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A series of mono- and di-methyl substituted p-nitrosoanisoles was synthesised either by direct nitrosation of the anisoles with NO+HSO4− or by mild oxidation (H2O2–Mo catalyst) of the appropriate amines. The nature of these compounds in solution was established by 1H and 13C NMR spectroscopy. Variable temperature 1H dynamic NMR studies provided thermodynamic data for the cases of dimer–monomer solution equilibria, and ΔG‡ data for the restricted rotation of the NO group in the monomers. Magnitudes of ΔG‡ were in the range 28–41 kJ mol−1 and were shown to be sensitive predictors of self-dimerisation tendencies of these compounds. This activation parameter, together with the 14,15N and 17O chemical shifts of the NO group, was shown to be a reliable predictor of self-dimerisation of nitroso compounds in general. The NO bond length and the wavenumber of the IR-active NO stretching mode were found to be somewhat less reliable predictors of this behaviour.

The Schiff bases 2,6-bis(phenyliminomethyl)pyridine and its derivatives (L) form bidentate chelate complexes with transition metals Re I , Pt IV and Pd II of general formulation fac -[ReBr(CO) 3 L], fac -[PtXMe 3 L] (X=Cl, Br or I), and cis -[Pd( p -CF 3 C 6 F 4 ) 2 L]. In solution these complexes are stereochemically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotations of the substituted aromatic rings attached to the metal-coordinated imino nitrogen. Rates and activation energies of these internal motions were measured by one-dimensional NMR bandshape analysis and two-dimensional EXSY NMR experiments over a range of solution temperatures. Free energy values, Δ G ‡ (298.15 K), for the fluxional shifts were in the range 76–97 kJ mol −1 with the Pd II complexes having the lowest energies. An X-ray crystal structure of fac -[ReBr(CO) 3 PMTFA] (PMTFA=2,6-pyridylene-bis(methylene-2-trifluoroaniline)) showed the pendant imine function to be in an E -conformation with its nitrogen trans to the pyridyl nitrogen.

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)].

A series of rhenium(I) tricarbonyl halide complexes of pyridyl mono- and di-carboxamides and -thioamides has been studied by dynamic NMR techniques. Oxygen coordination to ReI of the carboxamide ligand reduced the energy barrier to C–N rotation by 2–13 kJ mol–1 whereas sulfur–ReI coordination of the thioamide ligands led to a reduction of 28–33 kJ mol–1. In the pyridyl dicarboxamide and -dithioamide metal chelate complexes metallotropic shifts occur between the three donor atoms, O, N, O or S, N, S, energies [ΔG‡ (298.15 K)] being in the range 78–89 kJ mol–1; these high values arise from the strong electron donating properties of the -NR2 (R = Me, Et) groups.

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.

The ligands 2-(dimethoxymethyl)pyridine (L1) and 2,6-bis(dimethoxymethyl)pyridine (L2) formed bidentate chelate complexes with the isoelectronic transition metal moieties ReIX(CO)3 and PtIVXMe3 (X = halide). The complexes [ReX(CO)3L1] (X = Cl or Br), [PtXMe3L1] (X = Br or I) and [ReBr(CO)3L2] in organic solvents were shown by NMR to undergo fluxional processes which interconvert co-ordinated and pendant OMe groups. Rates and activation energies of these fluxions were measured by NMR methods (1-dimensional bandshape analysis or 2-dimensional exchange spectroscopy). Magnitudes of ΔG‡ (298.15 K) for the fluxions were in the range 59–85 kJ mol−1 with the order being ReI–L1 > ReI–L2 > PtIV–L1. A carbon–carbon bond rotation mechanism is proposed for the fluxions in the L1 complexes and concerted C–C bond rotation/metallotropic shift processes for the L2 complexes. Crystal structures of [ReCl(CO)3L1] and [PtIMe3L1] revealed distorted octahedral metal centres with N–M–O ‘bite’ angles of 73–75° and a trans relationship of the axial halide and pendant OMe group.

The complexes fac -[ReCl(CO) 3 L] [L= ortho or para substituted methyl or t butyl aromatic derivatives of 2,6-bis[1-(phenylimino)ethyl]pyridine (BIP)] were synthesised. In solution all complexes exist as bis-imine E , E and E,Z isomers, with the former greatly predominating (>85% in most cases). 1,4-Metallotropic shifts of the ReCl(CO) 3 moiety between the ligand N donor pairs occur in all cases. In the 1 H NMR spectra of the methyl- and the para - t Bu aromatic derivatives of BIP these shifts were detected as exchange between equivalent E , E forms of the complexes. In the case of the ortho - t Bu derivative, where the E , Z -isomer was more in evidence (21% at 303 K), E , E ⇌ E , Z isomerisation occurs rather more readily than the accompanying metallotropic shifts. Rates and activation energies for the detected dynamic processes were based on NMR bandshape analysis. The results indicated that Me and t Bu substitution of the phenyl rings in BIP had a negligible effect on the Re I N 1,4-metallotropic shifts, whereas ortho substitution of the rings, particularly by t Bu had a considerable influence on the relative proportions of E,E and E,Z conformers, presumably because of steric effects.

The following complexes were synthesised [W(CO)5L1], [{W(CO)5}2L1], [W(CO)5L2], [W(CO)5L3], [{PdCl2(PPh3)}2L1], [PdCl2(PPh3)L2] and [PdCl2(PPh3)L3] where L1=2,6-dithiaspiro[3.3]heptane (2,6-DTSH), L2=2,6-DTSH-2-oxide and L3=2,6-DTSH-2,2′-dioxide. In solution these complexes exhibit pyramidal inversion of the metal-coordinated sulphur atom(s), rates and activation energies of which were evaluated by total NMR bandshape analysis. ΔG‡ values were in the range 45–50 kJ mol−1. These magnitudes are discussed in terms of ring geometry distortions and sulphur lone pair interactions.

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.

Complexes of general formulae cis-[M(C6F4CF3)2L] (M = PdII, PtII; L = 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-pyridyl)-pyrimidine (TPP) were isolated as air-stable solids. In all cases cis square-planar complexes were formed with the nitrogen ligands acting as bidentate chelates towards each metal moiety. The complexes exhibited various modes of fluxionality in solution, namely 1,4-metallotropic shifts, a new ‘metal hurdling’ fluxion and, at below-ambient temperatures, restricted rotation of the pendant pyridyl ring adjacent to the metal chelate ring. Dynamic NMR experiments (one-dimensional bandshape analysis and two-dimensional EXSY experiments) provided activation energy data for these processes. Gibbs free energy values (ΔG≠ (298.15 K)) were in the ranges 74–113 (metal hurdling). 69–118 (metal 1,4-shifts) and 37–43 (pendant pyridyl rotations) kJ mol−1. Energies of any of these fluxions were considerably higher in the PtII complexes than in the PdII complexes. To aid understanding of the low temperature fluxionality of the TPT complexes, the complex [Pd(C6F4CF3)2(mstd)] (mstd = meso-stilbenediamine) was synthesised. At low temperatures, C6F4CF3 ring rotations and five-membered ring puckering in this complex were arrested, ΔG≠ (208K) for the latter process being 42.8 kJ mol−1.

The complexes fac-[PtXMe3(BIP)] (X Cl,Br or I;BIP 2,6-bis[1-(phenylimino)ethyl]pyridine) have been synthesised andcharacterised as involving BIP as a bidentate chelate ligand. In solution the metal-bound ligand exists in four conformationalforms, namely distal and proximal E,E and E,Z-isomers. 2D-EXSY NMR has been used to measure 1,4-metallotropic shifts ofthe PtXMe3 moiety between equivalent distal E,E forms, E,Z isomerisation of the pendant imine function and restricted C Crotation of the pendant arm of the BIP ligand. Activation energies, DG" (298.15 K), for all these processes are in the range 67–96kJ mol 1 and are essentially halogen independent. A crystal structure of fac-[PtIMe3(BIP)] shows that the pendant imine functionis in the E-conformation and is rotated at an angle of 80.1° to the plane of the pyridine.

The complexes [M(C 6 F 5 ) 2 (BIP)] (M = Pd II or Pt II , BIP = 2,6-bis[(1-phenylimino)ethyl]pyridine) have been synthesised and characterised as involving BIP as a bidentate chelate ligand. In solution they undergo 1,4 metallotropic shifts of the M(C 6 F 5 ) 2 moiety, E,Z isomerisation of the pendant imine bond, and restricted C-C rotation of the pendant portion of the BIP ligand. 1 H and 19 F dynamic NMR studies yielded activation energies for these three types of fluxion. Δ G ≠ (298 K) values for the three processes were 89.6, 86.6 and 47.4kJmol −1 respectively for the Pt II complex. Values for the Pd II complex were significantly lower in magnitude, namely 71.6, 70.4 and 41.8 kJ mol −1 respectively.

Complexes of general formula fac-[Re(CO)3(L–L)(pydz)][BF4] (L–L=neutral bidentate chelate ligand; pydz=pyridazine) were prepared in high yield from fac-[ReBr(CO)3(L–L)]. The pyridazine ligand coordinates to the metal moiety in a monodentate fashion, and undergoes a facile intramolecular 1,2-Re–N shift. The kinetics of the 1,2-metallotropic shift were measured by either one-dimensional NMR bandshape analysis or two-dimensional exchange spectroscopy. The free energies of activation, ΔG≠ (298 K), were found to be dependent on the nature of the chelate ligand, and were in the range 77–87 kJ mol−1. The origins of the chelate ligand effects are discussed.

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.

Under mild conditions [ReX(CO)3(THF)2] reacts with pyrazolylmethylpyridines to form the complexes fac-[ReX(CO)3(BPz)] and fac-[ReX(CO)3(BMPz)] (X  Cl, Br, or I; BPz = 2,6-bis(pyrazol-1-ylmethyl)pyridine ; BMPz = 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine) that involve BPz and BMPz acting as non-fluxional bidentate chelate ligands. Under more severe reaction conditions fac-[ReX(CO)3(BPz)] is converted to fac-[Re(CO)3(BPz)]X (X  Cl, Br, or I). Trimethylplatinum(IV)iodide reacts with BPz to form fac-[PtMe3(BPz)]I. The crystal structure of fac-[ReI(CO)3(BMPz)] confirms the bidentate chelate bonding of BMPz with a NReN angle of 85.8°.

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.

2,4,6-Tris(pyrazolyl)pyrimidines reacted with pentacarbonylhalogenorhenium(I) complexes to form stable mixtures of bidentate, chelate complexes of types fac-[ReBr(CO)3(tpzp)],fac-[ReBr(CO)3(tmpzp)] and fac-[ReX(CO)3(tdmpzp)](X = Cl, Br or I) where tpzp = 2,4,6-tris(pyrazol-1-yl)pyrimidine, tmpzp = 2,4,6-tris(4-methylpyrazole-1-yl)pyrimidine and tdmpzp = 2,4,6-tris(3,5-dimethylpyrazol-1-yl)pyrimidine. In solution the complexes involving co-ordination with the 2-pyrazolyl ring nitrogen rather than the 4- or 6-pyrazolyl ring nitrogens strongly predominated (> 91%). Fluxional exchange between the two types of co-ordination species of tmpzp and tdmpzp was detected and measured quantitatively by dynamic NMR methods. Activation energies ΔG‡(298 K) for the fluxions were in the range 53–74 kJ mol–1. Dinuclear transition-metal complexes of type [{ReBr(CO)3}2L](L = tmpzp or tdmpzp) were also isolated. Structures were attributed to these nonfluxional complexes in solution.

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.

The hybrid S/N/S donor ligands 2,6-bis(methylthiomethyl)pyridine (L 1 ) and 2,6-bis( p -tolylthiomethyl)pyridine (L 2 ) react with the [M(CO) 5 (THF)] (M = Mo or W) compounds to form complexes of general formula [M(CO) 4 L] (M = Mo, L = L 2 ; M = W, L = L 1 or L 2 ), where both L 1 and L 2 act in a S/N bidentate chelate fashion. In solution, these complexes undergo three fluxional processes, viz. inversion at the coordinated S atom, S 1 –S 2 switching, and combined inversion and S 1 –S 2 switching, leading to an interconversion of the four possible permutational isomers. Energy barriers for all three processes have been evaluated by standard one-dimensional band-shape analysis techniques. The mechanism of the S 1 –S 2 switch is discussed.

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.

3-Methylpyridazine (3-Mepydz) reacts with halogenotrimethylplatinum(IV), [PtXMe 3 ) 4 ], to afford complexes of type fac -[PtXMe 3 (3-Mepydz) 2 ] (X = Cl, Br or I). In solution these complexes undergo 1,2-metallotropic shifts between the contiguous nitrogen donors producing different linkage isomers. Solution abundances of these ismers are strongly influenced by the steric effects of the 3-methyl group on the pyridazine ring, with the (1,1)-isomer always being predominant (≈ 95%). Activation energies for the fluxion were determined by variable temperature 1 H NMR spectroscopy, values of the (1,1) → (1,2) isomer shift being in the range 74–78 kJ mol −1 .

Complexes of types [W(CO)5L], [cis-Rh(CO)2C1L] (L  benzo[c]cinnoline) and [W(CO)5L′] (L′ pyridazine, 3-methylpyridazine, and 4-methylpyridazine) have been synthesized and shown to exhibit, in organic solvents, intramolecular metal-nitrogen 1, 2-fluxional shifts, which have been quantitatively studied by 1D bandshape analysis and 2D-EXSY NMR experiments. Energy barriers for the metallotropic shifts in the pyridazine complexes are ca. 20 kJ mol−1 higher than in the benzo[c]cinnoline complexes. These differences are discussed in terms of the ground state and likely transition state species for this fluxional process.

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.

Under mild conditions 2,2′ : 6′,2″-terpyridine reacted with [{RuX2(CO)2}n](X = Cl, Br or I) and with [M(CO)4(nbd)][M = Cr, Mo or W; nbd = norbornadiene (bicyclo[2.2.1]hepta-2,5-diene)] to form the octahedral complexes trans, cis-[RuX2(CO)2(terpy)] and [M(CO)4(terpy)], respectively, in which the terpyridine acts as a bidentate chelate ligand. In solution these complexes are fluxional with the terpyridine oscillating between equivalent bidentate modes. Heating trans,cis-[RuX2(CO)2(terpy)](X = Cl, Br or I) in a high boiling solvent formed trans-[RuX2(CO)(terpy)] whereas cis-[RuX2(CO)(terpy)] was formed by heating the solid dicarbonyl complex.

Pentacarbonylhalogenorhenium(I) complexes reacted with 6-(pyrazol-1-yl)-2,2′-bipyridine (pbipy) to form stable octahedral complexes of type fac-[ReX(CO)3(pbipy)](X = Cl, Br or I) in which pbipy acts as a bidentate chelate ligand. Mono-and di-methyl derivatives of this ligand formed similar series of complexes. In all cases chemically distinct pairs of complexes involving both bipyridyl and pyridyl/pyrazolyl coordination were formed but could not be separately isolated. The ‘bipyridyl’ co-ordinated complexes predominate ( > 80%) in solution. These exchange with the pyridyl/pyrazolyl co-ordination species by a 1,4-metallotropic shift, the rates and mechanism of which were followed by variable-temperature NMR spectroscopy. Energy barriers (ΔG‡) for this process were in the range 66–76 kJ mol–1.

Two nitrosodithiofulvenes and a nitrosodiselenafulvene formed insoluble coordination compounds with palladium chloride, these being the first reported coordination compounds of nitroalkenes. Complexes of general formulae PdCl2 · RNO and PdCl2 · (RNO)2 were formed. Variable temperature NMR studies of the nitroalkenes demonstrate that the free energy barrier to rotation about the bond is low, values being in the range 58–93 kJ mol−1.

2,2′:6′,2″-Terpyridine reacted with trans-[M(C6F5)2(diox)2](M = Pd or Pt, diox = 1,4-dioxane) to form the square-planar complexes cis-[M(C6F5)2(terpy)] in which the terpyridine acts as a bidentate chelate ligand. In solution these complexes are fluxional with the terpyridine oscillating between equivalent bidentate modes by a mechanism consisting of a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the metal centre. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the 1H signals of the aromatic hydrogens and in the 19F signals of two C6F5 groups. The ΔG‡ values for the fluxion were ca. 71 and 94 kJ mol–1 for the complexes of PdII and PtII respectively. At below-ambient temperatures further changes in the 19F NMR spectra of both complexes were interpreted in terms of varying rates of rotation of the unco-ordinated pyridine ring, with the rates of rotation of the C6F5 rings being substantially slower at all temperatures and not separately measurable. The lowest-temperature spectra suggested the presence of a pair of degenerate rotamers each having the planes of both C6F5 rings and the unco-ordinated pyridine ring closely parallel and orthogonal to the remainder of the ligand ring system. The crystal structure of [Pd(C6F5)2(terpy)] confirms the bidentate chelate bonding of terpy with a N–Pd–N angle of 77.9°, and the pendant ring oriented at an angle of 46° to the adjacent co-ordinated ring.

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.

In the first part of the review mention will be made of some recent developments in NMR techniques which are potentially important for future 2D-EXSY and 2D-NOESY studies of molecular motions. The second part of the review illustrates recent applications of NMR methods to studies of stereochemical non-rigidity in liquid phase inorganic and organic systems