Lanthanide organometallic compounds 4 oxidation state

Organometallic compounds of the lanthanides in the oxidation state Ln(II) were first reported in the early 1980s... 

Although lacking 7T-bonded compounds in low oxidation states that characterize the d-block elements, the actinides have a rich organometallic chemistry. Their compounds frequently exhibit considerable thermal stability, but like the lanthanide compounds are usually intensely air- and moisture-sensitive. They are often soluble in aromatic hydrocarbons such as toluene and in ethers (e.g., THF) but are generally destroyed by water. Sometimes they are pyrophoric on exposure to air. Most of the synthetic work has been carried out with Th and U this is partly due to the ready availability of MCI4 (M = Th, U) and also because of the precautions that have to be taken in handling compounds of other metals, especially Pu and Np. 

Organolanthanide chemistry is dominated by the trivalent compounds. " Compounds in oxidation state (II) are restricted to derivatives of europium, samarium, and ytterbium, but they have considerable importance in both organic and organometallic syntheses because of their reducing properties. " Redox transmetalation reactions of organomercurials with lanthanide metals provide convenient syntheses of a number of diorganolanthanides, for example, R2M, R = CgFj or PhCC, M = Yb or Eu. " °... 

Mixed valency is not limited to consideration of individual ions in different oxidation states. Delocalized systems also exist that exhibit properties of mixed valency, as in many organometallic mixed valence compounds. For example, phthalocyanine. Pc (1), sandwich compounds of the lanthanides, Ln(Pc)2, are best described as containing the two Pc anionic ligands in mixed valent charge states of —2 and —1 with the metal ion Ln +. 

Controversy exists in naming sandwich organometallic actinide and lanthanide compounds of cyclooctatetraene dianion [8]annulene dianion and [8]annulene-metal(X) complex are chosen here because these names properly describe the delocalization of charge in the ligand and also emphasize the formal oxidation state of the metal in the complex. 

Since we essentially deal with transition-metal compounds in this chapter, we, in this section, present a brief introduction to the main characteristics of rare earth complexes, where the reader can find references to more detailed accounts. The lanthanides show a marked preference for the - -3 oxidation state, and present very similar coordination and organometallic chemistries, with small differences associated only with changes in the atomic size along the series. The interest for the incorporation of lanthanides in supramolecular architectures is associated with Iheir magnetic and light emitting properties. 

A particular difference lies in the organometallic chemistry in low oxidation states. Transition metals form stable binary carbonyls—for example, Fe(CO)s and Ni(CO)4 are volatile liquids, stable at elevated temperatures, and Os3(CO)i2 decomposes at 190 °C to other carbonyls such as Os6(CO)i8. In contrast, lanthanide carbonyls, prepared by cocondensation of lanthanide atoms with Ar/CO mixtures at 4.2 K, are only stable at these very low temperatures, decomposing above 20 K. The ability of metals in the middle of the d block to form carbonyls is related to their possessing vacant d orbitals that can accept electron pairs from the o-donor CO ligand, and also some filled d orbitals that can participate in n back-donations (back-bonding). In fact, CO is not a strong enough r-donor to bind well to lanthanides, and the lanthanide 4f orbitals are not suitable to 7r-bond to carbon monoxide. Molecular orbital (MO) calculations indicate that the bonds in these compounds are very weak indeed. ... 

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