Richard A. Anderson

Reaction of (C5Me5)2Yb with fluorocarbons: formation of (C5Me5)4Yb2(µ-F) by intermolecular C–F activation

Addition of C6F6 and other fluoroaromatics and fluoroalkenes, though not C2F6 or CF3 CH3, to (C5Me5)2 Yb gives the mixed-valence complex (C5Me5)4Yb2(µ-F) with a linear, asymmetric YbII–F–YbIII bond.

Electronic spectra and geometries of HgX–3 in water and an assessment of various computing procedures for revealing hidden spectra

The electronic absorption spectra, in the visible and ultraviolet regions, of HgX–3(X = Cl, Br or I) have for the first time been obtained, by computer techniques, free from any contributions of HgX2 or HgX2–4. Four different independent methods are described and evaluated for this system and for general use. For systems where the desired spectrum is completely hidden below other bands, here for HgBr–3 and HgCl–3, a combination of two methods is needed. Additional tests to validate the computed spectra are described and advised. The spectra of HgX–3 were resolved into their component Gaussian bands and the transitions identified and assigned. This permitted the identification of HgCl–3 as planar with D3h symmetry, and Hgl–3 as pyramidal, with C3v symmetry, suggesting that the solvated species are trigonal-bipyramidal and tetrahedral, respectively. The spectrum of HgBr–3 appears to show features of both symmetries, but it is closer to pyramidal geometry.

Effect of ionic strength on stability constants. A study of the electronic absorption spectra of the mercuric halides HgX+, HgX2, HgX–3 and HgX2–4 in water

Thermodynamic stability constants log KT3 and log KT4, at 20 °C, are reported for HgX2+ X–⇌ HgX–3 and HgX–3+ X–⇌ HgX2–4, respectively, in water, where X = I, Br and Cl. The electronic absorption spectra of these reactions are very sensitive to ionic strength (I), and give, by a new approach, log KT3= 3.79 ± 0.01, 2.23 ± 0.02 and 0.70 ± 0.03 and log KT4= 2.03 ± 0.02, 1.40 ± 0.03 and 0.50 ± 0.05, for X = I, Br and Cl, respectively. Values determined at constant ionic strength, with added NaClO4, compare well with literature values determined by other techniques. To compute stability constants from digitised absorption spectra does not rquire constant ionic strength conditions, and thus the relations between K and I have been studied and they are discussed in terms of activity coefficients. The results are compared with the extended Debye–Huckel equation, employing appropriate distance of closest approach a parameters. The effect of added perchlorate on the reaction HgX2⇌ HgX++ X– is reported, and the activity coefficients of the neutral species HgX2 and HgXY are found to be independent of concentration and probably essentially unity. The reaction between HgX2 and added X– always involves a concentration range in which HgX2, HgX–3 and HgX2–4 are in equilibrium. Plots of log K against I½ indicate when a two-species equilibrium becomes a three-species equilibrium, and the ionic strength so identified tallies with that noted in appropriate sets of spectra. Since the KT values agree well with existing results this spectroscopic technique has great potential for determining thermodynamic stability constants in non-aqueous solvents, especially when electrochemical methods are not available.

Structure of mercury(II) halides in solution and assignment of their resolved electronic spectra

The electronic spectra of HgX2(X = Cl, Br and I) have been measured in 23 solvents, most for the first time, the remainder to resolve discrepancies. The peak maxima correlated linearly with dielectric constant and solvent polarity (Kosowers Z value) for protic solvents, because they can solvate the linear mercury(II) halides in the equatorial position. The spectra in water and methanol were fitted to Gaussian bands, and were shown to contain no hidden bands. The resolved and observed band maxima were essentially identical, and thus all the observed bands were confidently assigned. The solvent dependence and intensities of the 1Σ+u and 1Δu excited states are consistent with linear, or weakly bent upper states, contrary to some previous suggestions for HgX2 in solution. The only exceptions were HgI2 solutions in cyclohexane and 2,2,4-trimethylpentane: the separation between the two excited states was greater, and essentially constant, in all other solvents, and in these two solvents the 1Δu state is thus assumed to be strongly bent. As the remaining dipolar aprotic solvents had at least one donor atom containing a lone pair of electrons, the solvation thus obtaining around HgX2, while not correlating with solvent polarity, also did not alter the HgX2 bond angle from 180°.

Electronic spectra of anionic mixed halide complexes in solution: identification, computation of spectra and stability constants and assignment of transitions of [HgX2Y]– and [HgX2Y2]2–

The electronic absorption spectra, in the ultraviolet region, of [HgX2Y]– and [HgX2Y2]2–(X = Cl, Br or I, Y = Cl or Br) are here reported for the first time. Established computer techniques were used to obtain and validate the spectra of [HgX2Y]– free from any contributions of HgX2 or [HgX2Y2]2–. Very high mole ratios of added halide (Y) to HgX2 were required for complete reaction to form [HgX2Y2]2–(minimum 40 000). From the spectra the (stoichiometric) stability constants were computed, constant ionic strength conditions not being required, and hence by extrapolation the thermodynamic stability constants log K′3(T) and log K′4(T) were obtained as: 3.17 ± 0.01, 1.92 ± 0.05 and 0.99 ± 0.03 for log K′3(T) for [HgBr2Cl]–, [HgI2Br]– and [HgI2Cl]–, respectively; and as 0.25 ± 0.15, 0.84 ± 0.04 and –0.64 ± 0.06 for log K′4(T) for [HgBr2Cl2]2–, [HgI2Br2]2– and [HgI2Cl2]2–, respectively. The log K′3(T) values were confirmed within experimental error by an independent graphical approach. The spectra of [HgX2Y]– and [HgX2Y2]2– were resolved into their component Gaussian bands and the transitions identified and assigned. This permitted the identification of [HgBr2Cl]– as near planar, with close to C2v symmetry, and [HgI2Br]– and [HgI2Cl]2– as closer to pyramidal, with Cs symmetry, suggesting that the solvated species are approximately trigonal-bipyramidal and tetrahedral, respectively. This allowed [HgX2F]– structures to now be suggested as basically pyramidal for only [HgI2F]–. The resolved spectra of the [HgX2Y2]2– species indicated a distorted tetrahedral structure, with C2v symmetry, but a detailed band assignment was not possible. General trends now arising from this work and our earlier studies on the spectra of HgX2, [HgX3]– and [HgX4]2– are briefly reviewed.

The electronic spectra of the mixed mercury dihalides. Part 1. Computational procedures for calculating spectra, for a new route to equilibrium and formation constants, and the resolved spectra

Three possible methods for computing the spectra of the mixed mercury dihalides, HgCll, HgBrl, and HgBrCl, free from contributions of other species, are described and discussed as to their general applicability. The spectra studied are either those of equimolar mixtures of HgX2 and HgY2, or of HgX2 with added halide Y–. A new procedure is described for computing the formation constants of these species and the spectra have been resolved into their component bands. Each species is shown to contain three bands, even though this is not always apparent from inspection of the calculated spectra. Peak maxima have been found at 37 800, 46 600, and 53 400 for HgCll, 37 800, 43 200, and 51 210 for HgBrl, and 43 500, 47 400, and 57 200 cm–1 for HgBrCl. The other parameters of the resolved bands are given.

The electronic spectra of the mixed mercury dihalides. Part 2. Identification, equilibrium and formation constants, and assignment of transitions

The electronic absorption spectra of HgXY (HgCll, HgBrl, and HgBrCl) in water at 20 °C are calculated over the complete wavelength range, and shown not to be the mean of those of HgX2+ HgY2. For the reaction HgX2+ HgY2⇌ 2HgXY the spectra have been derived by two different techniques, which give the same profile. Equilibrium constants (log K), independent of added Na[ClO4], are: HgCll, 1.40 ± 0.15; HgBrl, 1.26 ± 0.10; HgBrCl, 0.70 ± 0.10. The reaction is exothermic. The replacement reaction HgX2+ Y–→ HgXY + X– has not previously been studied spectroscopically to obtain the formation constants (log K), which are 0.98 ± 0.05, 0.52 ± 0.09, and 0.23 ± 0.09, respectively. Accurate molar absorption coefficients could not be computed, but the spectral profiles are identical with the other results. The formation constants have been obtained by a new graphical method. The three independent methods have shown earlier incomplete spectra to be unreliable. Further, all the equilibrium and formation constants reported are the most accurate to date. The resolved spectral bands are discussed and assigned. The absence of the expected allowed transition 1Σ+→1Σ+(X) is explained as due to the proximity of 1Σ+(X) and 1Δ(Y) interacting by spin–orbit coupling to produce the representation 1Δ, to which transitions are forbidden.

Electronic spectra and geometries of HgX3– in water

The electronic spectra, in the visible and u.v. regions, of HgX3–(X = Cl, Br, or I) have for the first time been obtained, by a computer technique, free from any contributions of HgX2 or HgX42–, and the resolved bands are assigned and permit identification of HgCl3– as planar with D3h symmetry, and HgI3– as pyramidal, with C3v symmetry, which suggests that the solvated species are trigonal-bipyramidal and tetrahedral, respectively; HgBr3– has possibly intermediate or pyramidal geometry.