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

Substitution effects at bridging vinyl ligands: Molecular structures of [Os3H(CHCHR)(CO)10] where R = OEt or ferrocenyl

The addition of cyanide ion to [Os3H2(CO)10] gives [Os3H2(CN)(CO)10]– as four isomers in solution. Subsequent protonation at nitrogen leads via intermediates to the aminomethylidyne complex [Os3H(µ-CNH2)(CO)10], the characterisation of which was assisted by the synthesis of the corresponding 13CNH2, C15NH2, and deuteriated analogues. Hydrogen exchange at the CNH2 ligand is slow in neutral solution but is both acid- and base-catalysed. Rather than displacing acetonitrile from [Os3H(CO)10(MeCN)(PMe2Ph)]PF6, cyanide ion adds at MeCN to give two main isomers of [Os3H (NCMeCN)(CO)9(PMe2Ph)]. Related complexes [Os3H(NCHCN)(CO)9L](L = CO or PMe2Ph) were obtained by reaction of [Os3H2(CO)9L] with cyanogen (C2N2) at room temperature. Rotation about the CN bonds in the iminyl complexes is sufficiently slow to allow the separation of isomers when L = PMe2Ph. The isomeric form [Os3H(NHCCN)(CO)10] was also obtained.

Triosmium clusters containing the ligands CNH2, NCHCN, and NCMeCN derived from cyanide ion, cyanogen, and acetonitrile

Oxidative addition of the silanes R3SiH (R3
				= Ph3, Et3, EtMe2) to the unsaturated cluster [Os3(μ-H){μ3-Ph2PCH2PPh(C6H4)}(CO)8] leads to the saturated clusters [Os3(μ-H)(SiR3)(CO)9(μ-dppm)]
				(SiR3
				= SiPh31, SiEt32 and SiEtMe23) and the unsaturated clusters [Os3(μ-H)2(SiR3){μ3-Ph2PCH2PPh(C6H4)}(CO)7]
				(SiR3
				= SiPh34, SiEt35 and SiEtMe26). Structures are based on spectroscopic evidence and a XRD structure of [Os3(μ-H)(SiPh3)(CO)9(μ-dppm)]
				1 in which all non-CO ligands are coordinated equatorially and the hydride and the silyl groups are mutually cis. From variable-temperature 1H NMR spectra of the SiEt3 compound 2, exchange of the P nuclei is clearly apparent. Simultaneous migrations of the SiEt3 group and of the hydride from one Os–Os edge to another generate a time-averaged mirror plane in the molecule. VT 1H NMR spectra of the somewhat less bulky compound [Os3(μ-H)(SiMe2Et)(CO)9(μ-dppm)]
				3 have been analysed. Two isomers 3a and 3b are observed with the hydride ligand located on different Os–Os edges. Synchronous migration of the hydride and SiMe2Et groups is faster than the observed interconversion of isomers which occurs by hydride migration alone. The synchronous motion of H and SiR3only occurs when these ligands are mutually cis as in the major isomer 3a and we propose that this process requires the formation of a transient silane complex of the type [Os3(η2-HSiR3)(CO)9(μ-dppm)]. Turnstile rotation within an Os(CO)3(η2-HSiR3) group leads to the observed exchange within the major isomer 3a without exchange with the minor isomer. This process is not observed for the minor isomer 3b because the hydride and the silyl group are mutually trans. Protonation to give [Os3(μ-H)2(SiR3)(CO)9(μ-dppm)]+ totally suppresses the dynamic behaviour because there are no edge vacancies.

Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(µ-H){µ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(µ-H)(SiR3)(CO)9(µ-dppm)]

Antony J. Deeming

Papers

Substitution effects at bridging vinyl ligands: Molecular structures of [Os3H(CHCHR)(CO)10] where R = OEt or ferrocenyl

Triosmium clusters containing the ligands CNH2, NCHCN, and NCMeCN derived from cyanide ion, cyanogen, and acetonitrile

Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(µ-H){µ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(µ-H)(SiR3)(CO)9(µ-dppm)]

Synthesis, structures, isomerism, and dynamic behavior of metalated derivatives of methyl-substituted pyrroles in triruthenium clusters

12 – Trinuclear Clusters of Ruthenium and Osmium: (i) Introduction and Simple Neutral, Anionic and Hydrido Clusters