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The N3 ligands 2-{(2′-pyridylmethylene)aminomethyl}pyridine, (PMAMP), 8-{(2′-pyridylmethylene)amino}quinoline, (PMAQ) and 2-[{(6′-methyl-2′-pyridyl)methylene}aminomethyl]pyridine (MPMAMP) are analogous to 2,2′:6′,2″-terpyridine in respect of their spatial arrangement of N donor atoms. When acting as bidentate chelates PMAMP and PMAQ bind exclusively to the PtIV moiety PtIMe3 such that the five-membered chelate rings incorporate the imine bond. This was deduced by detailed solution NMR studies and X-ray crystal structures. In contrast, the ligand MPMAMP, with 6-methyl substitution of the pyridyl ring attached to the carbon of the imine bond, preferentially, but not exclusively, coordinates to the nitrogen of the other pyridyl ring. The population ratio of the two complexes formed was 6:1 (303 K, CDCl3 solution), but there was no NMR evidence of exchange between these species, in the temperature range 303–413 K, resulting from 1,4-metallotropic shifts, as occurs in analogous complexes such as fac-[PtIMe3(2,2′:6′,2″-terpyridine)].

2-Nitrosopyridine, 1, 3-methyl-2-nitrosopyridine, 2, and 4-methyl-2-nitrosopyridine, 3, exist in organic solvents as monomer–azodioxy dimer equilibria with the dimers predominating at ambient temperatures. In the case of compounds 1 and 3 only the Z-dimers co-exist with the monomers, whereas for compound 2 both Z- (major) and E- (minor) dimers are present with the monomer. Variable temperature 1H NMR bandshape analysis and 2D-EXSY spectra of 1–3 provided thermodynamic and kinetic data for the dissociation equilibria Z- (or E-) dimer ⇌ 2 monomer, ΔH values being in the range 53–58 kJ mol−1 and ΔG‡ (298.15 K) values in the range 70–82 kJ mol−1. An X-ray crystal structure of 1 identified the compound as a Z-dimeric species with the two pyridine rings twisted by 62.7° and their heterocyclic N atoms pointing towards each other.

Syntheses and characterisation data are reported for the following ReI and PtIV bidentate chelate complexes of oligopyridines, [{ReBr(CO)3}2L] (L=2,2′:6′,2″:6″,2‴-quaterpyridine (QP), or 4′,4″-bis(4-tert-butylphenyl)-2,2′:6′,2″:6″,2‴-quaterpyridine (BPQP)), [(PtIMe3)2 (QP)], [ReBr(CO)3(QP)] and [ReBr(CO)3(BPQNP)] (BPQNP=4′,4‴-bis(4-tert-butylphenyl)-2,2′:6′,2″:6″,2‴:6‴, 2⁗ -quinquepyridine). The mononuclear complexes do not exhibit metallotropic shifts in solution as occur in the three-ring 2,2′:6′,2″-terpyridine complex analogues. This is attributed to the steric effects of the uncoordinated portions of these ligands, which also lead to severe restrictions to rotation of these pendant arms. Such movements were investigated by low temperature NMR spectra.

The mono- and di-metallic N6-ruthenium(II) complexes [Ru(terpy)L][PF6]2 1 and [{Ru(terpy)}2L][PF6]4 2, where L is 2,3-bis(2,2′-bipyridin-6-yl)pyrazine and terpy is 2,2′∶6′,2″-terpyridine, have been prepared. Extensive and careful 1H and 13C NMR spectroscopic studies at ambient temperature showed that complexation induces steric strain in the scaffolding of L and thereby fluxional behaviour in both complexes. In 1, rotation of the free 2,2′-bipyridyl (bpy) group of L relieves these interactions and results in two enantiomerically related conformations. In 2, a mutual twisting of both complexed bpy fragments results in helicene-like, P- and M-configured enantiomeric complexes. Dynamic low-temperature NMR studies of 1 and 2 in acetone-d6 solutions have provided activation energy data for both processes. The behaviour of 2 is paralleled by its crystal structure, which indicated a strong twisting of the bpy groups of L. Cyclic voltammetric and UV-spectroscopic studies conducted on both indicated a mild electrochemical and electronic coupling of the metal centres in 2.

σ N -Coordinated compounds [PtCl 3 (4-X-C 6 H 4 NO)] - K + have been prepared and studied by 13 C NMR spectroscopy and by variable temperature 1 H NMR spectroscopy. It is shown that the anomalous behaviour of the NO group on the 13 C chemical shifts is modified by coordination but where X = N(CH 3 ) 2 or OCH 3 the free energy barriers to rotation of the NO group determined by 1 H NMR studies are closely similar to those of the free ligands. Restricted rotation of the N(CH 3 ) 2 group was also studied in [PtCl 3 (2-CH 3 -4-N(CH 3 ) 2 -C 6 H 3 NO)] − K + .

Syntheses are given for the following transition metal complexes, [{ReBr(CO)3}2L] (L = L1, L2 or L3), [(PtClMe3)2L1], [(PtIMe3)2L] (L = L2 or L3), [ReBr(CO)3L1], [Pt(C6F5)2L3], [Pt(C6F4CF3)2L] (L = L1 or L2), [{Pt(C6F4CF3)2}2L] (L = L1 or L2) and [ReBr(CO)3PtIMe3L1] where the ligands, L, are the ‘back-to-back’ terpyridine ligands, 6′,6″-bis(2-pyridyl)-2,2′∶4′,4″∶2″,2‴-quaterpyridine (L1), 1,4-bis(2,2′∶6′,2″-terpyridin-4′-yl)benzene (L2) and 6′,6″-bis{2-(4-methylpyridyl)}-2,2′∶4′,4″∶2″,2‴-quaterpyridine (L3). All the complexes undergo 1,4-metallotropic shifts in solution at above-ambient temperatures and restricted rotations of the pendant pyridyl rings at below-ambient temperatures. Activation energies for these processes have been computed from variable temperature one-dimensional bandshape analysis and 2D-exchange spectroscopy (2D-EXSY) NMR experiments. The metallotropic shift energies are very metal-dependent being in the order PtIV

Pentacarbonylbromorhenium(I) reacted with 2,6-bis(benzimidazol-2′-yl)pyridine (bbip), 2,6-bis(1′-methylbenzimidazol-2′-yl)pyridine (bmbip), 2,6-bis(5′,6′-dimethylbenzimidazol-2′-yl)pyridine (bdmbip), and 2,6-bis(1′,5′,6′-trimethylbenzimidazol-2′-yl)pyridine (btmbip) to form stable octahedral complexes of type fac-[ [ReBr(CO)3L] (L = bbip, bmbip, bdmbip or btmbip in which L is acting as a bidentate chelate ligand. At above-ambient temperatures in solution the complexes are fluxional with the nitrogen ligand oscillating between equivalent bidentate bonding modes. Rates and activation energies for these fluxions have been investigated by NMR methods. At low temperatures rotation of the uncoordinated nenzimidazolyl unit is restricted and for fac-[ReBr(CO)3(btmbip)] in CD2Cl2 solution, two preferred rotamers exist in 89:11% relative abundances. A rotational energy barrier has been estimated. The X-ray crystal structure of fac-[ReBr(CO)3)(btmbip)] confirms the bidentate chelate bonding of btmbip with a NReN angle of 74.2°. The pendant benzimidazole unit is inclined at an angle of 60.7° to the pyridine ring.

Energy barriers associated with rotation of nitroso groups in monomeric nitrosobenzene and eleven monomeric substituted nitrosobenzenes in solution have been measured by total 1H NMR bandshape analysis. ΔG ‡ (298.15 K) values for the rotations are in the range 31–41 kJ mol–1. The values for 4-substituted compounds correlate well with the Hammett σp+ parameter for the substituent. The experimental energy barrier for nitrosobenzene is compared with theoretical calculations. Monomer–dimer equilibria of these compounds in solution have been investigated, with ΔH, ΔS and ΔG data calculated for the dissociation of both Z- and E- azodioxy dimers to monomers. Kinetic data, based on time-dependent and two-dimensional exchange spectroscopy (2D-EXSY) NMR measurements, have been obtained for the dissociation of 3-methylnitrosobenzene and 3,5-dimethylnitrosobenzene dimers. Energy profiles for the ground and transition states of both dimers and monomers (including exchange between both their rotameric forms) are discussed.

Syntheses are reported for the following transition metal complexes of derivatives of 2,2′∶6′,2″-terpyridine, [ReBr(CO)3L] (L = mcpt, mcpmt, mmtt or bmtt), [PtIMe3(mcpt)], [Pd(C6F5)2(mcpt)] and [Pt(C6F5)2(mcpt)] where mcpt = 4-methyl-4′-(4-chlorophenyl)-2,2′∶6′,2″-terpyridine, mcpmt = 4-methyl-4′-(4-chlorophenyl)-4″-methyl-2,2′∶6′,2″-terpyridine, mmtt = 4-methyl-4′-methylthio-2,2′∶6′,2″-terpyridine and bmtt = 4-tert-butyl-4′-methylthio-2,2′∶6′,2″-terpyridine. In organic solvents (CD2Cl2, CDCl2·CDCl2) they exist as bidentate chelate complexes which undergo double 1,4-metallotropic shifts (‘tick-tock’ twists). These shifts have been monitored by variable temperature one- and two-dimensional NMR methods and activation energy data obtained. These energy data are very metal-dependent, magnitudes of ΔG ‡ (298.15 K) being in the range 66–101 kJ mol–1 and showing the trend PtII PdII ≈ ReI > PtIV. The nature of the ligand does not greatly influence the ligand fluxionality but does affect the relative populations of the pairs of metal complexes found in solution (when L = mcpt, mmtt and bmtt). Above-ambient temperature 2-D-exchange spectroscopy (EXSY) NMR experiments provide firm evidence for the fluxions occurring by an associative mechanism, involving five-co-ordinate intermediates for palladium(II) and platinum(II) complexes, and seven-co-ordinate intermediates for platinum(IV) and, by analogy, rhenium(I) complexes.

Variable temperature NMR studies of a range of 3,5-disubstituted (R = R′ = Me, But) and 1,3,5-trisubstituted (R, R′, R″ = Me, CF3, Ph or Bui)-4-nitrosopyrazoles have led to the identification of individual rotational isomers at low solution temperatures arising from the slowing down of the rotation of the nitroso function with respect to the pyrazole ring. Rotational energy barriers [ΔG‡ (298.15 K)], calculated by total NMR bandshape analysis, were in the range 38–65 kJ mol-1. The –NO rotation process also occurs in the solid state and leads to disorder in the crystal structures of 1,3,5-trimethyl-4-nitrosopyrazole and 3,5-di-tert-butyl-4-nitrosopyrazole. The 1-H-3,5-disubstituted-4-nitrosopyrazoles exhibit rapid annular tautomerism in solution.

Ambient and low temperature 1H NMR spectra of a wide range of 3- and 4-monosubstituted, and some di- and tri-substituted C-nitrosobenzenes have enabled –NO substituent constants for the static and rotating ring molecules to be calculated. This has provided information on the shielding anisotropy of the NO group which in turn leads to the firm identification of the monomeric and dimeric solution species. In all cases lowering the solution temperature enhances the relative populations of dimers to monomers, with the (Z)-azodioxy dimer being preferred over the (E)-form, irrespective of the nature of the solid state dimeric structure.

Trimethylplatinum(IV) halide tetramers [(PtXMe 3 ) 4 ] reacted with pyrazolylbipyridyl ligands to afford stable octahedral complexes of type fac-[PtXMe 3 L] [L = (6-pyrazol-l-yl)-2,2′-bipyridine (pbipy), X = Cl, I; L = 6-(4-methylpyrazol-1-yl)-2,2′-bipyridine (mpbipy) or 6-(3,5-dimethylpyrazol-1-yl)-2,2′-bipyridine (dmpbipy), X = I]. Both bipyridyl and pyridyl/pyrazolyl coordinated complexes of pbipy and mpbipy are formed in solution with the ‘bipyridyl’ coordinated species greatly predominating (>95%). A fluxional exchange between the two coordination species was detected by 1 H NMR. The crystal structure of [PtIMe 3 (dmpbipy)] showed exclusive ‘bipyridyl’ coordination to Pt IV with the pendant pyrazolyl ring rotated by 74.34 and 77.39° from the plane of the neighbouring pyridyl ring in the two crystallographically independent molecules. Copyright © 1996 Elsevier Science Ltd

The octahedral complexes fac-[ReX (CO)3L] [X = Cl, Br or I; L = 2,6-bis-(4-methylpyrazol-1-yl)pyridine (bmppy), 2-(pyrazol-1-yl)-6-(4-methylpyrazol-1-yl)pyridine (pmppy) and 2-(4-methylpyrazol-1-yl)-6-(3,5-dimethylpyrazol-1-yl)pyridine (mpdmppy)] are stable octahedral complexes in which the substituted pyridines are acting as bidentate chelate ligands. In solution the bmppy complexes are fluxional with the nitrogen ligand oscillating between two equivalent bidentate bonding modes, whereas for the unsymmetrical ligands pmppy or mpdmppy the fluxionality interconverts two chemically distinct species with differing solution populations. Rates and activation energies of these fluxions have been investigated by NMR methods. Activation energies are dependent on the relative donor strengths of the nitrogen atoms and the steric requirements of the ligands with magnitudes of †G ‡ (298 K) being ≈ 55 (bmppy), ≈ 58 (pmppy) and ≈ 71 kJ mol−1 (mpdmppy).

4-Iodonitrosobenzene has been isolated as the (E)-azodioxy dimer for the first time. Its solid-state structure has been identified by X-ray crystallography and characterised by IR, MS and 13C NMR techniques. In CDCl3 or CD2C12 solution dynamic equilibria exist between monomeric 4-iodonitrosobenzene and its (Z)- and (E)-azodioxy dimers. Thermodynamic data (ΔG⊖, ΔH⊖ and ΔS⊖) for these equilibria have been calculated from low temperature 1H NMR spectra. Criteria for the dimerisation tendencies of all types of C-nitroso compounds are proposed.

The stable octahedral complexes fac-[ReX(CO)3L][X = Cl, Br or I; L = 2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (tmpzt) or 2,4,6-tris(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (tdmpzt)] have been prepared in which the substituted triazines act as bidentate chelate ligands. In solution the complexes are fluxional with the nitrogen ligand oscillating between two equivalent bonding modes. Rates and activation energies for the fluxion have been investigated by dynamic NMR methods. Activation energies ΔG‡(298.15 K) are in the range 47–72 kJ mol–1 and are dependent on the steric requirements of the pyrazole groups. At low temperatures changes in the 1H NMR spectra are interpreted in terms of varying rates of rotation of one of the unco-ordinated pyrazole rings. The non-fluxional dimetallic complexes [{ReX(CO)3}2L](X = Cl or Br; L = tdmpzt) were also isolated and structurally characterized by 1H NMR spectroscopy.

The octahedral complexes fac-[ReX(CO)3L][L = 6-(pyrazol-1-yl)-2,2′-bipyridine (pbipy), 6-(3,5-dimethylpyrazol-1-yl)-2,2′-bipyridine (dmpbipy) or 6-(4-methylpyrazol-1-yl)-2,2′-bipyridine (mpbipy); X = Cl, Br or I] have been studied by low-temperature NMR spectroscopy and in the first two cases by X-ray crystallography. The pendant pyrazol-1-yl ring is shown to be oriented considerably out of the plane of the adjacent co-ordinated pyridyl ring of the ligand. The crystal structure of [ReBr(CO)3(pbipy)] reveals this angle of orientation to be 36.3 and 48.1° in two crystallographically independent molecules. In solution, restricted rotation of the pendant ring occurs leading to distinct pairs of rotamers at low temperatures. Rotation barriers (ΔG‡) for the pbipy and mpbipy complexes are in the range 39–51 kJ mol–1 and the two rotamers have near-equal solution abundances. Methyl substitution at the 3 and 5 positions of the pyrazol-1-yl ring as in the dmpbipy complexes has a major effect on the orientation of the unco-ordinated pendant ring in the solid state as evidenced by the crystal structure of [ReBr(CO)3(dmpbipy)]. This in turn affects the relative rotamer populations in solution, with one rotamer being very dominant, and the barrier to pendant-ring rotation increases somewhat.

The octahedral tris-chelate complexes [M(tta)3] (M = AlIII, GaIII; tta = 2-thenoyltrifluoroacetone or 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione) were isolated as mixtures of cis- and trans-isomers. Their fluxionality in solution was measured by 19F NMR two-dimensional exchange spectroscopy and one-dimensional bandshape analysis. Rates of exchange between all four fluorine signals for a given temperature imply a common transition state for both the cis-trans isomerization and the Δ-Λ racemization processes. A bond rupture mechanism involving a trigonal bipyramidal or, more probably, a square planar transition state structure with a pendant axial ligand is proposed for the fluxion. The indium(III) complex [In(tta)3] was also prepared but its fluxionality was too rapid to be followed by 19F NMR spectroscopy. Activation energies [ ΔG † (298.15 K ] for the fluxions were 89.8 ± 0.3 (AlIII) and 75.4 ± 0.2 KJ mol−1 (GaIII). 1H and 19F NMR date for the octahedral complex [Co(tta)3], and the tetrahedral complexes [Be(tta)2] and [Zn(tta)2] are also reported.

2,6-Diacetylpyridine (DAP) reacted with pentacarbonylhalogeno rhenium(I) complexes under mild conditions to form stable octahedral complexes of the type fac -[ReX(CO) 3 (DAP)] (X = Cl, Br or I) in which the DAP acts as a bidentate chelate ligand. In solution these complexes are fluxional with DAP oscillating between equivalent bidentate bonding modes by a twist mechanism involving the breaking/making of two ReO bonds. Rates and activation energies of these fluxions have been investigated by NMR methods. 2-Acetylpyridine (AP) reacts with bromopentacarbonylrhenium(I) to form the non-fluxional chelate complex fac -[ReBr(CO) 3 (AP)].

The energy barriers associated with NO rotation in a series of p-nitrosoanilines were measured by the total bandshape NMR method. Energies [ΔG‡ (298.15 K)] are in the range 38–57 kJ mol−1, the magnitudes being lowered appreciably by the presence of methyl substituents flanking the nitroso group on either side. In the case of p-nitroso-N,N-dimethylaniline, NO rotation has been shown to occur also in the solid state with a slightly higher energy barrier to that in the dissolved state. For p-nitroso-N-methylaniline, a combination of 2D-exchange spectroscopy (EXSY) and 1D-bandshape analysis NMR methods enabled the barriers to NO rotation and NHMe rotation to be measured separately.

A new synthesis of the mesocyclic ligand 1,5-dithiacyclooctane (1,5-DTCO) is reported. Complexes of type [{W(CO)5}n(1,5-DTCO)] (n= 1,2) were isolated and their conformational and exchange dynamics in solution examined by NMR. The complex [W(CO)4(1,5-DTCO)], where the ligand acts as a bidentate chelate, undergoes facile boat-chair⇌chair-boat ring conformational exchange, NMR bandshape analysis of the motion yielding an activation energy, ΔG ‡ (298.15 K ), of 25.1 ± 0.2 kJ mol −1 . The complexes [ReX(CO)3(1,5-DTCO)] (X = Cl, Br) were isolated. An X-ray crystal structure of the X = Cl complex showed that the ligand adopts a boat-chair conformation, with a trans-annular S…S contact distance of 3.277 A. This ring conformation is retained in solution.

2,6-Bis(pyrazol-1-yl)pyridine (bppy) reacted with pentacarbonylhalogenorhenium(I) complexes under mild conditions to form stable octahedral complexes of type fac-[ReX(CO)3(bppy)](X = Cl, Br or I) in which bppy acts as a bidentate chelate ligand. In solution these tricarbonyl complexes are fluxional with bppy oscillating between equivalent bidentate bonding modes by a twist mechanism involving the breaking/making of two Re–N bonds. 2,6-Bis(3,5-dimethylpyrazol-1-yl)pyridine (tmbppy) and 2-(3, 5-dimethylpyrazol-1-yl)-6-(pyrazol-1-yl)pyridine (dmbppy) formed analogous fluxional complexes. The 1,4-Re–N metallotropic shifts of [ReX(CO)3(dmbppy)] interconvert chemically distinct complexes with different solution populations. Under more severe reaction conditions the complex cis-[ReBr(CO)2(tmbppy)] was formed in which the ligand adopts its more usual terdentate behaviour. Rates and activation energies of these ‘tick-lock’ twist fluxions have been investigated by one- and two-dimensional NMR methods. Activation energies are dependent on the relative donor strengths of the N atoms, with magnitudes of ΔG‡(298.15 K) being 55–56 (bppy), ≈69 (dmbppy) and 70–77 kJ mol–1(tmbppy).

Pyridazine (pydz) and 4-methylpyridazine (4Me-pydz) form stable bis(monodentate) complexes of general formulae fac-[PtXMe3L2](X = Cl, Br or I) and fac-[ReX(CO)3L2](X = Cl, Br or I). These complexes in above-ambient temperature solutions exhibit 1,2-fluxional shifts between the nitrogen donor pairs of each ring. All linkage isomers of the pydz complexes are equivalent whereas three NMR-distinct linkage isomers, namely (1,1), (1,2)/(2,1) and (2,2) species, occur in solutions of the 4Me-pydz complexes. Fluxional exchange kinetics have been measured by variable-temperature 1H NMR bandshape analysis and two-dimensional exchange spectroscopy (EXSY). Platinum-195 NMR data have been obtained for the complexes [PtXMe3(4Me-pydz)2](X = Br or I) and their fluxional dynamics investigated by 195Pt two-dimensional EXSY and total bandshape analysis. Energy barriers, ΔG‡(298.15 K) for the 1,2-M–N fluxion are in the ranges 69–74 kJ mol–1 for the PtIV complexes and 84–90 kJ mol–1 for the ReI complexes. The crystal structure of [PtClMe3(pydz)2] shows the cis pydz rings oriented in a propeller arrangement with their unco-ordinated nitrogens oriented away from the cis positioned chlorine.

The doubly trisulphur-bridged metallocene compound, 1,1′,2,2′-bis(1,2,3-trithia-[3])ferrocenophane 1 exists in solution as a mixture of chair—chair and chair—boat diastereomers arising from the orientations of the trisulphur linkages. Dynamic NMR studies have shown that the energies of the two bridge reversal processes, chair—boat to chair—chair and chair—boat to boat—chair, are, in terms of ΔG‡ (298.15 K) data, 93.9 and 89.0 kJ mol−1 respectively. These values are compared with data obtained for the singly trichalcogen-bridged metallocenophanes, and the mechanism of bridge reversal is discussed. Relative populations of the two diastereomers were strongly solvent dependent, with the less abundant chair-boat conformer more favoured in polar solvents. An NMR analysis of the products resulting from conversion of the doubly -trisulphur-bridged compound, 1, to SSeS-bridged species, namely 1,1′,2,2′-(1,2,3-trithia-[3])(1,3-dithia, 2-selena-[3])ferrocenophane 2 and 1,1′,2,2′-bis(1,3-dithia,2-selena-[3])ferrocenophane 3 identified each compound and its individual diastereomers.

2,2′:6′,2″-Terpyridine (terpy) reacts with trimethylplatinum halides [(PtXMe3)4](X = Cl, Br or I) to form stable octahedral complexes fac-[PtXMe3(terpy)](X = Cl, Br or I) in which the terpy molecule is acting as a bidentate chelate ligand. In solution the complexes are fluxional with the ligand oscillating between equivalent bidentate bonding modes by a mechanism consisting of ‘tick-tock’ twists of the metal moiety through an angle equal to the N–Pt–N angle of the octahedral centre. At below-ambient temperatures rotation of the unco-ordinated pyridine ring is severely restricted with the most favoured rotamers having the plane of the pendant pyridine ring at an angle of ca. 52° with respect to the adjacent co-ordinated pyridine ring plane. The X-ray crystal structure of [PtIMe3(terpy)] depicts the pendant pyridine N atom cis to iodine and this is the predominant species in solution at low temperatures. At above-ambient temperatures the complexes exhibit intramolecular Pt–Me exchange of axial and equatorial environments. Energy data based on accurate dynamic NMR fittings are reported for the three dynamic processes, namely pendant pyridine rotation, 1,4-Pt–N metallotopic shifts and Pt–Me scramblings.

Under mild conditions pentacarbonylhalogenorhenium(I) complexes react with 2,2′:6′,2″-terpyridine (terpy) to form stable octahedral tricarbonyl complexes fac-[ReX(CO)3(terpy)](X = Cl, Br or I) in which the terpyridine acts as a bidentate chelate ligand. Under more severe reaction conditions fac-[ReBr(CO)3(terpy)] can be converted to cis-[ReBr(CO)2(terpy)]. In solution the tricarbonyl complexes are fluxional with the terpyridine oscillating between equivalent bidentate bonding modes. At low temperatures rotation of the unco-ordinated pyridine ring is restricted and in CD2Cl2 solution two preferred rotamers exist in approximately equal abundances. Rotational energy barriers have been estimated for the X = Cl and I complexes. The X-ray crystal structure of fac-[ReBr(CO)3(terpy)] confirms the bidentate chelate bonding of terpy with a N–Re–N angle of 74.3°. The pendant pyridine ring is inclined at an angle of 52.9° to the adjacent co-ordinated ring and the unco-ordinated nitrogen is directed towards the axial carbonyl and trans to Br.

Trimethylplatinum(IV) iodide complexes with ditelluroether ligands, namely [PtlMe3(L–L)][L–L = MeTe(CH2)3TeMe, PhTe(CH2)3TePh or o-C6H4(TeMe)2] have been isolated as pure solids. 195Pt-{1H} and 125Te-{1H} NMR studies show that ambient-temperature solutions of the complexes with aliphatic ligand backbones consist predominantly of DL isomers. The complex [PtlMe3{MeTe(CH2)3TeMe}] also exists in significant amounts of both meso forms. In contrast, the meso-1 form is present in only very low abundance in [PtlMe3{PhTe(CH2)3TePh}] and [PtlMe3{o-C6H4(TeMe)2}]. Pyramidal tellurium inversion interconverts these isomeric species and precise rates of this process were measured at near-ambient temperatures by 195Pt-{1H} and 125Te-{1H} NMR two-dimensional exchange spectroscopy experiments, or by 1H one-dimensional NMR bandshape analysis at above-ambient temperatures in the case of [PtlMe3{o-C6H4(TeMe)2}]. Tellurium pyramidal inversion-energy barriers [ΔG‡(298.15 K)] were in the range 70–84 kJ mol–1, reflecting the nature of the the substituent at Te and the ligand backbone. Comparison of the tellurium inversion energy in [PtlMe3{MeTe(CH2)3TeMe}] with data for Se and S in analogous complexes of chalcogen ligands establishes the trend in inversion energies, namely Te > Se > S, on a firm quantitative basis.

Ambient and low temperature 1H NMR spectra of a wide range of 3- and 4-monosubstituted, and some di- and tri-substituted C-nitrosobenzenes have enabled –NO substituent constants for the static and rotating ring molecules to be calculated. This has provided information on the shielding anisotropy of the NO group which in turn leads to the firm identification of the monomeric and dimeric solution species. In all cases lowering the solution temperature enhances the relative populations of dimers to monomers, with the (Z)-azodioxy dimer being preferred over the (E)-form, irrespective of the nature of the solid state dimeric structure.

The complexes [M(CO) 5 (RSCH 2 SR)] (M  Cr, R  Et, i Pr, t Bu; M  W, R  Me, Et, i Pr, t Bu) were synthesised. Total bandshape NMR analysis was used to measure energies of inversion of the coordinated S atom in the tungsten complexes. Δ G ‡ (298.15 K) values were in the range 34–42 kJ mol −1 . An X-ray structure of [W(CO) 5 ( t BuSCH 2 S t Bu)] was obtained.

A synthetic route to the platinum(IV) complexes [(PtXMe3)2([graphic omitted]HMe)](X = Cl, Br or I) has been established. Solution NMR and other evidence favour the ligand adopting a boat conformation and bridging the two platinum(IV) centres across an –SCH2S–portion of the ring, with the ring methyl group in an equatorial environment. Only very weak evidence was obtained for the existence of the –SCHMeS– bridged ligand complex despite the occurrence of a fluxional process involving 60° pivots of the ligand about individual S–Pt bonds which must proceed via the –SCHMeS– bridged species. Activation energies and a proposed mechanism for the process are presented. Some rare examples of 2J(PtIVPtIV) scalar couplings are reported.

Dynamic NMR studies have shown that the energies of the bridge reversal fluxion in the [3]-ferrocenophanes [Fe(C5H4Te)2E] (E  S, Se, Te) are, in terms of ΔG‡ (298 K) data, 56.3, 55.4 and 51.8 kJ mol−1 respectively. These values, which are a function of total bridge length, are compared with values for the other trichalcogena-[3]ferrocenophanes. Relative magnitudes of torsional barriers about chalcogen-chalcogen bonds calculated from these data, showed the TeTe torsion energy to be 1.7 kJ mol−1 lower than the SeTe torsion and 2.2 kJ mol−1 lower than the STe torsion. The mechanism of the bridge reversal process was investigated by CNDO/2 calculations on [Fe(C5H4E)2E] (E  S, Se, Te), which showed that a transition state structure involving staggered Cp rings, akin to the half chair conformation of cyclohexane, is considerably more favoured than a structure with eclipsed Cp rings and a planar trichalcogen bridge.

Variable temperature 1H and 13C{1H} NMR studies on 1,1′,3,3′-tetra(alkyl)-ferrocenes and -ruthenocenes (alkyl = t-pentyl,t-butyl) have provided accurate barrier energies for restricted rotations of the substituted cyclopentadienyl rings. Energies (ΔG† (298.15 K)) are in the range 40–57 kJ mol−1 and are dependent on the ‘sandwich’ metal (Fe ⋙ Ru) and alkyl substituent (t-pentyl < t-butyl). Ring rotation in 1,1′,3.3′-tetra(phenyl)ferrocene was too rapid for measurement even at 173 K. Spectral changes were analysed primarily on the basis of exchange between a mirror pair of structures having both 5-membered rings eclipsed and the alkyl substituents staggered. In the case of 1,1′,3,3′-tetra(t-butyl)ferrocene a pair of staggered ring rotamers also contributed to the observed NMR bandshape changes.

The [3]metallocenophanes with trichalcogen chains as bridging groups, namely [M(C5H4E)2E′] (M = Ru, Os, E = E′ = S; M = Ru; E = E′ = Se), have been shown by NMR spectroscopy to be fluxional by a bridge reversal process. From two-dimensional NMR exchange experiments (2D-EXSY), accurate energy data for the bridge reversal fluxion have been obtained and compared with previous data for analogous [3]ferrocenophanes. The ΔG‡(298 K) values for the process lie in the range 67–93 kJ mol−1. Magnitudes increase in the order Fe < Ru < Os, reflecting the increasing strength of metal-ring bonding, and in the order Se3 < S3 due to the different torsional energies of the chalcogen-chalcogen bridge bonds.

The complexes cis-[M(CO)4(LL)], fac-[ReX(CO)3(LL)] and fac-[PtXMe3(LL)], (M = Cr, Mo, W; LL = [(C5H4SCH3)2Ru]; X = Cl, Br, I) have been synthesised. A 1H NMR study of their solution properties has shown that pyramidal inversion of the coordinated sulphur atoms is rapid on the NMR timescale at ambient temperature. At low temperatures (ca. −90° C) the motion is arrested and the tungsten complex exists as a mixture of meso and dl species in approximately equal proportions, whereas the rhenium complexes are predominantly (> 94%) in the meso form. Insolubility precluded low-temperature studies on the platinum complexes. Variable temperature bandshape analyses of the tungsten complex yielded a sulphur inversion (meso → dl) value of ΔG≠ (298 K) 32.0 ± 1.0 kJ mol−1. The massive predominance of one invertomer in the case of the rhenium complexes prevented a study of the sulphur inversion by bandshape analysis. The crystal structure of 1,1′-bis(methylthio)ruthenocene tetracarbonyltungsten has been determined. The WS bond lengths are 2.571(5) and 2.565(4) A, with a SWS bond angle of 81.1(1)°. The cyclopentadienyl rings are eclipsed and the SCH3 groups adopt a meso relationship.

The complexes fac-[ReX(CO)3{(C5H4ECH3)2Fe}] (X  Cl, Br, I; E  S, Se) have been synthesised. A 1H NMR study of their solution properties showed that pyramidal inversion of the coordinated chalcogen atoms was rapid on the NMR timescale at ambient temperatures, but at low temperatures (ca. −70°C) the complexes existed as mixtures of meso and dl species, with the former predominating, particularly in the sulphur ligand complexes. Low temperature 2D-EXSY studies and variable temperature 1D bandshape analyses yielded accurate chalcogen inversion energies for both meso-1 → dl -1 and dl -1 → dl -1 exchanges. Values of ΔG‡(298 K) for these interconversions were, respectively, in the regions of 43 and 46 kJ mol−1 (S inversion) and 63 and 66 kJ mol−1 (Se inversion). The crystal structure of chloro-1,1′-bis(methylthio)ferrocenetricarbonylrhenium has been determined. The crystals have space group P 1 , with a 9.030(1), b 14.252(2), c 7.554(1) A, α 76.58(1)°, β 104.73(1)°, γ 107.08(1)° and Z = 2. Least-squares refinement gave R = 0.030 for 2930 reflections. The ReS bond lengths are 2.504(4) and 2.512(4) A and the SMe groups adopt a meso relationship analogous to the preferred solution structure. The SReS bond angle is 80.0(2)° and the non-bonded SS separation

The complexes [M(CO)5( SC 2 H 4 SC RR′)] (M = Cr, Mo or W, R = R′= H; M = Cr or W, R = R′= Me, R = H, R′= Me, But or Ph) have been synthesized and characterized. Their solution stereodynamics were examined by 1H NMR spectroscopy. Pyramidal inversion of the metal-coordinated sulphur atoms occurs rapidly at ambient temperatures (ΔG‡ (298 K) = 40–44 kJ mol−1). An appreciably slower 1,3-metal—sulphur shift is also evident at temperatures where sulphur inversion is rapid on the proton NMR timescale. For the ligand complexes (R = R′ = H, Me) this shift is purely intramolecular in nature (ΔG† (298 K) values ∼ 78 kJ mol−1), in contrast to the 2-mono-substituted ligand complexes (R = H, R′ = Me, But, Ph) where it is accompanied by a dissociation process.

Solid-state and solution 13C NMR data for the monomers and dimers of 3- and 4-substituted nitrosobenzenes, and the crystal structure of E-(4-ClC6H4NO)2 are reported.

1 H/19F NMR two-dimensional exchange spectroscopy (EXSY) reveals that the 1,4-metallotropic shifts in bidentate chelate complexes of a substituted 2,2′ : 6′,2″-terpyridine ligand occur by an associative mechanism involving five-coordinate intermediates for PdII and PtII complexes, and seven-coordinate intermediates for PtIV and, by analogy, ReI complexes.

2,2′:6′,2″-Terpyridine (terpy) forms octahedral complexes fac-[ReBr(CO)3(terpy)], cis-[W(CO)4(terpy)] and fac-[PtClMe3(terpy)] in which the ligand oscillates between equivalent bidentate forms by a mechanism involving a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the octahedral centre and involving a seven-coordinate metal intermediate.

Under mild conditions 2,6-bis(p-tolylthiomethyl) pyridine (L1) reacted with halogenotrimethylplatinum(IV) to afford complexes of the type fac-[PtXMe3L1](X = Cl or Br). This potentially terdentate ligand acts here in a five-membered S/N chelate bidentate fashion. Solution dynamic NMR studies revealed the presence of three fluxional processes: pyramidal inversion of the co-ordinated sulfur atom, S-S switching with correlated pyramidal inversion, and S-S switching. The mechanism for the S-S exchange process was elucidated by two-dimensional NMR exchange spectroscopy. The crystal structure of fac-[PtBrMe3L1] confirms the bidentate chelate nature of the ligand in the solid state, with a N-Pt-S(1) angle of 81.5°. IR data for the Pt-C, Pt-N and Pt-X stretching modes in these complexes are also presented.

The syntheses of the complexes [PtXMe3{(C5H4ECH3)2Fe}] (E = S, Se; X = Cl, Br, I) are reported. A variable temperature NMR study of their solution properties revealed varying rates of pyramidal inversion of the coordinated chalcogen atoms. When inversion is slow on the 1H or 195Pt NMR time-scales, the dl forms of the complex predominate (⩾ 90% abundance). Activation energies (ΔG‡(298 K)/kJ mol−1) for the pyramidal inversions fall in the narrow ranges 41.7–43.4 (sulphur) and 58.0–58.8 (selenium). At above-ambient temperatures averaging of the ring proton NMR signals indicates that the ligand is executing 180° 'pancake' rotations with respect to the trimethylplatinum(IV) halide moiety, accompanied by exchange of the Pt-methyl environments. The NMR spectra also imply that the E-Pt-E portion of the ring is rapidly moving from one half-chair form to the other at all observed temperatures.

The dinuclear complexes [Re2X2(CO)6(RCH2EECH2R)] (X = Cl or Br, R = Ph or Me3Si, E = S or Se) have been prepared and characterized. A variable temperature 1H NMR study on these complexes demonstrated the pyramidal atomic inversion process at the coordinated sulphur and selenium atoms. Total band-shape fittings were used to yield activation parameters for the rate process, in which the two sulphur or selenium atoms undergo synchronous or correlated inversion.

Mononuclear chelate complexes fac-[{ReX(CO)3}MeSCH2SCH2SMe)] X = Cl, Br and I, have been isolated and examined by dynamic NMR methods. Energy data associated with the pyramidal inversion of the sulphur atoms and rotation of the chelated ligands have been obtained.

N.m.r. studies of tungsten pentacarbonyl complexes of type [W(CO)5L] where L refers to open-chain ligands (PhCH2SSCH2Ph or MeSCH2SMe) and cyclic ligands (β-[graphic omitted]HMe, or [graphic omitted]H2) have revealed no evidence of carbonyl scrambling accompanying the tungsten–sulphur 1,2- or 1,3-metallotropic shifts. This implies stereochemical rigidity of the square-pyramidal W(CO)5 moiety, on the two-dimensional 13C n.m.r. time-scale, during these intersulphur–metal commutations, energy barriers (ΔGDagger;) for any carbonyl rearrangement being in excess of ca. 90 kJ mol–1. The results further indicate that the metallotropic shifts involve lateral movements of the rigid W(CO)5 moieties with no tendency to distort to seven-co-ordinate tungsten(0) species in the transition states.

Mixed-chalcogen ligand complexes of type fac-[ReX(CO)3(MeSZSeMe)](X = Cl, Br, or l; Z= CH2CH2 or o-C6H4) have been prepared and their temperature-dependent 1H n.m.r. spectra studied. Pyramidal S and/or Se inversions were observed in all cases and quantitatively assessed for the complex fac-[ReBr(CO)3{o-MeS(C6H4)SeMe}] and the dithio-analogues fac-[ReX(CO)3{o-MeS(C6H4)SMe}](X = Cl, Br, or l). The effects of π conjugation and halogen type on the barrier energies are discussed.

Two-dimensional n.m.r. exchange spectroscopy has been used to measure the kinetics of four-site exchange associated with pyramidal sulphur inversion in the complex [PtIMe3(MeSCH2CH2SEt)]. Ethyl sulphur inversion occurs more readily than methyl sulphur inversion with ΔG‡ values for the processes differing by 3.3–7.2 kJ mol–1 depending on which pathways are considered. Application of the same n.m.r. method to the three-site exchange of the Pt-methyl groups has led to ΔG‡ values of 90.9 ± 0.5, 92.6 ± 0.4, and 97.7 ± 1.9 kJ mol–1. The first two values are attributed to ligand (180°)/PtMe3(120°) correlated rotations, the different values depending on which equatorial Pt-methyl group is interchanged. The third ΔG‡ value, arising from direct exchange of the two equatorial Pt-methyl environments, refers to independent ligand rotation and/or Pt-methyl scrambling.

Platinum(IV) complexes of the type [(PtXMe3)2([graphic omitted])](E = S or Se; X = Cl, Br, or I) have been synthesised and shown to possess dinuclear structures with highly strained Pt2X2 moieties. Variable-temperature 1H n.m.r. studies have clearly identified pyramidal inversions of the E atom pairs and scrambling of the Pt-methyl groups. Although chalcogen atom switching between the Pt atom pairs has not been unambiguously detected, this process constitutes the probable link between the other two fluxional processes. The ΔG† data for the pyramidal inversions are in the range 66–75 kJ mol–1. Such values, being in most cases ca. 20 kJ mol–1 higher than the usual single-site inversions, are clearly associated with appreciably slower, synchronous inversions of the chalcogen atom pairs.

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.