Early transition metal chemistry

Amido compounds are gaining increasing importance in early transition metal chemistry as supporting ligands, and structures with both phosphorus and nitrogen donor atoms are... 

The exchange and reduction chemistry is particularly prevalent in the early transition metals (groups III-VII). Some examples are shown in Eqs. (49-51) [150-152]. Along with / -elimination, these deleterious reactions are the major reason for poor to moderate yields (20-60%) in early transition metal chemistry. It has been suggested that MgX2 acts as a Lewis acid site that will promote the reduction of the transition metal s oxidation state... 

Nitrogen ligands are particularly versatile in high-valent, early transition metal chemistry. Deprotonation of amines, amides, and hydrazines generates multiply bonding ligands which help to stabilize the high oxidation states. Aspects of metal-amido and metal-imido chemistry have been reviewed.79,80... 

Trimethylphosphine is a ligand of proven utility in oiganometallic chemistry. However, because of its expense when purchased from commercial sources (Strero Chemicals) or its poor to moderate yidds when isolated as the silver iodide adduct [AgI(PMe3)l4, its potential is not fully realized. Frequently, large quantities of the phosphine are required, for example, as a reactive solvent or in the field of lanthanide and actinide chemistry, wherein the lability of the phosphine ligand may require crystallization from neat tri-methylphosphine. Similar considerations apply in exploratory synthetic early transition metal chemistry. In transition metal ylide chemistry, access to quantities of PMea is also very desirable, particularly when excess ylide is required. The volatility of PMe3 facilitates work-up of reaction mixtures and... 

Our group started a research program on the synthesis and reactivity of Cp2TiR (R = alkyl, aryl) compounds in the early seventies, and from about 1980 on, pentamethylcyclopentadienyl derivatives, Cp TiR, were studied. Based on our experience with Group IVB and other early transition metal chemistry we decided to expand our field to Group IIIB compounds Cp MR, and for economic reasons we chose yttrium. Later... 

Projecting forward, much new early transition metal chemistry still remains to be explored and the properties imparted by the macrocyclic structures should enhance the relative stabilities of snch species. The development of tel-luroether macrocycles remains a major synthetic challenge, but the lower electronegativity of the heavier gronp 16 element, combined with the additional stability provided by the rings should produce unprecedented coordination chemistry which is not possible with acyclic analogs. Finally,... 

In the traditional late transition metal chemistry, transmetallation with [(NHC)Ag] compounds is a common route for the synthesis of NHC com-pounds. However, a hard Lewis acidic center cannot compete for a soft NHC ligand with silver. Therefore, in the early transition metal chemistry, and especially with REM, the adducts of NHCs with alkali metals play the role of transmetallating agents. ... 

Whereas CP2MX2 compounds are ubiquitous ia early transition-metal organometallic chemistry, the thorium analogues are rather unstable. 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

Aminopyridinato ligands form a special class of anionic ligands in which an aromatic ring is part of an amidinate system. These ligands have frequently been employed in early transition metal and lanthanide coordination chemistry. Their diverse and interesting chemistry has been described in detail by Kempe et al. ° and will thus be covered here only briefly. Typical reaction pathways leading to titanium aminopyridinato complexes are outlined in Scheme 169. Metathetical as well as salt-free routes have been developed. 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

The last decade has seen the development of a rich and varied chemistry for or-ganometallic porphyrin complexes of the early transition metals (groups 3 and 4). However, there have been many fewer developments in the organometallic chemistry of the middle transition elements. Despite the paucity of its organometallic porphyrin compounds, molybdenum has played a very important role in... 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

In this contribution, we review the mechanism of polymerization and oligomerization involving early transition metals, taking as our basis recent results in advanced organometallic chemistry. First of all, some recent examples of the previous reviews concerning the Ziegler-Natta polymerization are cited [1-10]. Then, relevant new reports are surveyed in a systematic fashion. 

The molecular design of stereospecific homogeneous catalysts for polymerization and oligomerization has now reached a practical stage, which is the result of the rapid developments in early transition metal organometallic chemistry in this decade. In fact, Exxon and Dow are already producing polyethylene commercially with the help of metallocene catalysts. Compared to the polymerization of a-olefins, the polymerization of polar vinyl, alkynyl and cyclic monomers seems to be less developed. 

I am not going to discuss early transition metal oxide or alkoxide chemistry here. Instead I would like to use the few minutes allotted me to describe out work with low valent dimers of tantalum and tungsten. Professor McCarley will no doubt see his influence on our work and forgive this digression from the topic of his excellent presentation. 

The aim of this chapter is to review the chemistry of chalcogenolates in the last 10 years. The more recent reviews in this field included chalcogenolates of the s-block elements,13,14 early transition metal thiolates,15 metal complexes with selenolate and tellurolate ligands,16 copper(I), lithium and magnesium thiolates,17 functionalized thiolate complexes,18 19 pentafluorobenzenethiolate platinum group compounds,20 tellurium derivatives,21 luminescent gold compounds,22 and complexes with lanthanide or actinide.23... 

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