unstable, it orbitals of the ligand are shown on the right. Their effect is to stabilize the filled Aj, orbitals of the complex and so increase A...

The synthesis and structural characterization of the first example of an titanium complex (Scheme 670 Section 4.05.5) has been reported. Its reactivity toward cr-donors/7T-acceptor ligands demostrates that this complex does not behave like a low-valent titanium compound but rather bears a closer resemblance to Lewis-acidic Ti(iv) complexes. Based on theoretical calculations, this behavior can be attributed to a strong and appreciably covalent Ti-cycloheptatrienyl interaction which leads to highly stabilized frontier orbitals and consequently to a diminishing 7r-electron release ability.1706... 

Ammonia forms a number of complexes with osmium (see Table 5), though the only fully established unsubstituted osmium ammine which has been isolated is the hexaammine [Os(NH3) ] + there is, however, electrochemical evidence for the existence both of [Os(NHj)sf+ and of [Os(NHj) f +. Amongst the substituted ammines which have been isolated and characterized are several of osmium(II), all with supporting (or stabilizing) 7t-acceptor ligands, e.g. [Os(N-H3)5CO] IOs(NH3)3(NO)r+ and [Os(NH3)3(N3>P+. 

Complexes with several tt-donor or 7T-acceptor ligands... 

Regarding vibrational spectroscopic comparison, the coordinated B-H stretch cannot be observed in M(CO)5BH3 L, possibly because of overlap with the v o, which are highly split into six bands that occur from 1840 to 2075 cm". These values are significantly lower than those in M(CO)5(H2>, which typically show only two Vco at 2090-2100 and 1970-1980 cm . This indicates that the borane here is much more of a donor ligand and less of a 7t-acceptor ligand than Hj or silanes. 

In main group chemistry, we have encountered the octet rule, in which electronic structures can be rationalized on the basis of a valence shell requirement of 8 electrons. Similarly, in organometallic chemistry, the electronic structures of many compounds are based on a total valence electron count of 18 on the central metal atom. As with the octet rule, there are many exceptions to the 18-electron rule, but the rule nevertheless provides useful guidelines to the chemistry of many organometallic complexes, especially those containing strong 7T-acceptor ligands. 

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