Germán Cavigliasso

The Mayer bond order as a tool in inorganic chemistry

The bonding in molecules is most often described using the classical chemical ideas of covalency (bond multiplicity) and ionicity (atomic charges). The Mayer bond order is a natural extension of the Wiberg bond order, which has proved extremely useful in bonding analysis using semi-empirical computational methods, and the Mulliken population analysis to ab initio theories. The usefulness of the Mayer bond order has been tested in a number of inorganic molecules including sulfur–nitrogen rings, halogen–oxide molecules and transition metal dichloride molecules. The basis set dependence of the Mayer bond order is tested through the case studies presented. It is shown that the bond order can be fully or partially decomposed into the contributions from symmetry types for many interactions of interest to the inorganic chemist. The power of this approach is shown by examining the bonding in a variety of systems and is illustrated by detailed studies of the role of the ring size and electron count on the bonding in S–N rings, the role of hypervalency in the relative stabilities of mixed hydrogen and halogen peroxide isomers and the importance of s–d hybridization in the 3d transition metal dichloride molecules.

Density functional study of the vibrational frequencies of Lindqvist polyanions

Abstract The structures and vibrational spectra of the Lindqvist polyanions [Mo6O19]2−, [W6O19]2−, [Mo6O19]3−, [VMo5O19]3− and [VMo5O19]4− have been calculated using density functional theory. The agreement between the previously reported vibrational spectra and the calculated values is extremely good. For [Mo6O19]2−, a detailed comparison between the performance of commonly used functionals and basis sets and between Hartree–Fock and density functional methods has been performed. Whilst all density functional methods perform adequately, the Hartree–Fock method overestimates the strength of the metal–oxygen bonding and this is reflected in poor reproduction of the stretching frequencies. The results suggest that the local density approximation with Slater type orbitals and relativistic corrections is best able to model both the structure and vibrational spectra of these polyanions when treated as pseudo-gas phase species. The calculations are extremely computationally demanding requiring precise energies and geometries and cannot presently be considered a standard task for the study of these large, heavy element cluster anions. The vibrational analyses confirm the high symmetry for these anions suggested by previous geometry calculations.

Spectroscopic and catalytic characterization of a functional FeIIIFeII biomimetic for the active site of uteroferrin and protein cleavage

A mixed-valence complex, [Fe(III)Fe(II)L1(μ-OAc)(2)]BF(4)·H(2)O, where the ligand H(2)L1 = 2-{[[3-[((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl](pyridin-2-ylmethyl)amino]methyl]phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe(III)Fe(II) and Fe(III)(2) oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3/2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = -2JS(1)·S(2), where J = -5.6 cm(-1)) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe(III)Fe(II) complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin-lattice relaxation. However, a phosphate-bound Fe(III)(2) complex showed an EPR spectrum due to population of the S(tot) = 3 state (J= -3.5 cm(-1)). The phosphatase activity of the Fe(III)Fe(II) complex in hydrolysis of bis(2,4-dinitrophenyl)phosphate (k(cat.) = 1.88 × 10(-3) s(-1); K(m) = 4.63 × 10(-3) mol L(-1)) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe(III). It is interesting to note that aqueous solutions of [Fe(III)Fe(II)L1(μ-OAc)(2)](+) are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complexs catalytic promiscuity.

Formyl group modification of chlorophyll a: a major evolutionary mechanism in oxygenic photosynthesis

We discuss recent advances in chlorophyll research in the context of chlorophyll evolution and conclude that some derivations of the formyl side chain arrangement of the porphyrin ring from that of the Chl a macrocycle can extend the photosynthetic active radiation (PAR) of these molecules, for example, Chl d and Chl f absorb light in the near-infrared region, up to ∼750 nm. Derivations such as this confer a selective advantage in particular niches and may, therefore, be beneficial for photosynthetic organisms thriving in light environments with particular light signatures, such as red- and near-far-red light-enriched niches. Modelling of formyl side chain substitutions of Chl a revealed yet unidentified but theoretically possible Chls with a distinct shift of light absorption properties when compared to Chl a.

A matrix isolation and DFT study of the generation and characterization of monomeric vapour phase platinum chlorides

Abstract Molecular platinum monochloride (PtCl) and platinum dichloride (PtCl 2 ) have been prepared from platinum atoms and chlorine doped argon in a hollow-cathode sputtering device, matrix isolated in solid argon and characterized using electronic, infrared and X-ray absorption spectroscopies together with high level DFT calculations.

Activation and cleavage of the N–N bond in side-on bound [L2M-NN-ML2] (L = NH2, NMe2, NiPr2, C5H5, C5Me4H) dinitrogen complexes of transition metals from groups 4 through 9

The activation and cleavage of the N-N bond in side-on bound [L₂M-NN-ML₂] (L = NH₂, NMe₂, N(i)Pr₂, C₅H₅, C₅Me₄H) dinitrogen complexes of transition metals in groups 4 through 9 have been investigated using density functional theory. Emphasis has been placed on Ti, Zr, and Hf (group 4) complexes due to their experimental relevance. Calculations on these species have shown that for cases when the structural configuration corresponds to the terminal [ML₂] fragments adopting a perpendicular orientation with respect to the central [N-N] unit, a considerably higher degree of N-N activation is predicted relative to that observed in the experimentally characterized cyclopentadienyl analogues and in related systems involving end-on dinitrogen coordination. An examination of the orbital interactions between the metal-based fragments and the dinitrogen unit shows that both σ and π bonding are important in the side-on binding mode, in contrast to the end-on mode where metal-nitrogen π interactions are dominant. This analysis also reveals that the model amide systems possess the orbital properties identified as necessary for successful N-N hydrogenation. A significant result obtained for the amide complexes containing metals from groups 5 (V, Nb, Ta), 6 (Cr, Mo, W), and 7 (Mn, Tc, Re), is the presence of metal-metal bonding in configurations that are considerably distorted from planarity. As a consequence, these complexes exhibit strongly enhanced stability relative to species where metal-metal bonding is absent. In contrast, the d² metal-based configurations in the group 4 complexes of Ti, Zr, and Hf are unable to provide the six electrons required for complete reductive cleavage of the dinitrogen unit which is necessary to allow the metal centres to approach one another sufficiently for metal-metal bond formation.

Dinitrogen activation by Fryzuk's [Nb(P(2)N(2))] complex and comparison with the Laplaza-Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems.

The reaction profile of N(2) with Fryzuks [Nb(P(2)N(2))] (P(2)N(2)=PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) complex is explored by density functional calculations on the model [Nb(PH(3))(2)(NH(2))(2)] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N(2) with the three-coordinate Laplaza-Cummins [Mo{N(R)Ar}(3)] and four-coordinate Schrock [Mo(N(3)N)] (N(3)N=[(RNCH(2)CH(2))(3)N](3-)) systems. When the model system is constrained to reflect the geometry of the P(2)N(2) macrocycle, the N--N bond cleavage step, via a N(2)-bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol(-1). In comparison, formation of the single-N-bridged species is calculated to be exothermic by 119 kJ mol(-1), and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N--N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol(-1), but the N(2) cleavage step remains endothermic by 35 kJ mol(-1). Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol(-1) and that of the N--N bond cleavage step by 85 kJ mol(-1), making it a favourable process. Replacing Nb(II) with Mo(III) has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol(-1). The reaction profile for the model [Nb(P(2)N(2))] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems. For both [Mo(N(3)N)] and [Nb(P(2)N(2))], the intermediate dimer is calculated to lie lower in energy than the products, although the final N-N cleavage step is much less endothermic for [Mo(N(3)N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}(3)], and therefore this system is predicted to be most suitable for dinitrogen cleavage.

Molecular and electronic structures of six-coordinate W complexes and polyanions containing tri-oxo groups

Abstract The molecular and electronic structures of [WO 3 (OL) 3 ] (L: CN, H 2 , H) complexes and of the [W 2 O 9 ] 6− and [W 4 O 16 ] 8− polyanions have been investigated using density-functional methods. Structural and bonding analyses have been performed employing various molecular-orbital and population approaches. Correlations of the trans -influence type between bonds to unshared and to high-coordinate oxygen atoms have been observed for geometrical and electronic parameters, such as WO distances and orders. The ( trans ) bridging bonds have been found to be (separately) rather weak, as suggested by their (long) lengths, but the valency and orbital-interaction results have indicated that the overall bonding capacity and strength are similar for all oxygen atoms in each individual polyanion.

Bonding in [W4O16]8− isopolyanions

Abstract The electronic structures of the T d and C 2 h forms of the [W 4 O 16 ] 8− polyanion have been investigated using density–functional methods. The C 2 h isomer has been found to be more stable than the T d isomer, primarily due to somewhat stronger WO orbital interactions. Various analyses based on molecular-orbital and bonding-energy methods have indicated that these WO interactions are predominantly of WdOp character. Delocalised bonds with a closed-loop structure involving bridging-oxygen atoms have been observed in both isomers.

Electronic Structure and Metal-Metal Interactions in Trinuclear Face-Shared [M3X12]3-(M = Mo, W; X = F, Cl, Br, I) Systems

The molecular and electronic structures of trinuclear face-shared [M3X12]3-species of Mo (X = F, Cl, Br, I) and W (X = Cl), containing linear chains of metal atoms, have been investigated using density functional theory. The possibility of variations in structure and bonding has been explored by considering both symmetric (D3d) and unsymmetric (C3v) forms, the latter having one long and one short metal-metal distance. Analysis of the bonding in the structurally characterized [Mo3I12]3- trimer reveals that the metal-metal interaction qualitatively corresponds to a two-electron three-center sigma bond between the Mo atoms and, consequently, a formal Mo-Mo bond order of 0.5. However, the calculated spin densities suggest that the electrons in the metal-metal sigma bond are not fully decoupled and therefore participate in the antiferromagnetic interactions of the metal cluster. Although the same observation applies to [Mo3X12]3- (X = Br, Cl, F) and [W3Cl12]3-, both the spin densities and shorter distances between the metal atoms indicate that the metal-metal interaction is stronger in these systems. The broken-symmetry approach combined with spin projection has been used to determine the energy of the low-lying spin multiplets arising from the magnetic coupling between the metal centers. Either the symmetric and unsymmetric S = 3/2 state is predicted to be the ground state for all five systems. For [Mo3X12]3- (X = Cl, Br, I), the symmetric form is more stable but the unsymmetric structure, where two metal centers are involved in a metal-metal triple bond while the third center is decoupled, lies close in energy and is thermally accessible. Consequently, at room temperature, interconversion between the two energetically equivalent configurations of the unsymmetric form should result in an averaged structure that is symmetric. This prediction is consistent with the reported structure of [Mo3I12]3-, which, although symmetric, indicates significant movement of the central Mo atom toward the terminal Mo atoms on either side. In contrast, unsymmetric structures with a triple bond between two metal centers are predicted for [Mo3F2]3- and [W3C12]3-, as the symmetric structure lies too high in energy to be thermally accessible.