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 cluster [Os3(CO)11{AsMe2(C6H4Me-2)}] in refluxing octane gave the unexpected positional isomer of the methylbenzyne cluster [Os3H(CO)9(AsMe2)(µ3-C6H3Me-4)]. Similarly the isomeric clusters [Os3(CO)11L][L =(2- or 4-methoxyphenyl)dimethylarsine] both gave the 3-methoxybenzyne cluster [Os3H(CO)9(AsMe2)(µ3-C6H3OMe-3)]. A mechanism involving hydrogen-atom transfer between metal and µ3-benzyne with phenyl intermediates is proposed to account for these migration reactions. The complexes [Os3H(CO)9(AsMe2)(µ3-C6H3R)](R = 4-Me or 3-MeO) exist in solution as rapidly interconverting mixtures of isomers at room temperature which give separate 1H n.m.r. signals at low temperatures. The rapidly interconverting isomers differ in the relative arrangements of the C6H3R and the hydride ligands and not by positional isomerism of the benzyne ligands.

Selective formation of positional isomers of substituted µ3-benzynes in triosmium clusters by degrading aryldimethylarsines

Decarbonylation of [Mn2HBr(CO)5(dppm)2](dppm = Ph2PCH2PPh2) using Me3No and treatment of the resultant tetracarbonyl derivative in tetrahydrofuran with LiAlH4 gives deep purple crystals of the formally unsaturated compound [Mn2H2(CO)4(dppm)2]which inserts MeCHO or MeNC to give µ-EtO or µ-MeNCH complexes and reacts with certain orher ligands such as CO or P(OMe)3 with displacement of H2.

Synthesis and reactions of the di-µ-hydrido compound [Mn2H2(CO)4(Ph2PCH2PPh2)2]

Low temperature allene coupling and isomerization at triosmium clusters

NMR coalescence effects resulting from stereochemical non-rigidity and halide exchange in octahedral rhodium(III) and iridium(III) tertiary phosphine complexes

Antony J. Deeming

Papers

Selective formation of positional isomers of substituted µ3-benzynes in triosmium clusters by degrading aryldimethylarsines

Synthesis and reactions of the di-µ-hydrido compound [Mn2H2(CO)4(Ph2PCH2PPh2)2]

Low temperature allene coupling and isomerization at triosmium clusters

NMR coalescence effects resulting from stereochemical non-rigidity and halide exchange in octahedral rhodium(III) and iridium(III) tertiary phosphine complexes

Studies on nucleophilic additions at bridging vinyl ligands in triosmium clusters