Transition-metal derivatives structural parameters

The first transition metal derivatives of a Zintl ion was prepared by Teixidor et al. in 1983 in reactions between Pt(PPli4)4 and en solutions of the Eg (E = Sn, Pb) [25, 26]. Despite being the first examples in this important class of clusters, the complexes have yet to be isolated and their structures and compositions remain unknown. The authors propose that complexes have a (PPh3)2PtSng stoichiometry and a nido-ty structure. Based on comparisons with NMR parameters from the past 30 years and the stoichiometry of the reactions described by Teixidor et al., we believe that the Rudolph compounds are most likely 22-electron cZos )-Pf E9Pt (PPh3) complexes. Our rationale is given below. 

Recently, the borylene complex [p-B(NMe2) (r 5-C5H5)Mn(CO)2 2] (1) was subject to detailed computational studies and the theoretically predicted and the experimentally derived structural parameters were found to be in very good agreement (Table I).117 Density functional theoretical studies have concluded that borylenes BX can be viable ligands in the design of transition metal complexes, which are thermodynamically stable with... 

Structural parameters for transition metal adsorption on transition metals epitaxial structures. The given bond length is the shortest distance between the adatom and a substrate atom. du is the spacing between the first and second layers of the epitaxial system, di3 is the spacing between the second and third layers, etc. The adatom-adatom and adatom-substrate bond lengths are derived from the determined structural paramaters. 

The most studied bidentate Schiff bases containing an N,0 donor set are those derived from substituted salicylaldehyde derivatives, since they have been widely used as ligands for many transition metal complexes.73 They have also been investigated extensively because of their meso-morphism in the solid state. Those which have been structurally characterized are collected in Table 7 together with some structural parameters of interest. 

The energy level diagram for Ti3+ in fig. 3.4 shows the manner by which the 2D spectroscopic term is resolved into two different levels, or crystal field states, when the cation is situated in an octahedral crystal field produced by surrounding ligands. In a similar manner the spectroscopic terms for each 3d" configuration become separated into one or more crystal field states when the transition metal ion is located in a coordination site in a crystal structure. The extent to which each spectroscopic term is split into crystal field states can be obtained by semi-empirical calculations based on the interelectronic repulsion Racah B and C parameters derived from atomic spectra (Lever, 1984, p. 126). 

The crystal field spectra and derived A0 and CFSE parameters for several garnets containing octahedrally coordinated trivalent transition metal ions are summarized in table 5.3. The values of A0 and CFSE reflect the variations of metal-oxygen distances in the garnet structures. 

Chapter 5 summarizes the crystal field spectra of transition metal ions in common rock-forming minerals and important structure-types that may occur in the Earth s interior. Peak positions and crystal field parameters for the cations in several mineral groups are tabulated. The spectra of ferromagnesian silicates are described in detail and correlated with the symmetries and distortions of the Fe2+ coordination environments in the crystal structures. Estimates are made of the CFSE s provided by each coordination site accommodating the Fe2+ ions. Crystal field splitting parameters and stabilization energies for each of the transition metal ions, which are derived from visible to near-infrared spectra of oxides and silicates, are also tabulated. The CFSE data are used in later chapters to explain the crystal chemistry, thermodynamic properties and geochemical distributions of the first-series transition elements. 

The same conclusion concerning the M-NHC bond structure and strength can be drawn from bond parameters derived from X-ray structure determinations of transition metal NHC complexes [109,116,117]. 

In the above paragraphs, we have already introduced several approximations in the description of the shift and relaxation rates in transition metals, the most severe being the introduction of the three densities of states Dsp E ),Dt2g(E ), and Deg E ). The advantage is that these values can be supplied by band structure calculations and that the J-like hyperfine field can sometimes be found from experiment. We have no reliable means to calculate the effective Stoner factors ai that appear in Eq. (2), and the disenhancement factors ki in the expression for the relaxation rate, Eq. (4), are also unknown. It is often assumed that k/ can be calculated from some /-independent function of the Stoner parameter k (x), thus k/ = k((X/). A few models exist to derive the relation k((x), all of them for simple metals [62-65]. For want of something better they have sometimes been applied to transition metals as well [66-69]. We have used the Shaw-Warren result [64], which can be fitted to a simple polynomial in rx. There is little fundamental justification for doing so, but it leads to a satisfactory description of, e.g., the data for bulk Pt and Pd. 

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