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 diselenide PhSeSePh reacts with [Os3(CO)10(MeCN)2] to give isomers of [Os3(CO)10(µ-SePh)2] which convert thermally to a compound of apparent formula [OS3(CO)9(SePh)2], which was shown by X-ray diffraction to be the triple oxidative addition product [Os3(µ-Ph)(µ-PhCO)(µ3-Se)2(CO)8] in which the diselenide has cleaved into four separate ligand fragments as all three osmium(0) atoms are oxidised to osmium(II) atoms by an overall six-electron oxidation.

Triple oxidative addition of the diselenide PhSeSePh to the cluster [Os3(CO)10(MeCN)2] to give the osmium(II) compound [Os3(µ-Ph)(µ-PhCO)(µ3-Se)2(CO)8]

The cluster [Os3(CO)12] reacts with but-3-yn-1-ol, HCCCH2CH2OH, in hydrocarbon solvent at 130 °C to give the 2,3-dihydrofuran-4,5-diyl complex [Os3H2(CO)9(µ3-[graphic omitted])](3)(yield 48%). The probable route to this compound is indicated by the formation of the simple alkyne compound [Os3(CO)10(µ3-HCCCH2CH2OH)](1) from [Os3(CO)10(MeCN)2] and the alkynol at room temperature which at higher temperatures converts to compound (3) probably via its isomer [Os3H(CO)9(µ3-CCCH2CH2OH)](2). This isomerisation occurs because α-carbon atoms of µ3-alkynyl ligands are susceptible to nucleophilic attack which in this case occurs intramolecularly with cyclisation. The dynamic behaviour of clusters (1) and (3) is described; in both cases inversion of the chiral Os3(alkyne) group is observed.

Intramolecular nucleophilic attack at the α-carbon atom of a µ3-alkynyl ligand in a triosmium cluster

Linkage of two trimetallic clusters through the bridging ligands C3HO2 and C3O2. Crystal structures of three clusters containing the components Os3CHCCO2Os3, Ru3CHCCO2Os3, and Os3CCCO2Os3

The compound [Os3(CO)10(MeCN)2] reacts with keten to give [Os3(CO)11(CH2)] by CC bond cleavage while diketen undergoes decarboxylation to give the µ2-allene compound [Os3(CO)11(C3H4)] and vinylene carbonate undergoes decarboxylation to give the µ3-formylmethylidene compound [Os3(CO)10(CHCHO)].

Bridging ligands formed by decarbonylation of keten and by decarboxylation of diketen and vinylene carbonate at triosmium clusters

The complex HRu3(CO)10SEt (1) readily dissolves in concentrated sulphuric acid (98%) at room temperature to give the protonated species H2Ru3(CO)10SEt+(2). When this solution is heated at 100 °C for 15–20 min. carbon monoxide is evolved and the trihydrido-cation H3Ru3(CO)9S+(3) is produced. On dilution of this solution with water the neutral complex H2Ru3(CO)9S (4) is precipitated. The composition of this complex has been confirmed by proton magnetic resonance and mass spectroscopy and the most likely structure of (4) is considered to be a tetrahedron of three rutheniums and one sulphur atom with three terminal CO group per ruthenium and two H ligands occupying regions between the metal atoms. Similar experiments with D2SO4 revealed that neither (1) nor (3) undergo proton exchange in concentrated H2SO4 but do in more dilute solutions.

Antony J. Deeming

Papers

Triple oxidative addition of the diselenide PhSeSePh to the cluster [Os3(CO)10(MeCN)2] to give the osmium(II) compound [Os3(µ-Ph)(µ-PhCO)(µ3-Se)2(CO)8]

Intramolecular nucleophilic attack at the α-carbon atom of a µ3-alkynyl ligand in a triosmium cluster

Linkage of two trimetallic clusters through the bridging ligands C3HO2 and C3O2. Crystal structures of three clusters containing the components Os3CHCCO2Os3, Ru3CHCCO2Os3, and Os3CCCO2Os3

Bridging ligands formed by decarbonylation of keten and by decarboxylation of diketen and vinylene carbonate at triosmium clusters

The chemistry of polynuclear compounds. Part XXIV. Some cationic carbonyl derivatives of ruthenium