Publisher Summary The rapid development of the chemistry of transition-metal complexes containing terminal carbene (A) or carbyne (B) ligands has been followed more recently by much research centered on bridged methylene compounds (C). As often occurs in new and developing areas of chemistry, some confusion about the nomenclature of these complexes has arisen. Protonation or alkylation of several ethynyl–metal derivatives gives the corresponding vinylidene complexes in high yield. Several vinylidene complexes of the group VI metals have been obtained by heating σ–chlorovinyl derivatives with tertiary phosphines, phosphites, arsines, or stibines. Several complexes containing μ -C=CHR ligands have been obtained directly from 1-alkynes and two equivalents (or excess) of an appropriate precurso. A general route to complexes containing propadienylidene ligands is by loss of water or alcohols from suitable carbene or vinylidene precursors, or of oxo or alkoxy functions from ynolate anions. The majority of the chemistry of vinylidene and propadienylidene complexes is concerned with their synthesis and reactions. Vinylidene is one of the best π-acceptors known and is exceeded only by SO 2 and CS. The vinylidene ligand occupies an important place in the sequence of reactions linking a variety of well-known η 1 -carbon-bonded ligands. Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces.

Vinylidene and Propadienylidene (Allenylidene) Metal Complexes

Ruthenium and osmium vinylidene complexes and some related compounds

Publisher Summary This chapter focuses on supracyclopentadienyl derivatives in organometallic chemistry. Within organometallic chemistry—synthetic, analytical, physical, applied, and theoretical chemists are involved in research projects, dealing with highly substituted cyclopentadienylide (Cp)-metal complexes. Supra-Cp's are a major focus of this chapter, whereas for many others they are just a sideline where the bulkiness is a minor feature in the comparative study of certain phenomena. This chapter reveals unifying interest, sometimes with no cross-referencing to similar bulky Cp systems or even different metals carrying the same supra-Cp. The basic bulky Cp systems that are common in organometallic chemistry are discussed in the chapter, in their anionic form as ball-and-stick models with the hydrogens included. Other basic bulky Cp's described in the chapter are di(isobutyl)(tert-buty1)-Cp, penta(isopenty1)- Cp [isopentyl=( H,C2)2CH-], and tris(methoxycarbony1)-Cp. This chapter presents highly substituted Cp's, supracyclopentadienes that so far have escaped attention as ligands in organometallic complexes: tetra(terfbuty1)- Cp, 3,Mimethyl- and 3,5di-n-propyl- 1,2,4-triphenyl-Cp, and 1,2,3,5-tetraphenyl-4-p(7-cycloheptatrienyl)phenyl-Cp. Most of the bulky ring substituents are simple hydrocarbon moities, either alkyl or aryl groups, and consequently the supra-Cp's can be divided into two classes: peralkyl and peraryl systems. The use of bulky cyclopentadienes with functional group substituents causes problems in organometallic synthesis. Although there is a sizable number of Cp- metal complexes with only one functional group on the Carbon, ring examples for persubstituted systems are limited. This chapter also covers tetrasubstituted fulvalenes ethylene-bridged bis(tetrahydroindeny1) complexes and a dibornacyclopentadienyl ligand.

Bulky or Supracyclopentadienyl Derivatives in Organometallic Chemistry

Publisher Summary This chapter discusses the chemistry of molecular carbyne complexes; there is also much interest in them as models for species that are possibly present on metal surfaces in catalytic reactions. Surfacebound methine (CH) is a proposed intermediate in the Fischer–Tropsch synthesis, and alkylidyne groups are suggested as intermediates in heterogeneously catalyzed alkyne metathesis reactions. X-Ray structure determinations of carbyne complexes show that they have very short metal–carbon bond lengths consistent with a metal–carbon triple bond. In certain cases, ∞-protons of an alkylidene ligand may be abstracted by a base to give alkylidyne compounds. Addition of electrophiles to terminal isonitrile (CNR) ligands gives aminocarbyne complexes; the analogous reaction of terminal thiocarbonyl (CS) ligands gives thiocarbynes. Fischer and Himmelreich reported the abstraction of oxygen from an anionic carbamoyl compound with SOCl 2 to give an aminocarbyne compound. Reactions of cationic carbyne compounds with nucleophiles proceed exclusively by nucleophilic attack at the carbyne carbon atom, thus providing a synthetically useful route to certain carbene complexes. The field of transition-metal carbyne chemistry has matured to the point where a great deal is known about the structure, bonding, and reactivity of the carbyne ligand.

Transition metal complexes with terminal carbyne ligands

Publisher Summary This chapter discusses the carbene and carbyne complexes of ruthenium, osmium, and iridium. The importance of transition-metal carbene complexes and of transition-metal carbyne complexes is now well appreciated. The wealth of empirical information collected for transition-metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Theoretical studies of metal–carbene complexes have been undertaken by several groups. The chemistry of transition metal–carbyne complexes is rather less developed than the chemistry of carbene complexes. The development of the chemistry of carbene complexes of the Group 8A metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. Although transition-metal alkylidene complexes––that is, carbene complexes––containing only hydrogen or carbon-based substituents were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. In 1980, a stable dichlorocarbene complex of osmium (II) was described, and since then a large number of dihalocarbene complexes of ruthenium, osmium, and iridium has been prepared. Transition-metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds have proved generally applicable. M=C and M=C bonds are now well-established features of the chemistry of Ru, Os, and Ir. Many exciting possibilities exist for using these functions in further reactions.

F. Van Meurs

Papers

Substituent effects in π-(tricarbonylchromium)arenes: I. 1H NMR Spectroscopy of alkylsubstituted π-(tricarbonylchromium)benzenes

Substituent effects in IR-(tricarbonylchromium)arenes

The molecular structure of π-(tricarbonylchromium)-toluene

The molecular structure of (1′-t-butl-2′,2′-dimethylpropyl)-π-(tricarbonylchromium)benzene

The molecular structure of 4-tert-butyl-π-(tricarbonylchromium)benzoic acid