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First-row transition metals, electronic

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. [Pg.310]

The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. [Pg.374]

The second category is the transition metal ions, all of which in Fig. 1 are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate (6-9). Their labilities are strongly influenced by the electronic occupancy of their d orbitals. This is illustrated by the divalent first-row transition metal ions, which should exhibit similar labilities to Zn2+ on the basis of their rM instead, however, their labilities encompass seven orders of magnitude. On a similar basis, the trivalent first-row transition metal ions might be expected to be of similar lability to Ga3+, but instead they exhibit a lability variation of 11 orders of magnitude, with Cr3 being at the... [Pg.6]

Solvent exchange on the first-row transition metal [M(solvent)6]2+ species has been subjected to extensive study, as is exemplified by Table III, which contains data for four solvent systems which have been particularly intensively studied (46, 47, 97, 99, 103, 110-117). The solvent exchange mechanism progressively changes from Ia to Id as the number of d-electrons increases and rM decreases for H20, MeOH, and MeCN solvents, as judged from the changes in sign of AV. It is also seen that lability decreases with increase in AHi substantially, as... [Pg.30]

As a consequence of its closed-shell electron configuration, zinc has a negative electron affinity, that is, the removal of an electron from Zn is exothermic. The electronegativity of zinc (1.588 PU) is intermediate between those of the alkaline earth metals and the first row transition metals and remarkably similar to that of beryllium (1.57 PU). [Pg.314]

Among binary transition-metal pnictides, only the first-row transition-metal phosphides have been analysed by XPS extensively, whereas arsenides and antimonides have been barely studied [51-61]. Table 2 reveals some general trends in the P 2p3/2 BEs for various first-row transition-metal monophosphides, as well as some metaland phosphorus-rich members forming for a given transition metal. Deviations of as much as a few tenths of an electron volt are seen in the BEs for some compounds measured multiple times by different investigators (e.g., MnP), but these... [Pg.111]

Our focus is on the most comprehensively studied series, the monophosphides of the first-row transition metals, whose structures successively distort from NaCl-type (ScP) to TiAs-type (TiP), NiAs-type (VP), MnP-type (CrP, MnP, FeP, CoP), and NiP-type, forming stronger metal-metal and phosphorus-phosphorus bonding with greater electron count (Fig. 11) [63-65], The P atoms are six-coordinate, but... [Pg.112]

A measure of the Lewis acidity of a metal ion is determined by its affinity for a pair of electrons, and the greater this affinity, the more stable the complexes formed by the metal ion will be. However, removing electrons from a metal to produce an ion is also related to the attraction the metal atom has for electrons. Therefore, it seems reasonable to seek a correlation between the stability constants for complexes of several metals with a given ligand and the total energy necessary for ionization to produce the metal ions. The first-row transition metal ions react in solution with ethylenediamine, en, to form stable complexes. We will consider only the first two steps in complex formation, which can be shown as follows ... [Pg.686]

The effective atomic number rule (the 18-electron rule) was described briefly in Chapter 16, but we will consider it again here because it is so useful when discussing carbonyl and olefin complexes. The composition of stable binary metal carbonyls is largely predictable by the effective atomic number (EAN) rule, or the "18-electron rule" as it is also known. Stated in the simplest terms, the EAN rule predicts that a metal in the zero or other low oxidation state will gain electrons from a sufficient number of ligands so that the metal will achieve the electron configuration of the next noble gas. For the first-row transition metals, this means the krypton configuration with a total of 36 electrons. [Pg.741]

XAS data comprises both absorption edge structure and extended x-ray absorption fine structure (EXAFS). The application of XAS to systems of chemical interest has been well reviewed (4 5). Briefly, the structure superimposed on the x-ray absorption edge results from the excitation of core-electrons into high-lying vacant orbitals (, ] ) and into continuum states (8 9). The shape and intensity of the edge structure can frequently be used to determine information about the symmetry of the absorbing site. For example, the ls+3d transition in first-row transition metals is dipole forbidden in a centrosymmetric environment. In a non-centrosymmetric environment the admixture of 3d and 4p orbitals can give intensity to this transition. This has been observed, for example, in a study of the iron-sulfur protein rubredoxin, where the iron is tetrahedrally coordinated to four sulfur atoms (6). [Pg.412]

Not all bare metal ions are highly reactive Cr+ and Mn+ are the two first-row transition metal ions that have low reactivity. The electronic configuration of the ion must be important in reactivity but may not be the only property of the ion that influences reactivity, shown by the reactions of these bare ions with S8 and P4 in comparison with other transition and non-transition metal ions. [Pg.370]


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