Possible tests for the mechanisms of ligand exchange in solids: The deaquation-anation of [Cr(NH3)5(H2O)]X3 salts

The possibility of using correlations of ΔH⁺ and ΔH, and of ΔH⁺ and ΔS⁺ to gain insight into the mechanisms of ligand-exchange reactions in solids are discussed. These correlations are tested using literature values for the deaquation-anation reactions of [Cr(NH₃)₅(H₂O)]X₃, where X^? = Cl^?, Br^?, I^? or NO^?₃. The poor agreement in the activation parameters reported in the literature precluded a meaningful test of the ΔH?ΔH^* correlation. This poor agreement suggests that these activation parameters are strongly influenced by experimental factors that have not been controlled in studies to date. nevertheless, there is a linear correlation of ΔH² and ΔS² which gives an isokinetic temperature of 367 ± 11 K. This isokinetic behavior suggests that the same mechanism is operative throughout the series.     

Ionic solvation in water + co-solvent mixtures. Part 8. Total free energies of transfer and free energies of transfer with the “neutral” component removed of single ions from water into water + acetone

The acid dissociation constants of a wide range of acids in water+acetone mixtures have been combined with values for the free energy of transfer of the proton. ΔG⁰_t(H⁺ to calculate values for the free energy of transfer of ions which derive only from the charge on the ion. ΔG⁰_t(i)_c. As the values of ΔG⁰_t(H⁺) have been revised, revised values for the total free energies of transfer of cations and anions, ΔG⁰_t(M⁺) and ΔG^o_t(X^-), are given. New data for ΔG^o_t(MX_n) is also split into values for ΔG⁰_t(Mⁿ⁺) (where n=1 and 2) and ΔG⁰_t(X^?). These free energies of transfer, both total and those deriving from the charge alone, are compared with similar free energies in other mixtures water+co-solvent. Values for ΔG^o_t(i)_c do not conform to a Born-type relationship and show the importance of structural effects in the solvent even when only the transfer of the charge is involved.     

Collision spectroscopy with aligned and oriented atoms

Electron capture processes in the H⁺?Na(3s) and H⁺?Na(3p) collisions are experimentally investigated in the 0.3–3 keV energy range using a crossed beam experiment. The excited Na(3p) target is produced with a well-defined alignment using laser pumping. The time of flight technique enables the identification of all the H(n)+Na⁺ channels populated in the collision. Total cross section ratios σ_3p (n=2)/σ_3s (n=2),σ_3p (n=3)/σ_3s (n=2) and σ_3s (n=3)/σ_3s (n=2) for the production of H(n=2) and H(n=3) are measured in the H⁺?Na (3s) and H⁺?Na (3p) collisions. They reveal a strong dominance of the production of H(n=2) in the H⁺?Na(3p) collision, especially for energies below 1 keV.     

Elucidating the structures of isomeric silylenium ions (SiC3H 9 + , SiC4H 11 + , SiC5H 13 + ) by using specific ion/molecule reactions

Specific ion/molecule reactions are demonstrated that distinguish the structures of the following isomeric organosilylenium ions: Si(CH₃) ₃ ⁺ and SiH(CH₃)(C₂H₅)⁺; Si(CH₃)₂(C₂H₅)⁺ and SiH(C₂H₅) ₂ ⁺ ; and Si(CH₃)₂(i?C₃H₇)⁺, Si(CH₃)₂(n?C₃H₇)⁺, Si(CH₃)(C₂H₅) ₂ ⁺ , and Si(CH₃)₃(π?C₂H₄)⁺. Both methanol and isotopically labeled ethene yield structure-specific reactions with these ions. Methanol reacts with alkylsilylenium ions by competitive elimination of a corresponding alkane or dehydrogenation and yields a methoxysilylenium ion. Isotopically labeled ethene reacts specifically with alkylsilylenium ions containing a two-carbon or larger alkyl substituent by displacement of the corresponding olefin and yields an ethylsilylenium ion. Methanol reactions were found to be efficient for all systems, whereas isotopically labeled ethene reaction efficiencies were quite variable, with dialkylsilylenium ions reacting rapidly and trialkylsilylenium ions reacting much more slowly. Mechanisms for these reactions and differences in the kinetics are discussed.     

Volumetric, viscometric and refractive index behavior of some α-amino acids in aqueous tetrapropylammonium bromide at different temperatures

Densities, ρ, viscosities, η, and refractive indices, n _D, of glycine (Gly), DL-alanine (Ala), DL-valine (Val) (0.05, 0.10, 0.15, 0.20, 0.25 mol kg^?1), and L-leucine (Leu) (0.02, 0.05, 0.10 mol kg^?1) in water and in 0.20 mol kg^?1 aqueous tetrapropylammonium bromide (TPAB) have been measured at 298.15, 303.15, 308.15, and 313.15 K. The density data have been utilized to calculate apparent molar volumes, ?_v, partial molar volumes at infinite dilution, ?_v°, and partial molar volumes of transfer, ? _v°(tr) of amino acids. The viscosity data have been analyzed by means of Jones-Dole equation to obtain Falkenhagen coefficient, A, and Jones-Dole coefficient, B, free energy of activation of viscous flow per mole of solvent, Δµ₁°*, and solute, Δµ₂°*, and enthalpy, ΔH*, and entropy of activation, ΔS*, of viscous flow. The refractive index data have been used to calculate molar refractivity, R _D, of amino acids in aqueous tetrapropylammonium bromide solutions. It has been observed that ?_v°, B-coefficient and Δµ₂°* vary linearly with increasing number of carbon atoms in the alkyl chain of amino acids, and they were split to get contributions from the zwitterionic end groups (NH₃ ⁺, COO^-) and methylene group (CH₂) of the amino acids. The behavior of these parameters has been used to investigate the solute-solute and solute-solvent interactions as well as the effect of tetrapropylammonium cation (C₃H₇)₄N⁺ on these interactions.     

Sigma-bond metathesis reactions of Sc(OCD3)2+ with water,ethanol, and 1-propanol: measurements of equilibrium constants,relative bond strengths,and absolute bond strengths

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been used to examine the reactions of Sc(OCD₃)₂⁺ with water, ethanol, and 1-propanol. Sigma-bond metathesis resulting in the elimination of CD₃OH is the initial reaction observed, with further solvation of the metal center and subsequent elimination of hydrogen occurring as additional reaction channels. These processes are facile at room temperature and involve little or no activation energy. Measured equilibrium constants for the reaction Sc(OCD₃)₂⁺ +ROH ⇌ CD₃OScOR⁺ +CD₃OH with R =H, ethyl, and n-propyl are 0.013 ±0.004, 0.5 ±0.15, and 0.7 ±0.2, respectively. For the reaction ROScOCD₃⁺ +ROH ⇌ Sc(OR)₂⁺ +CD₃OH with R =H and ethyl the measured equilibrium constants are 0.013 ±0.004 and 0.3 ±0.1, respectively. ΔS is estimated for these processes using theoretical calculations and statistical thermodynamics, and in conjunction with the measured equilibrium constants we have evaluated ΔH for these reactions and the relative and absolute bond strengths of the Sc⁺–OR bonds, R =H, methyl, ethyl, and n-propyl. The relative bond strengths, D₂₉₈^o(CD₃OSc⁺–OR)–D₂₉₈^o(CD₃OSc⁺–OCD₃), for R =H, methyl, ethyl, and n-propyl are +11.9, 0, −0.1, and −1.4 kcal mol⁻¹, respectively. The absolute bond strengths for HOSc⁺–OCD₃, CD₃OSc⁺–OCD₃, CD₃OSc⁺–OC₂H₅, CD₃OSc⁺–OCH₂CH₂CH₃, and H₅C₂OSc⁺–OC₂H₅ are 115.0, 115.0, 114.9, 113.6, and 114.7 kcal mol⁻¹, respectively. Theoretical calculations with an LAV3P1 ECP basis set at the level of localized second-order Møller–Plesset perturbation theory were performed to evaluate ΔS and ΔG for the specific equilibria Sc(OH)₂⁺ +CD₃OH ⇌ CD₃OScOH +H₂O, CD₃OScOH +CD₃OH ⇌ Sc(OCD₃)₂⁺ +H₂O, and Sc(OCD₃)₂⁺ +C₂H₅OH ⇌ CD₃OScOC₂H₅⁺ +CD₃OH. The theoretically determined ΔG values agree reasonably well with the experimentally determined ΔG values. In accordance with earlier theoretical predictions, these metathesis reactions are consistent with an allowed four-center mechanism similar to that of a 2_σ +2_σ cycloaddition.     

The inductive effect of a radical center and transferability of the properties of functional groups in <Emphasis Type="Italic">n</Emphasis>-alkyl radicals

Electron density distribution in n-alkyl radicals (from ethyl to n-octyl) was studied by the B3LYP/6-311++G(3df,3pd) DFT method. The theory of atoms in molecules was used to show that the inductive effect of a free valence extends to two neighboring CH₂ groups. The electronegativities χ(C^?H₂) > χ(CH₃) > χ(CH₂) of groups and χ(C^?) > χ(H) > χ(C) atoms were qualitatively determined. The group method for calculating the enthalpies of formation of n-alkyl radicals Δ_f H°(n-C _n H_{2n+ 1}, n > 5) was substantiated.     

Fourier transform-ion cyclotron resonance study of the gas-phase acidities of germane and methylgermane; Bond dissociation energy of germane

An accurate gas-phase acidity for germane (enthalpy scale, equivalent to the proton affinity of GeH₃ ^?), ΔH _acid ^o(GeH₄) = 1502.0 ± 5.1 kJ mol^?1, is obtained by constructing a consistent acidity ladder between GeH₄, and H₂S by using Fourier transform-ion cyclotron resonance spectrometry, and 0 and 298.15 K values for the first bond dissociation energy of GeH₄ are proposed: D₀ ^o(H₃Ge-H) = 352 ± 9 kJ mol^?1; D ^o(H₃Ge-H) = 358 ± 9 kJ mol^?1, respectively. These results are compared with experimental and theoretical data reported in the literature. Methylgermane was found to be a weaker acid than germane by approximately 35 kJ mol^?1: ΔH _acid ^o = 1536.6 kJ mol^?1.     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Heat capacity of polynuclear Fe(HTrz)3(B10H10)·H2O and trinuclear [Fe3(PrTrz)6(ReO4)4(H2O)2](ReO4)2 complexes (HTrz=1,2,4-triazole, PrTrz=4-propyl-1,2,4-triazole) manifesting 1A1⇔5T2 spin transition

Low-temperature heat capacity of polynuclear Fe(HTrz)₃(B₁₀H₁₀)·H₂O (I) and trinuclear [Fe₃(PrTrz)₆(ReO₄)₄(H₂O)₂](ReO₄)₂ (II) spin crossover coordination compounds was measured in 80–300 K temperature range using a vacuum adiabatic calorimeter. For I, an anomaly of heat capacity with a maximum at T _trs=234.5 K (heating mode) was observed, Δ_trs H=10.1±0.2 kJ mol^?1 Δ_trs S=43.0±0.8 J mol^? K^?1. For II, a smooth anomaly between 150 and 230 K was found, Δ_trs H=2.5±0.25 kJ mol^?1 Δ_trs S=13.6±1.4 J mol^? K^?1. Anomalies observed in both compounds correspond to ¹A₁?⁵T₂ spin transition.     

Transition-metal mediated heteroatom removal by reactions of FeL+ [L=O,C4H6, c-C5H6, c-C5H5, C6H6, C5H4(=CH2)] with furan,thiophene, and pyrrole in the gas phase

Reactions of Fe⁺ and FeL⁺ [L=O, C₄H₆, c-C₅H₆, C₅H₅, C₆H₆, C₅H₄(=CH₂)] with thiophene, furan, and pyrrole in the gas phase by using Fourier transform mass spectrometry are described. Fe⁺, Fe(C₅H₅)⁺, and FeC₆H ₆ ⁺ yield exclusive rapid adduct formation with thiophene, furan, and pyrrole. In addition, the iron-diene complexes [FeC₄H ₆ ⁺ and Fe(c-C₅H₆)⁺], as well as FeC₅H₄(=CH₂)⁺ and FeO⁺, are quite reactive. The most intriguing reaction is the predominant direct extrusion of CO from furan by FeC₄H₆ ⁺, Fe(c-C₅H₆)⁺, and FeC₅H₄(=CH₂)⁺. In addition, FeC₄H ₆ ⁺ and Fe(c-C₅H₆)⁺ cause minor amounts of HCN extrusion from pyrrole. Mechanisms are presented for these CO and HCN extrusion reactions. The absence of CS elimination from thiophene may be due to the higher energy requirements than those for CO extrusion from furan or HCN extrusion from pyrrole. The dominant reaction channel for reaction of Fe(c-C₅H₆)⁺ with pyrrole and thiophene is hydrogen-atom displacement, which implies D^O(Fa(N₅H₅)⁺-C₄H₄X)>D^O(Fe(C₅H₅)⁺-H)=46±5 kcal mol^?1. D^O(Fe⁺-C₄H₄S) and D^O(Fe⁺-C₄H₅N)=D^O(Fe⁺-C₄H₆)=48±5 kcal mol^?1. Finally, 55±5 kcal mol^?1=D^O(Fe⁺-C₆H₆)>D^O(Fe⁺-C₄H₄O)>D^O(Fe⁺-C₂H₄)=39.9±1.4 kcal mol^?1. FeO⁺ reacts rapidly with thiophene, furan, and pyrrole to yield initial loss of CO followed by additional neutral losses. D^O(Fe⁺-CS)>D^O(Fe⁺-C₄H₄S)≈48±5 kcal mol^?1 and D^O(Fe⁺-C₄H₅N)≈48±5 kcal mol^?1>D^O(Fe⁺-HCN)>D^O(Fe⁺-C₂H₄)=39.9±1.4 kcal mil^?1.     

Observations of magic numbers in gas phase hydrates of alkylammonium ions

Hydration of alkylammonium ions under nonanalytical electrospray ionization conditions has been found to yield cluster ions with more than 20 water molecules associated with the central ion. These cluster ion species are taken to be an approximation of the conditions in liquid water. Many of the alkylammonium cation mass spectra exhibit water cluster numbers that appear to be particularly favorable, i.e., “magic number clusters” (MNC). We have found MNC in hydrates of mono- and tetra-alkyl ammonium ions, NH₃(C _m H_2m+1)⁺(H₂O) _n , m=1–8 and N(C _m H_2m+1) ₄ ⁺ (H₂O) _n , m=2–8. In contrast, NH₂(CH₃) ₂ ⁺ (H₂O) _n , NH(CH₃) ₃ ⁺ (H₂O) _n1 and N(CH₃) ₄ ⁺ (H₂O) _n do not exhibit any MNC. We conjecture that the structures of these magic number clusters correspond to exohedral structures in which the ion is situated on the surface of the water cage in contrast to the widely accepted caged ion structures of H₃O⁺(H₂O) _n and NH ₄ ⁺ (H₂O) _n .     

Studies of layered uranium (VI) compounds. VI. Ionic conductivities and thermal stabilities of MUO2PO4 · nH2O,where M = H,Li,Na,K,NH4 or 12Ca, and where n is between 0 and 4

We have measured the ionic conductivities of pressed pellets of the layered compounds MUO₂PO₄ · nH₂O, and correlated the results with TGA data. The conductivities (in ohm^?1 m^?1), at temperatures increasing with decreasing water content over the range 20 to 200°C, were approximately as follows: Li⁺4H₂O, 10^?4; Li⁺, Na⁺, K⁺, and NH₄⁺3H₂O, 10^?4, 10^?2, 10^?4, and 10^?4; H⁺, Li⁺, and Na⁺1.5H₂O, 10^?2, 10^?4, and 10^?4; Na⁺1H₂O, 10^?5; H⁺, K⁺, and NH₄⁺0.5H₂O, all 10^?5; and H⁺, Li⁺, Na⁺, K⁺, NH⁺₄, and $\text{1}\text{2}\text{Ca}{}^{\mathrm{2+}}\text{}\text{OH}{}_2\text{O}$, 10^?5, 10^?5, 10^?4, 10^?5, 10^?5, and 10^?6. A ring mechanism is proposed to account for the high conductivity found in NaUO₂PO₄ · 3.1H₂O. The accurate TGA data showed that most of the hydrates had water vacancies of the Schottky type, and should be represented as MUO₂PO₄(A ? x)H₂O, where x can be between 0 and 0.3.     

Hydrolysis of dioxouranium(VI): a calorimetric study in NaClaq and NaClO4 aq, at 25 °C

We report the results of a calorimetric study on the hydrolysis of UO₂²⁺ in different ionic media (NaClO_4 aq, NaCl_aq) at 25 °C. Experiments in NaCl were performed at different ionic strength, at I≤1 mol l⁻¹. The species considered in both ionic media were UO₂(OH)⁺, (UO₂)₂(OH)₂²⁺ and (UO₂)₃(OH)₅⁺, and in addition (UO₂)₃(OH)₄²⁺ and (UO₂)₃(OH)₇⁻ in NaCl_aq. The dependence on ionic strength of enthalpy changes in NaCl_aq was expressed by the simple linear equation ΔH_pq=ΔH°_pq+aI^1/2 (a, empirical parameter). Comparison with literature findings is given and some recommended values are reported.     

A flowing afterglow study of uranium tetraborohydride

Ionization-fragmentation of uranium(IV) tetraborohydride, U(BH₄)₄, by He⁺ and by N⁺/N₂⁺ yields, predominantly, U(BH₅)⁺ and U(B₂H₈)⁺, respectively. Attachment of thermal electrons yields U(BH₄)₄^? and ions of 1, 2, and 3 mass units less. Fluoride transfer with SF₆^?, BF₄^?, and UF_n^? (n = 5–7) and reactions with other small ions (O^?, O₂^?, NO₂^?, F^?, Cl^?, O₂⁺) are described.     

Kinetics of nucleophilic attack on coordinated organic moieties : XII. Addition of triphenylphosphine to [(C8H11)Co(C5H5)]BF4

A stopped-flow investigation of the reversible addition of Ph₃P to [(C₈H₁₁)Co(C₅H₅)]⁺ indicates the rate law, k_obs = k₁[Ph₃P] + k_?1. The low Δ2 of 21.0 ± 1.2 kJ mol^?1 and the negative ΔS2 of ?114 ± 5 J K^?1 mol^?1 are consistent with rapid addition to the enyl ligand. The higher Δ2 of 86.2 ± 5.1 kJ mol^?1 and the positive ΔS2 of +60 ± 17 J K^?1 mol^?1are as expected for the reverse dissociation. Preliminary studies show that the related complex [(C₇H₉)Co(C₅H₅)]⁺ is at least 65 times more electrophilic towards Ph₃P.     

Behavior of nitrite forms of nitrosylruthenium on extraction and back extraction of heterometalic Ru/Zn complexes with trioctylphosphine oxide

The state of ruthenium in conjugated phases upon extraction of trans-[Ru(¹⁵NO)(¹⁵NO₂)₄(OH)]^2? complex with tri-n-octylphosphine oxide (TOPO) in the presence of Zn²⁺ and subsequent back extraction with H¹⁵NO₃ and NH₃(concd.) solutions was studied by ¹⁵N NMR. Binuclear complexes [Ru(NO)(NO₂)_5?n (μ-NO₂) _n?1(μ-OH)Zn(TOPO) _n ] and [Ru(NO)(NO₂)_4?n (ONO)(μ-NO₂) _n?1(μ-OH)Zn(TOPO) _n ], where n = 2, 3, are predominant forms in extract. Kinetic restrictions for ruthenium extraction with TOPO solution in hexane and its back extraction with aqueous solutions of nitric acid and ammonia are eliminated in the absence of direct coordination of extractant to ruthenium. fac-Dinitronitrosyl forms [Ru(NO)(H₂O)₃(NO₂)₂]⁺, [Ru(NO)(H₂O)₂(NO₂)₂(NO₃)]⁰ (3 and 6 M HNO₃) and [Ru(NO)(H₂O)(NO₂)₂(NO₃)₂]^? (6 M HNO₃) prevail in nitric acid back extracts. Equilibrium constant at ambient temperature (0.05 ± 0.01) was assessed for the coordination of second nitrate ion to nitrosylruthenium dinitronitrato complex. Complex species [Ru(NO)(NO₂)₄(OH)]^2? and [Ru(NO)(NO₂)₃(ONO)(OH)]^2? prevail in ammonia back extract.     

Unimolecular and hydrolysis channels for the detachment of water from microsolvated alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+) clusters

We examine theoretically the three channels that are associated with the detachment of a single water molecule from the aqueous clusters of the alkaline earth dications, [M(H₂O) _n ]²⁺, M = Mg, Ca, Sr, Ba, n ≤ 6. These are the unimolecular water loss (M²⁺(H₂O) _n?1 + H₂O) and the two hydrolysis channels resulting the loss of hydronium ([MOH(H₂O) _n?2]⁺ + H₃O⁺) and Zundel ([MOH(H₂O) _n?3]⁺ + H₃O⁺(H₂O)) cations. Minimum energy paths (MEPs) corresponding to those three channels were constructed at the Møller–Plesset second order perturbation (MP2) level of theory with basis sets of double- and triple-ζ quality. We furthermore investigated the water and hydronium loss channels from the mono-hydroxide water clusters with up to four water molecules, [MOH(H₂O) _n ]⁺, 1 ≤ n ≤ 4. Our results indicate the preference of the hydronium loss and possibly the Zundel-cation loss channels for the smallest size clusters, whereas the unimolecular water loss channel is preferred for the larger ones as well as the mono-hydroxide clusters. Although the charge separation (hydronium and Zundel-cation loss) channels produce more stable products when compared to the ones for the unimolecular water loss, they also require the surmounting of high-energy barriers, a fact that makes the experimental observation of fragments related to these hydrolysis channels difficult.     

An algorithm for estimating the enthalpies of formation of amine molecular ions and immonium fragment ions

A method of estimating the enthalpies of formation of amino molecular ions, ΔH^? ([C_nH_2n+1NH₂]⁺˙) and of immonium ions ΔH([C_nH_2n+1N]⁺) is reported. It is based on the fact that CH₃ is isoelectronic with NH₂, CH₂ with NH and CH with N. Some calculated values of the enthalpies of formation of amine molecular ions and immonium ions are reported and estimates are made of the accuracy of such calculation.     

Thermal [1,5] sigmatropic rearrangements in (σ-7-cycloheptatrienyl)triphenyltin and (σ-5-cyclohepta-1,3-dienyl)triphenyltin: A 1H and 13C NMR reinvestigation

It has been confirmed by ¹H and ¹³C NMR spectroscopies that Sn(σ-C₇H₇)Ph₃ undergoes either 1,4- or 1,5-shifts of the SnPh₃ moiety around the cycloheptatrienyl ring with ΔH³ = 13.8 ± 0.4 kcal mol^?1, ΔS3 = ?5.6 ± 1.2 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.44 ± 0.14 kcal mol^?1. Similarly, (σ-5-cyclohepta-1,3-dienyl)triphenyltin undergoes 1,5-shifts with ΔH³ = 12.4 ± 0.6 kcal mol^?1, ΔS³ = ?11.2 ± 1.8 cal mol^?1 deg^?1, and ΔG³₃₀₀ = 15.76 ± 0.13 kcal mol^?1. It is therefore probable that Sn(σ-5-C₅H₅)R₃ and Sn(σ-3-indenyl)R₃ do not undergo 1,2-shifts as previously suggested but really undergo 1,5-shifts.