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The mononuclear complexes [M(CO) 5 (CH 3 E) n CH 4− n ] ( n = 2, M = Cr and W, E = S and Se; n = 3, M = Cr and W, E = S; n = 4, M = Cr, E = S) have been synthesized and characterized. In addition to the facile atomic inversions of the metal co-ordinated sulphur and selenium atoms, there are intramolecular 1,3-metallotropic shifts among all the available co-ordination sites on each ligand. From dynamic NMR studies activation energies have been determined for these metal commutations.

The complexes cis-[M(CO)4{(C5H4ECH3)2Fe}] (M = Cr, Mo or W; E = S or Se) have been synthesised. Pyramidal inversion of the coordinated S or Se atoms was arrested in most cases at low temperatures (∼ −80°C), when the DL forms of the complexes predominated (⩾ 78%). At higher temperatures combined 1D and 2D-EXSY NMR studies led to energies (ΔG‡(298 K) values) for chalcogen inversion in the range 31–50 kJ mol−1. Fairly large ΔS‡ values for these processes were attributed to the rapid reversal of the EM(CO)4E portion of the ferrocenophane ring. The crystal structure of 1,1′-bis(methylseleno)ferrocene tetracarbonyltungsten has been determined. The crystals have space group P21/a with a 17.580(3), b 9.665(1), c 11.059(1) A, β 107.33(1)° and Z = 4. Least-squares refinement gave R = 0.031 for 2369 independent significant reflections. The WSe bond lengths are 2.674(4) and 2.692(4) A, and the SeMe groups adopt a DL relationship. The SeWSe bond angle is 86.3° and the non-bonded SeSe separation 3.67 A. The cyclopentadienyl rings adopt an eclipsed conformation with the ring planes parallel.

Variable temperature NMR studies have shown that, at temperatures below —100° C, the bridge reversal process in [3]ferrocenophane is slow on the NMR time scale and studies have yielded an energy barrier of 40.4 kJ mol-1 for this process, a value lower than that for ring reversal in cyclohexane. Other [3] ferrocenophanes with six-membered ring analogues have also been studied and the data obtained have allowed interesting comparisons between the bridge and ring reversal processes to be made. A number of torsional barriers about carbon—chalcogen and chalcogen—chalcogen bonds have also been calculated.

The ligand tris(methylthio)methane, HC(SMe)3, has been used to form dinuclear platinum(IV) complexes of type [(PtXMe3)2{HC(SMe)3}](X = Cl or Br), where the ligand bridges the pair of Pt atoms via two of its SMe groups. Variable-temperature 1H n.m.r. studies in the range –90 to 70 °C revealed inversion of the pyramidal S atoms, a 1,3-metal pivoting fluxion, and a scrambling of the Pt-methyl environments by a PtMe3 rotation mechanism. The latter two fluxions are concerted and involve a common transition state. Activation energies for the various dynamic processes are in the range 45–72 KJ mol–1.

Platinum(IV) complexes of general type [PtXMe3(RSCH2CH2SR′)](X = Cl, Br, or I; R = Me, R′= Et or But; R = R′= But) have been synthesised. Solid-state structures for two of the complexes (X = Cl, R = Me, R′= Et; X = I, R = Me, R′= But) have been established by X-ray crystallography. All the complexes have been extensively studied in solution by variable-temperature one- and two-dimensional n.m.r. spectroscopy. The thioalkyl dependence of the energy barriers for pyramidal sulphur inversion was found to be SMe > SEt SBut. At above-ambient temperatures ligand 180° rotations and PtMe3 120° rotations occur, these two fluxions showing high degrees of correlation.

Syntheses of the trimethylplatinum(IV) tetramers [(PtMe 3 SMe) 2 (PtMe 3 X) 2 ] (X = Cl, Br or I) and [(PtMe 3 SMe) 3 (PtMe 3 Cl)] are reported. Both classes of tetramer, by virtue of the two types of bridging ligand, contain two distinct platinum environments. This leads to four different Pt-methyl environments for the structures [(PtMe 3 SMe) 2 (PtMe 3 X) 2 ] and three such environments for the tetramer [(PtMe 3 SMe) 3 (PtMe 3 Cl)]. Variable temperature 1 H NMR studies show that methyl scrambling occurs in each PtMe 3 moiety with activation energies (Δ G ‡ (298 K)), in the range 61–72 kJ mol −1 . PtMe 3 groups bonded to two X ligands and one SMe ligand undergo slightly less facile fluxion than those groups attached to one X and two SMe groups. The implications of this fluxionality for symmetrical tetramers (e.g. [(PtMe 3 ) 4 ] are discussed.

Accurate NMR band shape fittings applied to the temperature-variable spectra of (η5-C5H5)2TiS5 have yielded the following activation parameters for the 6-membered TiS5 ring inversion process: Ea = 69.1 ± 2.3 kJ mol−1, ΔH‡ = 66.2 ± 2.3 kJ mol−1, ΔG‡ = 76.3 ± 4.2 kJ mol−1 and ΔS‡ = −33.9 ± 6.4 J K−1 mol−1.

The complexes trans-[MCl2( SC 2 H 4 SC HR)2] (M = Pd(II) and Pt(II), R = Me, tBu and Ph) have been synthesized. In CDCl3, solution these complexes exist as two out of six possible NMR-distinct isomeric forms. These isomers are not interconverted by pyramidal S inversions since the resulting invertomers, having R groups in sterically crowded environments, are energetically unfavoured. The palladium(II) complexes show evidence of 1,3-metal shifts coupled with ligand dissociation/recombination, whereas the platinum(II) complexes are stereochemically rigid at all temperatures up to 60°C.

The first metal carbonyl derivatives of a coordinated episulphide ring are reported. Two classes of complex have been obtained wherein the bisepisulphide behaves either as a bridging ligand between two metal pentacarbonyl moieties as in for example , or as a chelating ligand in a mononuclear metal pentacarbonyl complex such as , for which the X-ray crystal structure is reported.

The fluxional molecules bis(trimethylsilyl)(trimethylstannyl)indene, (trimethylyl)(trimethylstannyl)indene and bis(trimethylstannyl)indene have been studied by 1H n.m.r. spectroscopy over the temperature range 0–170°C. The Arrhenius parameters for the metallotropic shift of the trimethylstannyl groups have been determined. Although, in the case of bis(trimethylstannyl)indene, the abundances of the two fluxional isomers were grossly dissimilar (ratio > 20/1), the very sensitive temperature dependence of the spectra enabled accurate rate parameters to be deduced. The spectral line shape changes in all cases were found to be completely compatible with 1,3-metallotropic shifts of the trimethylstannyl group.

Trimethylplatinum(IV) halides [(PtXMe3)4] react with equimolar amounts of the ligand (MeS)2CHCH(SMe)2(L) to give chelate complexes of type [PtXMe3L] as major products and dinuclear complexes [(PtXMe3)2L] as minor components. The chelate complexes in CDCl3 solution exist as cis and trans configurational isomers, with the trans predominating. At temperatures below ca.–30 °C, pyramidal inversion of the co-ordinated S atoms is slow on the 1H and 195Pt time-scales and six invertomer species (viz. trans/DL-1,2,3, and 4 and cis-1 /meso-1 and DL) have been detected, identified, and characterised by 1H and 195Pt n.m.r. spectroscopy. The pyramidal inversion rates and energies of the co-ordinated S atom pairs in all six invertomers have been quantitatively measured by a combination of 1H bandshape analysis and 195Pt two-dimensional EXSY experiments. The resulting ΔG‡(298.15 K) values lie in the range 54–65 kJmol–1 and are governed primarily by the orientation of the contiguous unco-ordinated S-methyl group. The X-ray crystal structure of [PtlMe3{(MeS)2CHCH(SMe)2}] depicts the ligand with its co-ordinated S-methyls trans to each other and trans to their contiguous unco-ordinated S-methyls. This structure corresponds to the most populous CDCl3solution invertomer (trans/DL-2).

Dynamic n.m.r studies have yielded accurate energy data for S and Se pyramidal inversion in complexes of type [PtXMe3(L–L)][X = Cl, Br, or I; L–L = MeE(CH2)nEMe, E = S or Se, n= 2 or 3]. The mechanism of the inversion process and the effects of atomic mass, ring size, and nature of halogen on the barrier energies are discussed.

Dynamic NMR studies have yielded accurate energy data for the bridge reversal fluxion of [3]ferrocenophanes with Group VI bridging atoms. This process, whilst appearing very analogous to the chair-to-chair reversal of corresponding 6-membered heterocyclic rings, appears to be a much higher energy process, its associated Δ G ≠ values being in the range 59 to 81 kJ mol −1 depending on the types of Group VI bridging atoms. These data allow estimates to be made for the first time of the relative magnitudes of torsional barriers about single bonds involving like and unlike Group VI atoms. For example, the SS torsion is shown to be 3.9 kJ mol −1 higher in energy than the SSe torsion and 5.8 kJ mol −1 higher than the SeSe torsion. The probable mechanism of the bridge reversal process is discussed.

Inversion barriers at sulphur and selenium atoms in organic sulphide and selenide complexes of PdII and PtII. [MY2{XR(CH2SiMe3)}2](M = PdII or PtII, Y = Cl or Br, X = S or Se, R = Me or Ph), have been measured by accurate n.m.r. band-shape methods. The energies vary within the range 51–84 kJ mol–1 and depend to various extents on the nature of the pyramidal atom, the attached transition metal, and the organic ligand. Selenium barrier energies are 15–20 kJ mol–1 higher than corresponding sulphur barrier energies. Detailed line-shape studies indicate that the inversion mechanism is intramolecular and occurs predominantly at single S or Se atoms. Any synchronous double inversion is of low probability and does not contribute significantly to the observed changes in n.m.r. line shape.

The complexes cis -[M(CO) 4 (RSCH 2 CH 2 SR)] (M = Cr, Mo, W, R = t Bu; M = W, R = Me, Et, i Pr, t Bu) and cis -[M(CO) 4 ( cis -RSCHCHSR)] (M = Cr, Mo, W; R = Me, t Bu) have been synthesised, and sulphur inversion studied using variable temperature 1 H and 13 C { 1 H} NMR techniques. For complexes of the saturated ligands, signals due to both DL and meso invertomers were observed at −90°C, with the relative populations of these invertomers being dependent on the steric bulk of the alkyl groups attached to sulphur. Sulphur inversion barriers, which have been calculated via total bandshape analysis of the variable temperature NMR spectra of these complexes, show a marked dependence on the steric requirements of the dithioether ligand. For the complexes of the unsaturated ligands, only the meso -invertomers were observed at limiting low temperatures (ca. −90°C), and sulphur inversion barriers were therefore not able to be calculated in these cases.

The mono- and bis-(metal pentacarbonyl) complexes [{;M(CO)5};m{;MeS(CH2S)nMe};] (M = Cr, W;m = 1,n = 2;m = 2,n = 2,3) have been synthesised and studied by dynamic NMR methods. Total band shape fitting of temperature variable1H NMR spectra of the complexes has enabled the determination of very accurate data for pyramidal ligand atom sulphur inversions. At higher temperatures line shape changes that would accord with 1,3 intramolecular shifts of the M(CO)5 moieties between ligand sulphur atoms were observed. Concurrent dissociation, however, prevented access to the high quality spectra required for accurate determination of kinetic data of such shifts.

The n.m.r. spectra of 2,3,5,6-tetrafluoropyridine, perfluoro(4-methylpyridine) and 3,5-dichloro-2,4,6-trifluoropyridine oriented in nematic phases have been accurately analysed and structural data obtained. In the case of perfluoro(4-methylpyridine) the rotation of the CF3 group, whilst being assumed to be faster than the reorientational motion of the molecules in the nematic phase, is internally restricted with the staggered rotamers being energetically preferred.The average orientations of these solute molecules are compared with those of other substituted pyridines.

Mononuclear complexes of the type [M(CO)5(Me2[graphic omitted]H2)], where M = Cr, Mo, or W and E = S or Se, have been prepared. Detailed variable-temperature n.m.r. studies have established the existence of pyramidal inversion about the co-ordinated sulphur or selenium ligand atom and an intramolecular 1,2-metal shift between adjacent chalcogen ligand atoms. The energy barriers associated with both rate processes have been evaluated by dynamic n.m.r. methods. The metal pentacarbonyl complexes of naphtho[l,8-cd]-dithiole and -thiaselenole, [M (CO)5(C10H6EE′)](M = W, E = E′= S; M = Cr, E = S, E′= Se) have also been prepared but no fluxionality was evident.

The complexes cis -[Mo(CO) 4 (RSCH 2 CH 2 SR)] (R = Me, Et, i Pr, t Bu); cis -[Mo(CO) 4 ( cis - t BuSCHCHSBu t )] and cis -[Mo(CO) 4 (MeSCH 2 CH 2 SBu t )] have been prepared, and the pyramidal sulphur inversion investigated by variable temperature 1 H and 13 C{ 1 H} NMR spectroscopy, and variable temperature 1 H 2D NMR exchange spectroscopy. Sulphur inversion barriers calculated for these complexes show a marked dependence on the steric requirements of the dithioether ligand. Molybdenum-95 NMR chemical shifts have been rationalised in terms of the relative strengths of molybdenumsulphur bonding, and qualitatively correlated with the sulphur inversion barriers.

The monomer-dimer equilibria of 2,6- and 3,5-dimethylnitrosobenzenes in CDCI3 solution were investigated by 1H and/or 13C NMR spectroscopy. The mixed systems nitrosobenzene + 2,6-dimethylnitrosobenzene and 2,6-dimethylnitrosobenzene + 3,5-dimethylnitrosobenzene were also studied, and mixed azodioxy dimers were identified. In all systems exchange occurs exclusively between dimer and monomer species, rates and activation energies being calculated from time-dependent 1H 1D spectra and/or 1H 2D-EXSY spectra measured at different temperatures.

The rates and associated activation energies for metal migration in [M(CO)3(η6-C8H8)] (M = Cr or W) have been precisely determined by variable temperature 1H two dimensional NMR exchange spectroscopy. For M = W the 1,2 and 1,3 shifts are almost equally favoured, whereas for M = Cr the 1,3-migration pathway is slightly preferred. Planar monocyclooctatriene(η4-C8H8) and/or bicyclo[4.20]octatriene are considered to be probable intermediates for this fluxionality.

Nematic phase n.m.r. spectra of pyridine-4-aldehyde have been interpreted and the relative geometry of the ring hydrogens derived. The spectra show unambiguously that the rotation of the aldehyde substituent is slower than, or of the same order as, the reorientational motion of the solute molecules in the liquid crystal phase.

A general method for evaluating rate constants in complex exchange networks with N-sites from two-dimensional EXSY (NOESY) NMR spectra is proposed. A computer program D2DNMR capable of performing signal intensity to exchange rate calculations (and vice versa), based on a matrix formalism, is outlined. The method is illustrated by 195 Pt 2D NMR studies of the A ⇌ B ⇌ C spin system arising from pyramidal sulfur inversion in platinum(IV) complexes of type [Pt X Me 3 (MeSCH 2 CH 2 SMe)] ( X = Cl , I ). Comparison with 1 H NMR bandshape analyses of the same compounds shows high agreement between the rate constants and activation parameters determined by both techniques. Mechanisms of 195 Pt spin-lattice relaxation are briefly discussed.

Complexes of general type [ PtXMe3(MeECHCHEMe)](E = S or Se; X = Cl, Br, or I) have been isolated and their solution and solid-state structures established. Static structures in solution were characterised by 1H, 13C, 77Se, and 195Pt n.m.r. parameters; the dynamic stereochemistry of these complexes, arising from low-temperature chalcogen inversion and high-temperature fluxional rearrangements, was studied quantitatively by variable-temperature 1H n.m.r. spectroscopy. Correlations were found between l95Pt and 77Se chemical shifts, and between 195Pt shifts and 1J(PtSe) values in these complexes. 1J(CPt) values can be interpreted in terms of trans influence effects. The solution invertomer populations are very halogen- and temperature-dependent. In the case of selenium complexes, the predominant invertomer in solution corresponds to the single solid-state configuration as determined by X-ray crystallography. For the chloro-complex this is the meso-1 form and for the iodo-complex the meso-2 species. The effects of π-conjugation on the energies of the chalcogen inversion and platinum-methyl scrambling processes are discussed by comparing ΔG‡(298.15 K) values with those for analogous complexes with aliphatic and aromatic chalcogen ligands.

The application of accurate NMR band shape studies has enabled the barrier energy for the bridge reversal process to be determined for the first time in a [3]-ferrocenophane [(η-C 5 H 4 ) 2 FeS 3 ].

The title complexes have been synthesised for the first time. For the complex with R = H and E = S, X-ray and n.m.r. data indicate that the six-membered trithian ring adopts a boat conformation. Novel fluxional rearrangements in this complex have been studied by dynamic n.m.r. methods. One of these processes involves a series of 60° pivots of the cyclic ligand about individual S → Pt bonds which average the ligand-methylene and equatorial Pt–Me environments. The energy (ΔG‡) of this process has been computed to be 58.6 kJ mol–1 and is halogen independent. A higher-energy process associated with ligand dissociation–recombination involves a scrambling of the Pt–Me groups. For the complex with R = H and E = CH2, where the sulphur-pivot process cannot take place, only Pt–Me scrambling occurs. For the complex with R = Me and E = S, where the ligand ring adopts the usual chair conformation, the ligand-methyl groups prevent the pivot process from occurring at a measurable rate.

Pyramidal inversion of the co-ordinated sulphur atoms in the cis and trans configurational isomers of [Cr(CO)4{(MeS)2CHCH(SMe)2}] has been examined by n.m.r. spectroscopy. The two dimensional NOESY method was used to identify individual invertomers of each isomer at low temperature. One-dimensional band-shape analyses performed at higher temperatures yielded ΔG‡ values in the ranges 45–50 and 50–54 kJ mol–1 for the trans and cis isomers respectively. No evidence for cis–trans interconversion was obtained, probably because of the limited thermal stability of the complex.

Variable temperature 1H NMR spectroscopy has been used in the study of 1,3-intramolecular shifts of the M(CO)5 moiety in complexes of the general formula [M(CO)5L], (M = Cr or w), L = SCH 2 SCH 2 SC H2, SCH 2 SCH 2 CH 2 C H2 and SCH(Me)SCH 2 CH 2 C H2. For the 1,3,5-trithian complexes precise energy barriers for the process have been obtained by detailed computer simulation of the static and dynamic spectra. Our results suggest that the magnitude of ΔG≠ (298.15 K) for the 1,3-shift is largely dependent upon the skeletal flexibility of the ligand system. In this context we have investigated the X-ray crystal structure of the highly substituted trithian complex [W(CO)5{β- SCH(Me)SCH(Me)SC H(Me)}].

The series of complexes [PdX2{[graphic omitted]H2)n}2s](n= 4–6; X = Cl, Br, or I) and [PdX2{S[graphic omitted]H2}2](X = Cl, Br, or I) have been synthesised and their stereodynamics investigated by variable- temperature 1H n.m.r. spectroscopy. Whereas conformational changes of the various sized ligand rings were always fast on the n.m.r. chemical shift time- scale, the rates of inversion of the co- ordinated selenium atoms were much slower and directly measurable by bandshape fitting methods. ΔG‡ values for selenium inversion were in the range 66–78 kJ mol–1. Values were observed to increase consistently with decreasing ligand ring size and are explained in terms of the extents of angle strain required to achieve the transition- state structure of the inversion process. The inversion energies of all four series of complexes reflect a cis halogen influence of 5.1–7.3 kJ mol–1.

Variable-temperature 1H n.m.r. spectra of the mononuclear complexes [M(CO)5(Me3SiCH2EECH2SiMe3)](M = Cr, Mo, or W;E = S or Se) have established the occurrence ot two distinct internal dynamic phenomena. The co-ordinated S or Se atom undergoes facile pyramidal inversion whilst at above-ambient temperature a novel 1,2-metal shift between the adjacent pair of chalcogen atoms of the ligands occurs. Energy data for both processes have been computed by total n.m.r. band-shape analyses.

The mononuclear six-membered ring complexes [(PtXme3)(MeSCH2SCH2SME)] X  Cl, Br and I) have been synthesised and studied by dynamic NMR methods. The separate energy barriers associated with the pyramidal inversion of the sulphur atoms, platinum-methyl scrambling and chelate ligand rotation were measured by complete band-shape analyses.

We report the preparation and characteristisation of the 1:2 complexes [SnX 4 (R 2 E) 2 ] (R = C 6 H 5 CH 2 , Me 3 SiCH 2 ; E = S, Se; X = Cl, Br) and the 1:1 complexes [SnCl 4 (bmt)] (bmt = 3,4-bis(methylthio)-toluene and [SnCl 3 (C 6 H 5 )(dth)] (dth = 2,5-dithiahexane). Variable temperature 1 H NMR studies have confirmed the previously observed cis/trans isomerism about tin in the [SnX 4 L 2 ] complexes, and we have additionally discovered at low temperature the cessation of the pyramidal atomic inversion about sulphur and selenium. This latter phenomenon is characterised by the observation of prochiral centres in the [SnX 4 L 2 ] complexes and mixtures of dl and meso isomers in the chelate complexes.

Mixed chalcogen ligand complexes of trimethylplatinum(IV) halides of general type [PtXMe3(MeSRSeMe)][R =(CH2)2 or o-C6H4] have been prepared and the energy barriers associated with the pyramidal inversions of the individual chalcogen atoms measured by total band-shape dynamic n.m.r. methods. At elevated temperatures an intramolecular scrambling of all of the Pt-methyls occurs, associated with a fluxional movement which apparently effects a 180° rotation of the bidentate ligand.

The complexes fac -[ReX(CO) 3 (RSCH 2 CH 2 SBu t )] (X = Cl, Br, I; R = t Bu, Me) have been prepared by the reaction of [ReX(CO) 5 ] with the appropriate dithioether in tetrahydrofuran. Sulphur inversion in these complexes has been studied by variable temperature 1 H and 13 C 1 H NMR spectroscopy. These methods showed that, for R = t Bu, when sulphur inversion was slow on the NMR timescale only the meso -2 invertomer was present as a significant population in solution. Where R = Me, evidence for the presence of two invertomers, namely dl -2 and dl -3, was obtained from the low temperature NMR spectra. Some halogen and invertomer dependence of the NMR chemical shifts, most notably in the 13 C 1 H NMR parameters, was observed.

The barriers to conformational interconversion of the five-membered rings of the title compounds of general type SnRR′(SCH2CHR″S)(R, R′= Me or Ph; R″= H or Me) have been measured for the first time by low-temperature n.m.r. studies. Room-temperature measurements indicate that the chelate rings exist as half-chair conformations with the –CH2CHR″– moiety in a fully staggered configuration, as in the solid state. These findings are at variance with a previous n.m.r. study of this type of system. The ΔG‡ values for the interconversion of the two half-chair conformations lie in the range 30–32 kJ mol–1 and are essentially independent of the substituents on the SnIV atom and on the five-membered ring.

Total NMR band shape fitting methods have provided accurate energy data for inversion barriers at sulphur and selenium in complexes of types cis-[MX2L] (M = PdII, PtII; X = Cl, Br, I; L = MeS(CH2)2SMe, MeS(CH2)3SMe, o-(SMe)2C6H3Me, cis-MeSCH=CHSMe) and [PtXMe{MeE(CH2)2E′Me}] (E= E′= S or Se and E = S, E′= Se; X = Cl, Br, I). Barrier energies were found to decrease by 10–12 kJ mol−1 in going from aliphatic through aromatic to olefinic ligand back-bone. This can be explained in terms of (3p - 2p) π conjugation between the inverting centre and the ligand back-bone. The effects of ligand ring size, nature of halogen atom and the metal oxidation state on the barrier energies are discussed.

Stereochemical non-rigidity in the complexes [M(CO)5( SCH 2 SCH 2 SCH 2 SC H2)] (M = Cr or W) has been studied by dynamic nuclear magnetic resonance spectroscopy. In the temperature range ca. −100 to 120°C two intramolecular processes were observed, namely pyramidal atomic inversion about sulphur atoms, and a commutation of the M(CO)5 moiety between coordination sites on different sulphur atoms. Accurate energy barriers for both processes have been obtained by detailed computer simulations of the static and the dynamic spectra. The magnitudes of ΔG≠ (298.15 K) are compared with those reported for related complexes. The exact nature of the M(CO)5 shift cannot be unambiguously assigned from the observed spectral line-shape changes. Two possible mechanisms are proposed and discussed.

The monomer-dimer equilibrium of nitrosobenzene in CDCI3 solution was studied in the temperature range -50 to 30 °C using 13C, 15N and 17O NMR spectroscopy. Decreasing temperature favours the dimeric azodioxy species, and below ca 10 °C both cis and trans dimers were detected in addition to the monomer. The relative abundance of the trans dimer is low (⩽6%) at all temperatures and this is the first report of its existence in solution. The monomer and dimer species were characterized by their 13C and 15N shifts, and exchange between them was measured quantitatively by 13C 2D-EXSY spectra. Exchange occurred exclusively between the cis-dimer and monomer and between the trans-dimer and monomer, activation energies, ΔG≠(298.15 K) being 65.8±0.1 and 70.0±0.3 kJ mol−1, respectively.

Presentation des etudes spectrometriques RMN concernant des complexes du platine (II) et du platine (IV) avec des coordinats mono, bi- ou tridentes: mecanismes d'interconversion stereochimique et parametres d'activation

The complexes [PtXMe3{(MeS)2CHCH(SMe)2}](X = Cl, Br, or I) in C6D5NO2 solution exist as trans and cis-1 isomers which, in the temperature range 333–423 K, exhibit novel fluxionality. This involves a trans⇌cis-1 interconversion which proceeds via a mechanism which exchanges all co-ordinated and unco-ordinated S–methyl environments and all Pt–methyl environments. The dynamics of this process were measured by 1H n.m.r. band-shape analysis and two-dimensional exchange spectroscopy. Both methods yielded energy data in good agreement, ΔG‡(363 K) values being in the range 88.7–91.2 kJ mol–1 for the three halogen complexes. Since ΔG‡ values based on S–methyl and Pt–methyl exchanges were identical within experimental error a single concerted mechanism is operating. This is, in essence, a 1,3-metal-pivot process resulting from a rearrangement of individual Pt–S bonds and involving 109° clockwise or anticlockwise twists of the pendant –CH(SMe)2 group about its adjacent C–C bond, bringing the other gem S-methyl into co-ordination with the metal. The fluxion proceeds via a highly non-rigid seven-co-ordinate platinum(IV) intermediate which effects exchange of all Pt–methyl environments. This is the first example of a metal-pivot process in mononuclear platinum(IV) complexes.

The stereodynamics of ferrocenylsulphide-palladium(II) and -platinum(II) complexes, Fe(C5H4SR)2MX2, (M = PdII, PtII; X = Cl, Br; R = Ph, i-Pr and i-Bu), have been examined by variable temperature NMR. At temperatures down to ca. −100° C, the pyramidal inversion of the S atoms could be slowed down sufficiently to yield accurate energy data, while the reversal of the ferrocenophane ring remained fast on the NMR time scale. ΔG≠ data for the S inversion process were in the range 47 to 65 kJ mol−1 and were influenced to varying extents by the nature of the transition metal, the halogen, and the R substituent on the sulphur.

The dinuclear complex [Re2Cl2(CO)6S eCH 2 CMe 2 CH 2 S e] has been prepared and characterized in the solid state by X-ray crystallography. Proton NMR spectroscopy has shown the structure to be stereochemically rigid in solution.

Adducts of the type Me 3 PtXL (X = C1, Br, I; L = MeSCH 2 CH 2 SMe) exhibit pyramidal inversion at individual sulphur atoms, whereas the adducts (Me 3 PtX) 2 L′ (L′ = MeSCH 2 SMe, X = C1, Br, I) exhibit both ring inversion, and synchronous inversion about both sulphur atoms.

The complexes fac-[PtMe3L]Cl, where L = 1,4,7-trithiacyclonane (1), 1,4,7,10-tetrathiacyclododecane (2), and 10-oxa-2,4,7-trithiacyclododecane (3), have been prepared, and the X-ray crystal structure of (1), isolated as a dihydrate, has been obtained; variable-temperature 1H and 13C-{1H} n.m.r. studies of (2) have revealted a novel ligand commutation involving exchange between metal-co-ordinated and unco-ordinated sulphur atoms of the macrocycle.