ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

Ru-, Rh-, and Pd-Catalyzed C-C Bond Formation Involving C-H Activation and Addition on Unsaturated Substrates: Reactions and Mechanistic Aspects

The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C∧NPt(O∧O),where C∧N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2‘-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O∧O is a β-diketonato ligand. Reaction of K2PtCl4 with a HC∧N ligand precursor forms the chloride-bridged dimer, C∧NPt(μ-Cl)2PtC∧N, which is cleaved with β-diketones such as acetyl acetone (acacH) and dipivaloylmethane (dpmH) to give the corresponding monomeric C∧NPt(O∧O) complex. The thpyPt(dpm) (thpy = 2-(2‘-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 A, consistent with moderate π−π interactions and no evident Pt−Pt interactions. Most of the C∧NPt(O∧O) complexes...

Synthesis and characterization of phosphorescent cyclometalated platinum complexes.

Publisher Summary This is one of two articles in this volume that is concerned with the borane-carborane structural pattern. In the other, Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon 7∼ complexes, and various neutral or charged hydrocarbons. Boranes and carboranes may be regarded as cluster compounds in the sense defined by Cotton; they contain a finite group or skeleton of atoms held together entirely, mainly, or at least to a significant extent by bonding directly between those atoms, even though some other atoms may be associated intimately with the cluster. Examples of their structural pattern, however, can be found far beyond the confines of what is normally regarded as cluster chemistry, so this survey includes many systems not commonly referred to as clusters.

Structural and Bonding Patterns in Cluster Chemistry

The potential of palladacycles: more than just precatalysts

The bridging acyl compounds [HOs3(COR)(CO)10](R = Ph, PhCH2, n-C5H11, n-C6H13, or Pri), derived from aldehydes, and the bridging enolato-compounds [HOs3(OCHCR2)(CO)10](R = H, Me, or Ph), derived from ketens, give decarbonylated products at 150 °C, some by interconversion of these two forms.

Interconversion and degradation of acyl and enolato-ligands in triosmium clusters

The cluster [Os3(CO)10(MeCN)2] reacts rapidly at room temperature with cyclopentadienylidenetriphenylphosphorane (Ph3PC5H4) to give the oxidative-addition compound [Os3(µ-H)(µ-Ph3PC5H3)(CO)10]1 which was shown by X-ray structure determination to contain the C5 ring bonded through a single carbon atom to two osmium atoms. We consider the compound to be zwitterionic with a phosphonium substituent at the five-membered organic ring which is co-ordinated as a µ-alkylidene to the metal atoms which carry a formal negative charge. In solution the compound exists as two slowly interconverting isomers, 1a and 1b, which were separated by TLC and which differ only in the substitution position of the phosphonium group. Decarbonylation gives the compound [Os3(µ-H)(µ3-Ph3PC5H3)(CO)9]2, which also exists as isomers. The X-ray structure of the major isomer shows it to contain the C5 ring co-ordinated to the three osmium atoms through two σ Os–C bonds and an η2 bond. A zwitterionic description is likewise appropriate. The bonding in these compounds suggests new types of cyclopentadienylidene (C5H4) co-ordination.

New modes of attachment of C5 rings in organometallic compounds: X-ray crystal structures of the clusters [Os3H(Ph3PC5H3)(CO)x](x= 9 or 10) derived from cyclopentadienylidenetriphenylphosphorane

The cluster [Os3H2(CO)10] reacts with neat vinyl acetate at room temperature to give complexes [Os3H (CHCH2)(CO)10](5%) and [Os3(CO)10(CH2CHOCOMe)](1)(43%). Complex (1) has also been obtained (30%) from [Os3(CO)10(MeCN)2]. The X-ray crystal structure of complex (1) has been determined. Crystals are monoclinic, space group P21/n, with a= 17.218(3), b= 13.407(3), c= 8.597(3)A, β= 93.67(4)°, and Z= 4; the structure has been refined to R= 0.0565 for 2 562 observed reflections. The structure is related to that of [Os3(CO)12] but with two carbonyl ligands at one osmium atom being replaced by a chelating vinyl acetate. The ester group is co-ordinated axially [ν(CO) 1 622 cm–1] and the η2-vinyl group equatorially. The triosmium cluster [Os3(CO)10(CH2CMeCH2COMe)][ν(CO) 1 642 cm–1] obtained from 4-methylpent-3-en-2-one and [Os3(CO)10(MeCHCHCH2COMe)][ν(CO) 1 640 cm–1] obtained from hex-5-en-2-one have been shown by their i.r. and 1H n.m.r. spectra to be isostructural with complex (1).

Chelation of vinyl acetate and related unsaturated ketones: molecular structure of [Os3(CO)10(CH2CHOCOMe)]

A zwitterionic triosmium cluster bearing a metallated azulene ligand coordinated perpendicularly as an alkylidene bridge

Vinylene carbonate [graphic omitted]O reacts at room temperature with [Os2H3(CO)10] to give a good yield of the vinyloxy cluster [Os3H(OCHCH2)(CO)10](1). A similar decarboxylation occurs on reaction with [Os3(CO)10(MeCN)2] to give the formylmethylidene cluster [OS3µ3-CHCHO)(µ-CO)(CO)9](2). The single-crystal X-ray structure of (2) is closely related to those of [Os3(µ-CHR)(µ-CO(CO)10](R = H or SiMe3) with two osmium atoms bridged both by CO and an alkylidene group. In (2) this group is part of a four-electron-donating µ3 bridge. Hydrogenation of (2) or decarbonylation of (1) leads to the formylmethylidene compound [Os3H2(CHCHO)(CO)9](3).

Antony J. Deeming

Papers

Interconversion and degradation of acyl and enolato-ligands in triosmium clusters

New modes of attachment of C5 rings in organometallic compounds: X-ray crystal structures of the clusters [Os3H(Ph3PC5H3)(CO)x](x= 9 or 10) derived from cyclopentadienylidenetriphenylphosphorane

Chelation of vinyl acetate and related unsaturated ketones: molecular structure of [Os3(CO)10(CH2CHOCOMe)]

A zwitterionic triosmium cluster bearing a metallated azulene ligand coordinated perpendicularly as an alkylidene bridge

Decarboxylation of vinylene carbonate at triosmium clusters; X-ray crystal structure of the µ3-formylmethylidene compound [Os3(CHCHO)(CO)10]