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Experimental results (recent IR, DFT calculations and modern multinuclear NMR measurements on Rh-containing clusters, together with earlier VT multinuclear NMR measurements) show that the use of the Ligand Polyhedral Model (LPM) to provide a general mechanism for ligand fluxionality in Transition Metal Carbonyl Clusters (TMCCs) in solution cannot be sustained; instead there are numerous examples of only partial CO-migration over either part or sometimes the whole of the Rh-polyhedron as well as rhodium and carbonyl polyhedral rearrangements of Rh9- and Rh10-TMCCs containing an interstitial P when, in the high temperature limiting spectra, all the metals and all the carbonyls become equivalent and show time-averaged values of 1J(Rh–P) and 2J(P–CO) respectively.

Addition of excess CF 3 CO 2 H (HTFA) to [Rh 2 Pt 2 (CO) 7 (PPh 3 ) 3 ], I , under nitrogen results in the formation of a salt ( X 2+ Y 2− ) , which contains only the second example of a di-cationic carbonyl hydride tetra-nuclear cluster , [H 2 Rh 2 Pt 2 (CO) 7 (PPh 3 ) 3 ] 2+ , X 2+ , and a presently partially characterized polymetallic anion Y 2− . The di-cation X 2+ has been characterized by mass spectrometry and a variety of multinuclear NMR methods. Since there is no difference in the electron count for I and X 2+ , it is probable that both I and X 2+ adopt similar butterfly metallic frameworks with a Rh–Rh hinge; in X 2+ , there are two bridging hydrides to the same wing-tip Pt but the phosphine site occupancies on the Rh 2 Pt 2 -framework in I and X 2+ are different.

The fragmentation and redistribution reactions of [Rh4(CO)12−x{P(OPh)3}x] (x = 1–4) with carbon monoxide have been studied using high-resolution, high-pressure NMR spectroscopy. Under the conditions of efficient gas mixing in a high-pressure NMR bubble column, [Rh4(CO)9{P(OPh)3}3] fragments to give mainly [Rh2(CO)6{P(OPh)3}2]; [Rh4(CO)11{P(OPh)3}] is also observed, implying redistribution of the phosphite ligand and/or recombination of the dimers to tetrameric clusters. Fragmentation of [Rh4(CO)10{P(OPh)3}2] is found to be pressure-dependent giving predominantly [Rh2(CO)6{P(OPh)3}2] at low CO pressure (1–40 bar), and increasing amounts of [Rh2(CO)7{P(OPh)3}] at higher (40–80 bar) pressure. Using Syngas (CO : H2 (1:1)) instead of CO in the above fragmentations, homolytic addition of H2 to the dimer [Rh2(CO)6{P(OPh)3}2] to give [RhH(CO)3{P(OPh3}] and [RhH(CO)2{P(OPh)3}2] is observed. The distribution of tetrameric species obtained is similar to that obtained under the same partial pressure of CO. On depressurisation/out-gassing of the sample, the original mixture of tetrameric clusters is obtained. Copyright © 2008 John Wiley & Sons, Ltd.

The progressive addition of anhydrous pyridine, (py), to a solution of [Rh(4)(CO)(12)] in CH(2)Cl(2) under CO, even at low temperature, results in immediate disproportionation to give cis-[Rh(CO)(2)py(2)][Rh(5)(CO)(15)]; further addition of pyridine results in the progressive replacement of CO's by py on the same apical rhodium in [Rh(5)(CO)(15)](-) to give cis-[Rh(CO)(2)py(2)][Rh(5)(CO)(15-x)py(x)] (x = 1, 2). The analogous reactions with 2,2'-bipyridine (bipy) give only [Rh(CO)(2)bipy][Rh(5)(CO)(13)bipy]. IR and low temperature, multinuclear NMR measurements have been used to establish the structures of all the above anions and the structures of [Rh(5)(CO)(13)(bipy)](-) and [Rh(5)(CO)(13)py(2)](-) are subtly different. Under N(2), [Rh(4)(CO)(12)] reacts with py to give [Rh(6)(CO)(16-y)py(y)] (y = 1, 2).

Both Rh4(CO)12 and Rh6(CO)16 exhibit CO-fluxionality and modern, variable temperature, NMR methods allow the unambiguous assignment of the three terminal CO resonances and, for Rh4(CO)12, show that the mechanism of CO-fluxionality, which has been controversial for a long time, unambiguously involves the merry-go-round process; Rh6(CO)16, which was previously thought to be static, is also shown to be fluxional, although the rate of CO-exchange is much less than found for substituted derivatives, and possible pathways for this CO-exchange are discussed.

The promotional effect of low concentrations of water on the catalytic activity in the Co-catalysed methoxycarbonylation of oct-1-ene to straight and branched chain C 9 methylesters has been demonstrated. Under rigorously anhydrous conditions, only moderate octene conversions are noted. The controlled addition of water leads to enhanced octene conversions, by factors of up to 2.5, ester product selectivity remaining unchanged. Addition of dry pyridine to the anhydrous system results in the known promotional effects on both activity and selectivity towards linear esters. Further addition of water leads to enhanced oct-1-ene conversions, particularly at low pyridine loadings, whilst maintaining the improved product selectivity associated with the presence of pyridine. The role of water, which influences the rate as strongly as pyridine, has been attributed to increasing the effective concentration of HCo(CO) 4 , thereby enhancing the hydride mechanistic pathway for methoxycarbonylation. Certain amide/water compositions have also been found to result in significant promotional effects relative to the performance of standard Co-based catalysts. Thus, 4- and 3-amidopyridine ( iso -nicotinamide and nicotinamide, respectively) display enhancements in rate and selectivity approaching that of pyridine itself whereas, in total contrast, 2-amidopyridine (picolinamide) acts as an effective poison of both olefin isomerisation and methoxycarbonylation activity.

Progressive displacement of acac from [Pd(acac)2] occurs on reaction with different tetramic acids (HL � /Hata, Habta) to give [Pd(acac)(h 2 -L)] and [Pd(h 2 -L)2] but, when HL� /Haceta, only [Pd(acac)(h 2 -L)] is formed, even when the ratio of Pd � /HL is 1:4; reaction of all these tetramic acids with an aqueous solution of K2[PdCl4 ]g ives [Pd(h 2 -L)2]. In solution, NMR studies in non polar solvents show that there is only one isomer of [Pd(acac)(h 2 -L)] whereas there are usually two isomers of [Pd(h 2 -L)2] which are probably due to the presence of cis - and trans -isomers of the complex as a result of both the enolate ligands adopting a O,O?-mode of coordination via the functionalities associated with C 4 and the acyl group at C 3 in the pyrrolidine ring. In pyridine solution, [Pd(h 2 -abta)2] forms a Lewis base adduct, [Pd(py)4](abta)2, which has been characterised by X-ray analysis and shown to contain a

The sequence of reaction steps and the role of each reactant, required for the transformation of the Pd(0) precursor [Pd(dtbpx)(dba)] [dtbpx = 1,2-(CH2PBut2)2C6H4; dba = trans,trans-(PhCHCH)2CO], 1, into [Pd(dtbpx)H(MeOH)]+, 2a, the active Pd(II)-hydride catalyst for the methoxycarbonylation of ethene to methylpropanoate, have been delineated using a combination of spectroscopic and crystallographic methods. The preparation and characterisation of a variety of related complexes are described including some unusual examples involving bidentate sulfonate complexes and mono-cationic and neutral palladium hydride complexes. X-Ray crystal structures have been determined for [Pd(dtbpx)(η2-O2)], 3, [Pd(dtbpx)(η2-BQ)] (BQ = benzoquinone), 4, [Pd(dcpx)(dbaH)]+ [dcpx = 1,2-(CH2PCy2)2C6H4], 7, and [Pd(dtbpx)(η2-MeSO3)]+, 9b.

Tetrabutylammonium carbonyltribromoplatinate(II) [(C4H9)4N][PtBr3(CO)] crystallises in the monoclinic system in the space group P21/n with a = 1487.84(13), b = 897.41(8), c = 1849.6(2) pm, β = 106.558(2)° and four formula units per unit cell. The [PtBr3 (CO)]- anion is symmetrical (C2v) and almost planar with Pt-Br bond lengths of 242.40(12) pm (trans to CO), 243.25(10) pm and 242.99(9) pm (trans to Br), and a Pt-C bond length of 185.6(11) pm.

A detailed spectroscopic study has allowed the solution structure and dynamic properties of all the intermediates in the Pd-catalyzed methoxycarbonylation of ethene to be established. [Pd(L−L)H(solv)]+ 1 (L−L = 1,2-(CH2PBut2)2C6H4; solv = MeOH, 1a; PrnOH, 1b; THF, 1c; EtCN, 1d) is static, and the two inequivalent P atoms do not become equivalent through solvent exchange over all the temperatures studied. 2, contains a strong β-agostic C−H interaction which is remarkably stable and is not displaced even in strongly coordinating solvents such as EtCN. Cα and Cβ of the ethyl group in 2 become equivalent via a stereospecific interchange involving [Pd(L−L)H(η2-C2H4)]+ without making the two P atoms equivalent; at higher temperatures these two inequivalent P atoms do become equivalent probably via a T-shaped intermediate. For [Pd(L−L)(C(O)Et)(solv)]+, 6, there is no β-agostic C−H interaction and multiple 13C-labeling of the C(O)Et group shows that the inequivalent P atoms become equivalent via movement of the i...

One of the outstanding problems and challenges in CO activation concerns the lack of a detailed mechanistic understanding of the roles of catalytic promoters used in a number of homogeneously catalysed carbonylation reactions. These problems, and attempts at their resolution, are highlighted with reference to (i) the varied range of promoters that have found use in composite catalysts for the direct synthesis of oxygenates such as ethylene glycol and ethanol from CO/H2, (ii) the promotional effects of N-bases in the catalytic methoxycarbonylation of alkenes to esters, and (iii) some preliminary 13C NMR spectroscopic evidence which is enabling the definition of a dual role of Ru-promoters as iodide abstraction agents in the Ir-catalysed carbonylation of methanol to ethanoic (acetic) acid. The detection, and characterisation in solution, of an iodide-bridged Ru–Ir dimer is facilitating the development of a plausible, internally consistent model on which to base the catalysis.

A series of substituted hexarhodium carbonyl clusters [Rh6(CO)15L] (L = PR3 (R = alkyl, aryl), P(OPh)3, NCMe, I−) has been studied by variable temperature and two-dimensional, X-{103Rh}, (X = 13C, 31P) HMQC and 13C EXSY NMR spectroscopy in solution. At low temperatures, the spectra are consistent with retention of the solid state structure. Different localised exchanges of terminal (COt) and face-bridging (COfb) CO's are found to occur over different atoms of the Rh6-octahedron at higher temperatures and the different pathways of the exchanges are discussed. When L = PR3 (R = alkyl, aryl), the lowest energy scrambling surprisingly involves exchange of COt and COfb, associated with the substituted rhodium (text-decoration:underline'S-type), with concomitant exchange of L between the two terminal sites on the substituted rhodium, followed by other localised stereospecific exchanges involving CO's associated with unsubstituted rhodium atoms (text-decoration:underline'U-type). For the other substituted clusters (L = P(OPh)3, NCMe, I−), only text-decoration:underline'U-type exchanges are observed. The kinetics of these exchanges are reported at different temperatures and for the text-decoration:underline'S-type exchange mechanism, the rate is found to vary with the nature of PR3.

Displacement of norbornadiene (nbd; bicyclo[2.2.1]hepta-2,5-diene) from [Rh(PPh3)(2)(nbd)]ClO4 by hydrogenation in the presence of PPh3 and formamide or Me-substituted derivatives, results in the formation of O-bonded formamide complexes [Rh(PPh3)(3)(OCHNHxMe2-x)]ClO4 (x=0, 1, 2) rather than N-bonded derivatives. These have been characterised by spectroscopic measurements and, in the case of [Rh(PPh3)(3)(OCHNHMe)]ClO4, by X-ray crystallography. All undergo oxidative addition with H-2, and the rates of ligand exchange in the Rh-I and Rh-III complexes have been determined by magnetisation-transfer measurements.

A complete NMR study involving both 1D and 2D 13C-{103Rh} and 31P-{103Rh} HMQC measurements, on [Rh6C(CO)14(PPh3)]2- are reported and discussed, together with the multiple Rh quantum effects found for resonances associated with edge- and face-bridging CO's. As found in [Rh6C(CO)15]2-, the carbonyl ligands in [Rh6C(CO)14(PPh3)]2- undergo CO-intermolecular exchange with 13CO at different rates; for the edge-bridging CO's, the lower the value of 1J(Rh–CO), the faster the rate of intermolecular exchange with 13CO.

The dissolution of PtO2 in concentrated H2SO4 under an atmosphere of CO results in the formation of hexacarbonyldiplatinum(I), [{Pt(CO)3}2]2+ (1), the first homoleptic, dinuclear, cationic platinum carbonyl complex, of which a prolonged evacuation leads to reversible disproportionation to give cis-[Pt(CO)2]2+(solv) (2) and Pt(0). 1 has been completely characterized by NMR (13C and 195Pt), IR, Raman, and EXAFS spectroscopy. The structure of 1 is rigid on the NMR time scale at room temperature. NMR: δ(13CA) 166.3, δ(13CB) 158.7, δ(195Pt) −211.0 ppm; 1J(Pt−CA) = 1281.5 Hz, 1J(Pt−CB) = 1595.7 Hz, 1J(Pt−Pt‘) = 550.9 Hz. The strongly polarized, sharp Raman band at 165 cm-1 (ρ = ca. 0.25) indicates the presence of a direct Pt−Pt bond. The IR and Raman spectra in the CO stretching region are entirely consistent with the presence of only terminal CO's on a nonbridged Pt−Pt bond with D2d symmetry. ν(CO)IR: 2174 (E), 2187 (B2), and 2218 cm-1 (B2); ν(CO)Raman: 2173 (E), 2194 (B2), 2219 (B2), 2209 (A1) and 2233 cm-...

The molecular structures of (NMe 4 ) 4− x [H x Ni 12 (CO) 21 ] · S ( x =1, S=Me 2 CO; x =2, S=2THF) are reported. Both anions are similar and consist of a central Ni 6 (CO) 3 ( μ -CO) 6 -layer flanked on both sides by two Ni 3 (CO) 3 ( μ -CO) 3 -layers such that the Ni 12 -fragment is hexagonal close-packed. The separation between the central Ni 6 -plane (B) and the two outer Ni 3 -planes (A, C) is consistent with H-occupancy of either one ( x =1) or both ( x =2) of the two octahedral metal cavities. Variable temperature solid state 1 H MAS and 13 C CP/MAS NMR spectra are reported for both clusters. For the dihydride, the solid state 1 H- and 13 C-NMR spectra remain unchanged from low to room temperature and are consistent with the X-ray structure; at room temperature, there are two hydride resonances consistent with them being on the plane of symmetry on opposite sides of the Ni 6 -layer and the highfield shift is consistent with them not being in the centre of the Ni 6 octahedral cavity. For [H 2 Ni 12 (CO) 21 ] 2− , there is close agreement between the solution and solid state 13 CO chemical shifts except for the inner bridging carbonyls which appear in solution as one resonance whereas in the solid state occur as two resonances due to the bridging CO’s being either above or below the Ni 6 -plane. At higher temperatures, there is evidence for H movement (probably oscillation about the plane of symmetry) but 13 C spectra indicate that the CO’s remain essentially static. For the monohydride, three of the bridging CO’s are almost co-planar with the Ni 6 -plane, one, C6, is significantly bent towards the hydride and two CO’s are in intermediate positions; the low temperature solid state 13 CO-NMR spectra appear to be entirely consistent with this structure. At room temperature, there is some coalescence of the 13 CO resonances, suggesting some carbonyl movement, but there is no evidence for H-migration from the occupied interstitial octahedral site to the unoccupied octahedral site.

Triple-rhodium quantum effects in the HMQC spectra of [Rh6(C0)15L][L = MeCN, I, PBun3, P(0Ph)3, P(4-XC6H4)3; X = H. 0Me, F] modulate the intensity and position of the correlations in the rhodium dimension; cross peaks are displaced from the true chemical shift, additional cross peaks are seen, and the intensity of the coherences varies as a function of the mixing time and coupling constant, going to zero at the conventional valueof 1/(2J).

Two novel mixed metal rhodium–platinum clusters have been synthesized by the reaction of [Rh4(CO)12] with [Pt(PPh3)3]. The structure of [Rh2Pt3(µ-CO)5(CO)4(PPh3)3] 1 has been established by a single crystal X-ray diffraction study. The tetranuclear cluster [Rh2Pt2(µ-CO)3(CO)4(PPh3)3] 2 has been characterised by FAB mass spectrometry and shown by various multinuclear NMR techniques (13C, 31P, {31P–31P} COSY and {13C–103Rh} and {31P–103Rh} heteronuclear multiple quantum coherence, HMQC) to adopt a butterfly type structure. The carbonyl migration pathways in 2 have been established by a combination of EXSY and 1-D variable temperature measurements; independent localised exchange about the hinge rhodium atoms and interchange of bridging and semi-bridging carbonyls on the Pt–Rh bonds occur.

Clusters provide interesting models for surfaces and sometimes provide a controlled step-wise sequence of cluster growth. Although the metal geometry in homometallic clusters can now be predicted reliably from electron counting rules, it is much more difficult to predict both the metal site occupancy in heterometallic clusters and the carbonyl and non-carbonyl ligand site occupancies on the metallic framework; this is especially true for the ubiquitous hydrogen and structural elucidation of these clusters has had to rely heavily on neutron diffraction and NMR measurements, which are particularly useful when the metal has a spin. Similarly, the ease of migration and the exact migrational pathway(s) of CO and/or H are also difficult to predict, although NMR measurements in solution often allow the detailed pathway of ligand migrations to be unambiguously established. The purpose of this Perspective is to compare and contrast the dynamic and structural information available from solid state multinuclear NMR measurements with data obtained from solution NMR and other related spectroscopic and structural measurements on tetra- and higher-nuclearity transition metal carbonyl clusters.

Multinuclear NMR data ( 13 C, 31 P, 13 C–{ 31 P}, 13 C–{ 103 Rh} and 31 P–{ 103 Rh}) for a series of mono- and di-substituted derivatives of Rh 6 (CO) 16 containing neutral two electron donor ligands [Rh 6 (CO) 15 L, (L=NCMe, py, cyclooctene, PPh 3 , P(OPh) 3 ,1/2( μ 2 , η 1 : η 1 -dppe)); Rh 6 (CO) 14 (LL), (LL= cis -CH 2 CMe-CMeCH 2 , dppm, dppe, (P(OPh) 3 ) 2 )] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the 13 CO NMR spectra of Rh 6 (CO) 15 (PPh 3 ) and Rh 6 (CO) 14 (dppm) clusters have been made on the basis 13 C–{ 103 Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of 3 J(P–C) for terminal carbonyl ligands is discussed and the values of 3 J(P–C) are found to be mainly dependent on the bond angles in the P–Rh–Rh–C fragment; these data enable the fine structure of the complex multiplets in the 13 C–{ 1 H} and 31 P–{ 1 H} NMR spectra of Rh 6 (CO) 14 (dppm) to be simulated. Variable temperature 13 C–{ 1 H} NMR measurements on Rh 6 (CO) 15 (PPh 3 ) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh 3 on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ 3 -CO's.

The progressive addition of hydrazine and the following increasingly substituted hydrazines L (N2H4 Ia, H2NNHMe Ib, H2NNHPh Ie, H2NNMe2 Id, or MeHNNHMe Ic) to [Rh2(CO)4Cl2] resulted in the initial formation of [{Rh(CO)2Cl}2(µ-L)] 1 followed by the formation of cis-[Rh(CO)2L(Cl)] 2; analogous mononuclear complexes were formed directly on addition of the more heavily substituted hydrazines Me2NNMe2 IIf and H2NNPh2 IIg, but addition of 1,2-N2H2Ph2 IIIh to [Rh2(CO)4Cl2] resulted in disproportionation of the hydrazine and formation of cis-[Rh(CO)2(NH2Ph)Cl] 3 and cis-[Rh(CO)2(PhNNPh)Cl] 4. The above complexes have been spectroscopically characterised by IR and 13C/15N NMR measurements. X-Ray structural analysis on [{Rh(CO)2Cl}2(µ-L)] (L = H2NNHMe or MeHNNHMe) confirmed that L adopts a µ-η1∶η1-mode of bonding with a cisoid arrangement of the Rh(CO)2Cl groups about the N–N bond.

The products resulting from the progressive addition of pyridine (py) to a solution of [Rh2(µ-Cl)2(CO)4] 1 have been found to depend both upon the solvent and the atmosphere (CO or N2). In CH2Cl2 under N2, cis-[Rh(CO)2Cl(py)] 2, [Rh(CO)2Cl(py)2] 3 and [Rh2(µ-CO)3Cl2(py)4] 4 were obtained successively; under CO, 4 was converted into 3 and under N2 disproportionation of 4 slowly occurred to give 3 and trans-[Rh(CO)Cl(py)2] 5 which reacted with CO to give 3. In more polar solvents (thf or MeOH), 1 reacted under N2 to give the lightly solvent-stabilised complex [Rh(CO)2Cl(solv)] 6 (solv = thf a or MeOH b) and, in the presence of AgClO4, cis-[Rh(CO)2(solv)2]+ 7 (solv = thf a or MeOH b); additionally, when solv = MeOH there was spectroscopic evidence for the formation of [Rh2(µ-CO)x(MeOH)y]2+ 8 (x = 2, y = 4 or x = 3, y = 6) which reacted with CO to give [Rh(CO)2(MeOH)2]+. Complex 7 reacted with py to give successively cis-[Rh(CO)2(py)2]+ 9 and [Rh(CO)(py)3]+ 10; under CO 10 was converted into 9. The stereochemistry of all the above complexes has been established through a combination of IR and multinuclear (13C, 15N, 103Rh) NMR measurements and X-ray crystallography for 2 and 4. Analogous reactions have been carried out using trans-[Rh2(µ-Cl)2(CO)2(CxHy)2] 11 (CxHy = C2H4 a or C8H14 b) under a nitrogen atmosphere and spectroscopic measurements on the reaction of 11a with pyridine were consistent with the successive formation of [Rh2(µ-Cl)2(CO)2(C2H4)2(py)2] 12, which lost ethylene and rearranged to give [Rh2(µ-CO)2Cl2(py)2] 13. Reaction of 11b with pyridine gave immediately trans-[Rh2(µ-Cl)2(CO)2(py)2] 14 and both 13 and 14 further reacted with pyridine to give 5.

Pure samples and as-prepared mixtures of Rh 9 and Rh 10 carbonyl clusters with interstitial P atoms have been studied quantitatively by 31 P MAS and 1 H– 31 P CP/MAS NMR. Information on the 31 P chemical shift tensor of the Rh 9 and Rh 10 clusters has been derived from spinning sideband simulations. The chemical shift anisotropy is slightly larger in the Rh 10 clusters (340–400 ppm) than in the Rh 9 clusters (230–300 ppm), while the asymmetry parameters are similar ( η =0.1–0.4). The results contribute to the understanding of the relationship between the shielding anisotropy and the structure of the cluster cavity. © 1997 Elsevier Science B.V.

o-Bis(dimethylsilyl)benzene (1) reacts with cyclopentadienylbis(ethylene)cobalt at room temperature. This reaction results in overall double SiH activation, generating the disilacobaltacycle complex Cp(C2H4)C o(o-Me 2 SiC 6 H 4 S iMe2) (3). Complex 3 was characterized by X-ray diffraction. 1,2,4,5-Tetrakis(dimethylsilyl)benzene reacts with CpCo(C2H4)2 to give 4,12,6,10-benzo-1,1,3,3,7,7,8,9,9-nonamethyl-1,3-disilacobaltacycle-7,8-disilacyclopent-6-ene (4). Complex 4 was characterized by X-ray diffraction and found to contain a 6,10-benzo-7,7,8,9,9-pentamethyl-7,9-disilacyclopent-6-ene via 1.1-double silylation.

The 13 C cross polarisation magic angle spinning NMR spectra of [Ni 6 (CO) 12 ] 2- at natural 13 C abundance have been recorded with four different cations, [NMe 4 ] + , [NEt 4 ] + , [AsPh 4 ] + and [N(PPh 3 ) 2 ] + and the crystal structure of the previously unreported [AsPh 4 ] + salt of [Ni 6 (CO) 12 ] 2- has been determined. Carbonyl resonances of the anion at natural 13 CO abundance with excellent signal-to-noise ratio (ca. 1∶1 for a single scan) are only observed for [NMe 4 ] 2 [Ni 6 (CO) 12 ] and the possible reasons for this are discussed.

This review deals with the structures, preparation and spectroscopic properties of hydrazine and substituted hydrazine metal complexes and includes examples of η1-, η2- and μ2:μ2-bonded hydrazines.

Reaction of N-acetyl-3-butanoyltetramic acid (Habta)(N-acetyl-3-butanoyl-1,5-dihydro-4-hydroxy-2H-pyrrol-2-one) with [Rh(acac)(CO)2](acac = acetylacetonate) in a 1 : 1 ratio gave [Rh(abta)(CO)2]1 which underwent displacement of CO by either P(OPh)3 or PPh3 to give [Rh(abta)(CO)L][L = P(OPh)3 or 2 PPh34] and [Rh(abta){P(OPh)3}2]3; the reaction of 4 with PPh3 gave the five-co-ordinate complex [Rh(abta)(CO)(PPh3)2]5. The solid-state structure of 3 has been determined by X-ray diffraction. It shows that rhodium adopts a slightly distorted square-planar geometry with the abta enolate ligand adopting an O,O′ mode of co-ordination via the functionalities associated with C4 and the acyl group at C3 in the pyrrolidine ring. Under 13CO, 3 is in equilibrium with 2 and 1 as shown by 13C NMR spectroscopy. No evidence has been found for the formation of five-co-ordinate complexes through the addition of P(OPh)3 to 2.

Solid-state 1H and D NMR measurements have been made for all transition-metal carbonyl clusters containing interstitial hydrides/deuterides previously characterised by neutron diffraction. There is a close agreement between the values of δ(1H/D) in solution and the solid state except for [Co6H(CO)15]–. The values of δ(1H) for interstitial hydrides are in the range δ+ 18.5 to –26.8; the shift to high field is shown to be due to an increasing displacement of H from the centre of the metal octahedral cavity, consistent with surface tensor harmonic theory. Whereas migration of H readily occurs in both [Rh13Hx(CO)24](5–x)–(x= 2 or 3) and [Ru2Rh2H2(CO)12] in solution, solid-state 1H NMR measurements on [Rh13Hx,(CO)24](5–x)–(x= 2 or 3) showed that there is no evidence for such migration in the solid state and for [Ru2Rh2H2(CO)12] oscillation of H about the metal–metal edge(s) occurs rather than migration to different edges as found in solution.

Natural abundance 15 N NMR measurements, using the INEPT pulse sequence, have been used in conjunction with 31 P NMR studies to elucidate the structures of several rhodium complexes containing monomethyl- and 1,1-dimethyl hydrazine.

The cluster [Pd6Ru6(CO)24]2– was obtained in good yield from the reaction of [Ru3H(CO)11]– with [Pd(NCPh)2Cl2]. The salt [NEt4]2[Pd6Ru6(CO)24] crystallized in the triclinic space group P(no. 2) with a= 11.514(5), b= 11.602(5), c= 11.933(5)A, α= 73.01 (3), β= 78.25(3), γ= 65.26(4)°, Z= 1. Data were collected at room temperature, giving 5968 unique reflections. The structure was solved by direct methods. The final discrepancy indices were R= 0.024 and R′= 0.026 for 4129 independent reflections with I > 3σ(I). The metal skeleton consists of a trigonally distorted octahedron of palladium atoms, capped by six ruthenium atoms with twelve terminal, six edge- and six face-bridging carbonyl ligands. This ruthenium–palladium cluster adopts the same metallic polyhedron as [Fe6Pd6H(CO)24]3– but possesses two valence electrons less. The solution 13C NMR spectra of both anions are in agreement with the solid-state structures and electrochemical analysis shows that they produce labile congeners: [Pd6Ru6(CO)24]2– undergoes a one-electron oxidation with formation of the relatively stable monoanion; [Fe6Pd6H(CO)24]3– can be oxidized and reduced, to give corresponding anions with charges of 2– to 5–.

NMR studies (13C and 103Rh) show that mixing of equimolar amounts of [RhX2(CO)2]− and [RhY2(CO)2]− (X≠Y  Cl, Br, or I) results in approximately statistical X/Y exchange to give cis-[RhXY(CO)2]−. Products resulting from the addition of X2 to [RhX2(CO)2]− (X  Cl, Br, or I) have been characterized by NMR and IR spectroscopic studies, and include the anions cis- and trans-[RhX4(CO)2]− which slowly lose CO to give the bis(halogen)-bridged dimer [Rh2X 8(CO)2]2−; the dimer progressively reacts with halide to give the mono(halogen)-bridged dimer [Rh2X9(CO)2]3−, and this is followed by the formation of[RhX5(CO)]2−. The tendency for replacement of the halide trans to CO by methanol increases in the order X  Cl

The hydride complexes trans-[PdH(X)(PPh3)2](X = Cl or Br) have been prepared in good yield by hydrogenolysis of trans-[PdX(COPh)(PPh3)2] or trans-[PdXPh(PPh3)2]. They have been fully characterised by NMR and IR spectroscopic measurements and shown to have the trans configuration. The mechanism of the hydrogenolysis reaction and the role of these hydride species in catalytic reactions are discussed.

This review deals with the structures, preparation and spectroscopic properties of hydrazine and substituted hydrazine metal complexes and includes examples of η1-, η2- and μ2:μ2-bonded hydrazines.

One of the outstanding problems and challenges in CO activation concerns the lack of a detailed mechanistic understanding of the roles of catalytic promoters used in a number of homogeneously catalysed carbonylation reactions. These problems, and attempts at their resolution, are highlighted with reference to (i) the varied range of promoters that have found use in composite catalysts for the direct synthesis of oxygenates such as ethylene glycol and ethanol from CO/H2, (ii) the promotional effects of N-bases in the catalytic methoxycarbonylation of alkenes to esters, and (iii) some preliminary 13C NMR spectroscopic evidence which is enabling the definition of a dual role of Ru-promoters as iodide abstraction agents in the Ir-catalysed carbonylation of methanol to ethanoic (acetic) acid. The detection, and characterisation in solution, of an iodide-bridged Ru–Ir dimer is facilitating the development of a plausible, internally consistent model on which to base the catalysis.

The properties of a variety of HNCC's supported on holey carbon films have been investigated by transmission and analytical electron microscopy; a review of how these techniques can be used to study these crystallographically well-characterized clusters is also presented. Images of the metallic skeleton corresponding to a single cluster unit have been observed for [Ni38Pt6(CO)48H]5−, [Ni34C4(CO)38H]5−, [Pt38(CO)44Hx]2−, [Rh17S2(CO)32]3−, [Ni12Sn(CO)22]2−, and [Ni22Ge(CO)22]2−, and analytical data at low electron beam intensities are consistent with their crystallographically established structures. At increasing current densities, agglomeration into larger particles occurs and these larger particles (2–5 nm) gradually lose the chemical identity of the precursor. Ultimately at current densities of the order of 107 A/m2 metal segregation and/or metal alloy particles having close-packed crystal structures are formed together with graphitic carbon from decomposition of the CO ligands. Possible mechanisms for this beam damage are discussed. There is evidence of cluster/substrate interaction resulting in increased stability compared to that observed in solution and in the normal crystalline state.

59 Co 2D nutation NMR experiments were conducted at 47.485 MHz in order to determine the quadrupole coupling constants ( e 2 Qq/h ) of 59 Co nuclei in solid Co 4 (CO) 12 . The 2D spectrum observed clearly indicates the presence of two inequivalent 59 Co nuclei, Co(apical) and Co(basal). The magnitude of e 2 Qq/h and the asymmetry parameter of the electric field gradient, η, were determined to be 9 ± 2 MHz and 0 for Co(basal), and 15 ± 3 MHz and 0.8 for Co(apical). The large value of η for Co(apical) suggests that the apical Co occupies the site on a pseudo-C 3 axis in the crystalline lattice. These facts are consistent with our previous proposal that a symmetry conserved reorientation of the Co 4 -tetrahedron occurs in the solid state.

By the use of the INEPT pulse sequence it has been possible to obtain 15N and 103Rh NMR data on [RhH2(PPh3)2(bipy)]Cl (bipy = 2,2′-bipyridyl) containing natural abundance 15N and prove the stereochemistry; 1H and 31P NMR data are also reported for [RhH2(PPh3)2(bipy)]Cl which can be easily prepared in the pure state by the reaction of [RhCl(PPh3)3] or [Rh2Cl2(PPh3)4] with bipy ([Rh]: [bipy] = 1:1) under an H2 atmosphere.

Analytical studies employing energy dispersive analysis of X-rays (EDX), electron energy loss spectroscopy (EELS) and electron diffraction pattern techniques have been carried out on a series of transition metal carbonyl clusters supported on thin (ca. 20 nm) amorphous carbon films. High resolution micrographs show that the clusters are dispersed evenly upon the support in small groups or indeed singly. Exposure of the specimens to the electron beam resulted in various phenomena; agglomeration of the clusters, loss of materials in the form of gases (usually CO) and metallic ions (in some cases), and final degradation into dense particles of a metallic nature. Differences in respective stabilities of the clusters was found to depend upon a number of variables including type of metal, the presence of interstitial atoms (and their type when present), type of cation and electronic charge of the cluster anion. During this work it was noticed that the clusters, when supported on the amorphous carbon, were much more stable in air than when in solution or solid states in the laboratory which led to the suspicion that there was a stabilizing interaction between the support and the cluster. Evidence has been collected to corroborate this theory.

The fragmentation and recombination reactions of the clusters [Rh4(CO)12–xLx][L = P(OPh)3, x= 1–4] under applied pressure of CO or CO–H2(1:1) gases have been studied by multinuclear NMR spectroscopy using a high-pressure, high-resolution cell and by high-pressure, IR spectroscopy. Fragmentation to mixtures of dinuclear species [Rh2(CO)8 –xLx](x= 0–2) is observed. The proportions of the variously substituted dimers in the fragmented mixture depends both on the cluster originally present in the solution and on the rhodium concentration. The dimers undergo a fluxional process analogous to that found in [Co2(CO)8]. Recombination on depressurisation can occur either to predominantly the original Rh4 derivative or to mixtures in which several Rh4 species are present in comparable amounts depending on the rhodium to phosphorus ratio. No evidence has been obtained for the formation of hydride species [RhH(CO)4–xLx](x= 0 or 1).

The reaction of [Rh7(CO)16]3- with SnCl(2).2H2O in a 1 : 1 molar ratio under N2 results in the formation of the new heterometallic cluster, [Rh12Sn(CO)27]4-, in very high yield (ca. 86%). Further controlled additions of SnCl(2).2H2O, or solutions of HCl, or [RhCl(COD)]2, give [Rh12(mu-Cl)2Sn(CO)23]4-. Similarly, addition of HBr to [Rh12Sn(CO)27]4- gives the related cluster [Rh12(mu-Br)2Sn(CO)23]4-. Notably, if the addition of SnCl(2).2H2O to [Rh12Sn(CO)27]4- is carried out under a CO atmosphere, the reaction takes a different course and leads to the formation of the new cluster, [Rh12Sn(mu3-RhCl)(CO)27]4-. All the above clusters have been shown by single-crystal X-ray diffraction studies to have a metal framework based on an icosahedron, which is centred by the unique Sn atom. Their chemical reactivity and 13C-{103Rh} HMQC NMR spectroscopic characterization are also reported.

All the intermediates in the "carboalkoxy" pathway, and their interconversions giving complete catalytic cycles, for palladium-diphosphine-catalyzed hydroalkoxycarbonylation of alkenes, and for alkene-CO copolymerization, have been demonstrated using (31)P{(1)H} and (13)C{(1)H} NMR spectroscopy. The propagation and termination steps of the "hydride" cycles and the crossover between the hydride and carboalkoxy cycles have also been demonstrated, providing the first examples of both cycles, and of chain crossover, being delineated for the same catalyst. Comparison of the propagation and termination steps in the pathways affords new insight into the selectivity-determining steps. Thus, reaction of [Pd(dibpp)(CH(3)CN)(2)](OTf)(2) (dibpp = 1,3-(iBu(2)P)(2)C(3)H(6)) with Et(3)N and CH(3)OH affords [Pd(dibpp)(OCH(3))(CH(3)CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh(3), indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf at 243 K to give [Pd(dibpp){CH(2)CH(2)C(O)OCH(3)}]OTf, that is, there is no intrinsic barrier to alkene insertion into the Pd--C(O)OMe bond, as had been proposed. Instead, termination is proposed to be selectivity determining. Methanolysis of the acyl intermediate [Pd(dibpp){C(O)CH(3)}L]X (L = CO, CH(3)OH; X = CF(3)SO(3) (-) (OTf(-)), CH(3)C(6)H(4)SO(3) (-) (OTs(-))) is required in the hydride cycle to give an ester and occurs at 243 K on the timescale of minutes, whereas methanolysis of the beta chelate, required to give an ester from the carbomethoxy cycle, is slow on a timescale of days, at 298 K. These results suggest that slow methanolysis of the beta chelate, rather than slow insertion of an alkene into the Pd--carboalkoxy bond, as had previously been proposed, is responsible for the dominance of the hydride mechanism in hydroalkoxycarbonylation.

Two novel heterometallic octahedral clusters [Rh(4)Pt(2)(CO)(11)(dppm)(2)](1) and [Ru(2)Rh(2)Pt(2)(CO)(12)(dppm)(2)](2) were synthesized by the reaction of [Rh(2)Pt(2)(CO)(6)(dppm)(2)] with [Rh(6)(CO)(14)(NCMe)(2)] and Ru(3)(CO)(12), respectively. Solid state structures of 1 and 2 have been established by a single crystal X-ray diffraction study. Two dppm ligands in 1 are bonded to one platinum and three rhodium atoms, which form an equatorial plane of the Rh(4)Pt(2) octahedron. Two rhodium and two platinum atoms bound to the diphosphine ligands in 2 are nonplanar to give an octahedral C2 symmetric Ru(2)Rh(2)Pt(2)(dppm)2 framework. The (31)P NMR investigation of and (1D, (31)P COSY, (31)P-[(103)Rh] HMQC) and simulation of 1D spectral patterns showed that in both clusters the structures of the M(6)(PP)(2) fragments found in the solid state are maintained in solution.

Multiple-metal spin transitions which distort the HMQC spectra of rhodium carbonyl clusters are discussed. These effects are seen whenever the detector nucleus, e.g. 13C or 31P, couples to more than one metal spin and are not restricted to detector ligands occupying edge- or face-bridging sites. These effects are illustrated in, but not limited to, the 13C-{103Rh} and 31P-{103Rh} HMQC spectra of [Rh6(CO)15L], (where L = P(4-F-C6H4)3), [Rh4(CO)11{P(OPh)3}], [Rh6C(CO)15]2 − and [Rh2(carboxylate)2PPh3]. The effect is to modulate the intensity and position of the correlations in the metal dimension; cross peaks are displaced from the true chemical shift, additional cross peaks are seen and the intensity of the coherences varies as a function of the preparation delay, d2, and coupling constant, and may go to zero at the conventional value of 1/(2J). Analyses of the relevant spin systems are given together with experimental strategies to overcome these effects. Copyright © 2004 John Wiley & Sons, Ltd.

Drent and Budzelaar have proposed two catalytic cycles for cationic Pd–diphosphane complex catalyzed CO–alkene coupling reactions such as CO–ethene copolymerization and methoxycarbonylation of ethene. The hydride cycle is initiated by alkene insertion into a Pd–hydride bond whilst the carbomethoxy cycle is initiated by CO insertion into a Pd–methoxy bond. The two cycles may operate in parallel, for example, CO–alkene copolymerization, or one cycle may dominate, for example, the Lucite process for the methoxycarbonylation of ethene to methylpropanoate. Where the two cycles operate in tandem there exists the possibility of crossover between the cycles which has been inferred from chain end-group analysis. 2] Product selectivity is determined in both the initiation and termination step(s). The chainpropagation steps, insertion of CO into a metal–alkyl bond, and insertion of alkene into a metal–acyl, bond have been extensively investigated, whilst van Leeuwen has recently discussed termination by alcoholysis of the Pd–alkyl and Pd–acyl intermediates. We have recently (1) fully characterized the intermediates in and (2) delineated the initiation process of the hydride cycle. The hydride cycle is thus well established, however, a complete cycle for the Pd–carbomethoxy mechanism has not yet been demonstrated because of the absence of suitable Pd–carbomethoxy precursors. We report herein a simple, general procedure for the preparation of the key cationic monocarbomethoxy Pd–diphosphane compounds and present an NMR study that characterizes, for the first time, all the intermediates in the carbomethoxy cycle. We also report (1) the delineation of the propagation steps of the hydride cycle for this catalyst, and (2) for the first time, characterization of all the complexes involved in termination of the hydride cycle with concomitant crossover and initiation of the carbomethoxy cycle. It should be noted that ligands of the type used in this work give highly active catalysts in this field of chemistry.

All the intermediates involved in the platinum catalysed methoxycarbonylation of ethene have been characterised by in situ NMR; the low activity of platinum catalysts in this reaction is shown to be due to trapping of the active intermediates by carbon monoxide at every step in the catalytic cycle and to the ready reversibility of the product forming reactions.