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At the beginning of my independent research career in 1972, I received an offer from the central research department at DuPont where a talented group of organometallic chemists was exploring the synthesis of new early-transition-metal organometallic species. Of greatest interest were compounds with some potential application in homogeneous catalysis. Fascinating work was being carried out by Fred Tebbe (among others) on titanocene dihydride and dialkyl complexes and by Ulrich Klabunde on tantalocene trihydrides.

Tantalum! Now there was a tough metal, not only literally, but also in terms of its organometallic chemistry; put simply, little organometallic chemistry was known. The prospect of new tantalum chemistry, a far cry from rhodium in my graduate student days, was attractive to me as an initial project.

Some of the simplest early organo-transition-metal species are binary alkyl complexes, MRx, where the metal M is in group x. For example, compounds such as M(CH2SiMe3)4 and M(CH2CMe3)4 (M = Ti, Zr, or Hf) were prepared by Geoffrey Wilkinson and/or Michael F. Lappert between 1970 and 1973, while Wilkinson reported the isolation of W(CH3)6 in 1972. (Compounds in which the alkyl contains one or more protons on the carbon that is with respect to the metal, such as M(CH2CH3)4, are still unknown.) G. W. A. Fowles, D. A. Rice, and J. D. Wilkins (following work by Juvinall in 1964) showed that TaMe3Cl2 could be prepared readily from TaCl5 and ZnMe2 and that it was an isolable and well-behaved species (but, of course, sensitive to moisture and air). I was delighted to find that TaMe5 could be prepared, although it was highly volatile and unstable, decomposing intermolecularly, sometimes explosively. In contrast, red crystalline Ta(CH2Ph)5 proved to be stable at room temperature.

THROUGH THE LOOKING GLASS A heat-tinted, fusion-welded tantalum tube, shown here magnified through a 35x bright field.
It was natural to attempt to complete the series by preparing Ta(CH2CMe3)5. Fortunately, the products of an attempt to synthesis Ta(CH2CMe3)5 from Ta(CH2CMe3)3 Cl2 and LiCH2CMe3 were neopentane and (Me3CCH2)3Ta=5CHCMe3, the first example of a "carbene" complex that contains a proton on the carbene carbon atom. This compound is quite stable thermally, melting at about 70 °C and distilling readily in a good vacuum. This event in 1974 marked the beginning of high-oxidation-state alkylidene chemistry. It also dramatically illustrated that four bulky covalently bound ligands could stabilize pseudotetrahedral species against bimolecular decomposition.

The reaction between (Me3CCH2)3 Ta=CHCMe3 and butyllithium to give a lithium salt of [(Me3CCH2)3Ta [CCMe3]- further suggested that group 6 high-oxidation-state alkylidyne species could be made, as was illustrated four years later at MIT with the synthesis of (Me3CCH2)3 WCCMe3, another distillable, thermally stable compound. Deprotonation of [(5-C5H5)2TaMe2]+ to yield (5-C5H5)2Ta (CH2)Me, the first isolable methylene complex, further emphasized the relationship between tantalum and phosphorus. Furthermore, reactions of tantalum alkylidene complexes suggested that the metal is in its highest possible oxidation state and that the alkylidene carbon is a strong nucleophile. Therefore, they became known as "nucleophilic" alkylidene or carbene complexes.

One of the most mysterious reactions in the 1970s that captured the interest of chemists was olefin metathesis, a catalytic reaction that involved unknown Mo, W, or Re catalysts. The reaction consists of "chopping up" carbon-carbon double bonds in olefins and redistributing the alkylidenes (usually =CHR moieties) to give a mixture containing all possible olefins formed in that manner. My question upon my move to MIT in 1975 was, "Do the high-oxidation-state species that have been discovered have anything to do with olefin metathesis?"

It took five years to prove that they do. For example, we were able to show that (PMe3)(t-BuO)2ClTa=CH-t-Bu reacts with styrene in the presence of PMe3 to provide the isolable benzylidene complex, (PMe3)2(t-BuO)2ClTa=CHPh, and that species of this type would metathesize cis-2-pentene to 2-butenes and 3-hexenes in the presence of PMe3 to the extent of 25-30 turnovers. This was the first time that an alkylidene analogous to the initial alkylidene could be isolated upon reaction with an olefin and the species actually responsible for metathesis identified conclusively. An important message was that bulky alkoxide ligands are beneficial to sustained metathesis reactions involving tantalum or niobium alkylidenes. Although tantalum was a reluctant player in the metathesis game, the principles learned from studies of tantalum alkylidene complexes were key to the later development of well-defined tungsten, molybdenum, and rhenium alkylidene complexes for the metathesis of olefins.

Although I maintain only a small interest in tantalum organometallic chemistry today, tantalum started virtually all of the areas in which I am active; it still tantalizes.

Richard R. Schrock is the Frederick G. Keyes Professor of Chemistry at MIT. He is the discoverer of high-oxidation-state organometallic compounds that contain metal-carbon double or triple bonds that are now used as catalysts for alkene or alkyne metathesis, respectively.


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Name: Named after the Greek mythological figure Tantalus.
Atomic mass: 180.95.
History: Discovered in 1802 by Swedish chemist Anders Gustav Ekeberg.
Occurrence: Primarily obtained from columbite, tantalite, and euxenite.
Appearance: Grayish silver metal.
Behavior: It is very similar chemically to niobium, and the two freely replace each other in minerals.
Uses: The biggest use of Ta is for producing capacitors. Tantalum carbide is used in cutting tools.

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