Derek W. Smith

Applications of the angular overlap model

2. d d Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 2.1 Six-Coordinate Systems wi th Ground States o f Nondegenerate Octahectral Parentage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 2.1.1 Chromium(II I ) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 2.1.2 Cohalt(III) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 2.1.3 Nickel(II) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 2.2 Six-Coordinate Iron(II) Sys tems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 2.3 Five-Coordinate Sys tems (other than d 9) . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.3.1 The Pentaeldorovanadate(IV) Ion . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.3.2 Cobalt(H) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.3.3 Nickel(II) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 2.4 Triatomic Dichlorides of the Transi t ion Metals . . . . . . . . . . . . . . . . . . . . . . 97 2.5 Copper(II) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.5.1 CuN4X 2 Chromophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 2.5.2 Copper(II) Halide Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 2.5.3 Copper(II) Systems with Oxygen-donor Ligands . . . . . . . . . . . . . . . . . . 102 2.5.4 Five-Coordinate Copper(II) Sys tems . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.5.5 Miscellaneous Copper(II) Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 104 2.6 Other Aspects o f d d Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

ELECTRONEGATIVITY EQUILIBRATION AND ORGANOMETALLIC THERMOCHEMISTRY : THE STRENGTHS OF CARBON-CARBON BONDS IN METAL ALKYLS

Abstract Atomic fractional charges in a number of metal alkyls have been estimated by electronegativity equilibration. Assuming that the bond energy terms E (M–C) and E (C–H) are constant, but that E (C–C) depends on the fractional charges on the carbon atoms, the methylene increments in the enthalpies of formation of the alkyls MR 2 (M=Zn, Cd and Hg) can be rationalised, allowing reasonable estimates of the heats of formation in the absence of experimental data. The treatment has been extended to HgRX and to ER n (E=Group 13 or 14 element); the results provide clear evidence for the electronegativity sequence C>Si Sn∼Pb. Ab initio MO calculations at the MP2/6-311G** level support the proposition that the carbon–carbon bond is significantly weakened when one carbon is bonded to an atom of much lower electronegativity; this may contribute to the instability of complexes L n MEt with respect to L n M(H)(η 2 -C 2 H 4 ).

Stability Index Diagrams: Pictorial Representations of the Relative Stabilities of Oxidation States for Metallic Elements

Diagrams that plot the free energies of formation of aqueous species, or enthalpies of formation of ionic solids, against the oxidation number n for metallic elements have been exploited by a number of authors in order to display the relative stabilities of oxidation states. The lack of experimental data for unstable oxidation states, and difficulties in estimating required enthalpies/free energies of formation, have limited the utility of such diagrams. The stability index An of a metallic element M in the +n oxidation state is defined by the equation: An = DeltaHof(Mn+, g) - an(n + 1)/(r + b) where a is an empirical constant (taking the recommended value 1150 kJ mol-1A), and r is the metallic radius of M in A. b is a constant which reflects the covalent radius of the element bonded to M in a binary compound, and is allocated a representative value of 1.0 A, although this can be varied in order to compare, e.g., fluorides with iodides. Stability index diagrams plot An against n. Examples given for elemen...

ATOMIZATION ENTHALPIES OF METALLIC ELEMENTAL SUBSTANCES USING THE SEMI-QUANTITATIVE THEORY OF IONIC SOLIDS : A SIMPLE MODEL FOR RATIONALIZING PERIODIC TRENDS

Semi-quantitative interpretations of periodic trends in atomization enthalpies bring students to a better understanding of ionization enthalpies, ionic radii, and much else.