Description: This article discusses the butterfly dimer. Abstract: Treatment of CrCl2(THF)2 with NaOSitBu3 afforded the butterfly dimer [(tBu3SiO)Cr]2 (μ-OSitBu3)2 (1 2), whose d(CrCr) of 2.658(31) Å and magnetism were indicative of strong antiferromagnetic coupling. A Boltzmann distribution of low-energy 1A1, 3B1, 5A1, 7B1, and 9A1 states obtained from calculations on [(HO)2Cr]2(μOH)2 (1'2) were used to provide a reasonable fit of the μeff vs T data. Cleavage of 1 2 with various L (L=4-picoline, p-tolunitrile, tBuCN, tBuNC, Ph2CO, and PMe3) generated (tBu3SiO)2CrL2(1-L2). The dimer was oxidatively severed by Ph2CN2 to give (tBu3SiO)2Cr(N2CPh2)2 (2) and by RN3 at 23 ˚C to afford (silox)2Cr=NR (3-R) for bulky R (adamantyl (Ad), 2,6-iPr2-C6H3, 2,4,6-Me3-C6H2 = Mes, 2,6-Ph2-C6H3) and (tBu3SiO)2Cr(=NR)2 (4-R) for smaller substituents (R = 1-Naph, 2-Anth). X-ray structural studies were conducted on 1 2, square planar 1-(OCPh2)2, pseudo-Td 2 and pseudotrigonal 3-(2,6-Ph2-C6H3), whose S = 1 ground state was discussed on the basis of calculations of (H3SiO)2Cr = NPh (3"-Ph).

Description: This article discusses the correlation-consistent composite approach. Abstract: The correlation-consistent composite approach (ccCA), an ab initio composite technique for computing atomic and molecular energies, recently has been shown to successfully reproduce experimental data for a number of systems. The ccCA is applied to the G3/99 test set, which includes 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities. Improvements on the original ccCA formalism include replacing the small basis set quadratic configuration interaction computation with a coupled cluster computation, employing a correction for scalar relativistic effects, utilizing the tight-d forms of the second-row correlation-consistent basis set extrapolation of MP2 energies, ccCA results in an almost zero mean deviation for the G3/99 set (with a best value of -0.10 kcal molˉ¹), and a 0.96 kcal molˉ¹ mean absolute deviation, which is equivalent to the accuracy of the G3X model chemistry. There are no optimized or empirical parameters included in the computation of ccCA energies. Except for a few systems to be discussed, ccCA performs as well as or better than Gn methods for most systems containing first-row atoms, while for systems containing second-row atoms, ccCA is an improvement over Gn model chemistries.

Description: This article discusses disproportionation of gold(II) complexes. Abstract: A computational study of gold(II) disproportionation is presented for the atomic ion as well as complexes with chloride and neutral ligands. The Au²⁺ atomic ion is stable to disproportionation, but the barrier is more than halved to 119 kcal/mol in an aqueous environment vs 283 kcal/mol in the gas phase. For dissociative disproportionation of chloride complexes, the loss of chlorine, either as an atom (∆Gaq = +20 kcal/mol) or as an anion (∆Gaq = +15 kcal/mol) represents the largest calculated barrier. The calculated transition state for associative disproportionation is only 9 kcal/mol above separated Auᴵᴵ complex, a nonpolar solvent is preferred. With the exception of [Au(CO₃]²⁺, disproportionation of AuᴵᴵL₃ complexes to AuᴵL and AuᴵᴵᴵL₃ is exergonic in solution phase for the ligands investigated. The driving force is provided by the very favorable solvation free energy of the trivalent gold complex. The solvation free energy contribution to the reaction (∆Gsolv) is very large for small and polar ligands such as ammonia and water. Furthermore, calculations imply that choosing ligands that would yield neutral species upon disproportionation may provide an effective route to thwart this decomposition pathway for Auᴵᴵ complexes. Likewise, bulkier ligands that ...

Description: This article discusses the effects of covalency on the p-shell photoemission of transition metals. Abstract: The effect of the solid-state environment for an Mn cation in MnO on the Mn 2p- and 3p-shell x-ray photoemission spectra (XPS) has been investigated using ab initio relativistic wave functions for an embedded MnO6 cluster model of MnO. These wave functions include many-body effects due to the angular momentum coupling and recoupling of the open-shell electrons. They also include the covalent mixing of the metal d orbitals with ligand p orbitals. The treatment of covalency has not been included previously in ab initio theoretical studies of the 2p-shell XPS of transition-metal complexes. In this work, covalent interactions between the metal and ligands are treated on an equal footing with spin-orbit splittings. The four-component spinors used in these wave functions are optimized separately for the ground and for the 2p- and 3p-hole configurations. This orbital relaxation leads to a "closed-shell" interatomic screening of the Mn core hole. The different orbital sets optimized for the ground and core-ionized configurations mean that mutually nonorthogonal orbital sets are used to determine the matrix elements for the XPS relative intensities. The authors' treatment of the transition intensities is rigorous, ...

Description: This article discusses intertrimer and intratrimer metallophilic and excimeric bonding. Abstract: The interactions present in cyclic trinuclear coinage metal pyrazolates were studied computationally. Cuprophilic interaction was found to bind the singlet ground state of the dimer of trimers {[Cu(Pz)]₃}₂, overcoming electrostatic repulsion. The large variation in intertrimer separations found in the literature for coinage metal pyrazolates is consistent with the relatively weak metallophilic interaction. The emissive triplet excited-state geometry of {[M(Pz]₃}₂ is predicted by density functional calculations to show major geometric distortion caused by Jahn-Teller instability and excimeric M-M bonding. Large calculated Stokes' shifts, which are also observed for experimental models, are consistent with significant excited-state distortions for these materials. The major finding derived from the present study is that the intertrimer M⋯M contraction in the emissive T₁ state is much more than the intratrimer contraction in all {[M(Pz)]₃}₂ models, giving rise to a lower T₁ → S₀ phosphorescence energy in these models than in analogous monomer-of-trimer models. The observations made here point to a great potential for rationally tuning the emission properties of trinuclear coinage metal complexes through choice of the metal and ligands.