How well does the Hartree–Fock model predict equilibrium geometries of transition metal complexes? Large‐scale LCAO–SCF studies on ferrocene and decamethylferrocene

Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba)

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C₅R₅)₂] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

Ferrocene derivatives with planar chirality and their enantioseparation by liquid-phase techniques

In the last decade, planar chiral ferrocenes have attracted a growing interest in several fields, particularly in asymmetric catalysis, medicinal chemistry, chiroptical spectroscopy and electrochemistry. In this frame, the access to pure or enriched enantiomers of planar chiral ferrocenes has become essential, relying on the availability of efficient asymmetric synthesis procedures and enantioseparation methods. Despite this, in enantioseparation science, these metallocenes were not comprehensively explored, and very few systematic analytical studies were reported in this field so far. On the other hand, enantioselective high-performance liquid chromatography has been frequently used by organic and organometallic chemists in order to measure the enantiomeric purity of planar chiral ferrocenes prepared by asymmetric synthesis. On these bases, this review aims to provide the reader with a comprehensive overview on the enantioseparation of planar chiral ferrocenes by discussing liquid-phase enantioseparation methods developed over time, integrating this main topic with the most relevant aspects of ferrocene chemistry. Thus, the main structural features of ferrocenes and the methods to model this class of metallocenes will be briefly summarized. In addition, planar chiral ferrocenes of applicative interest as well as the limits of asymmetric synthesis for the preparation of some classes of planar chiral ferrocenes will also be discussed with the aim to orient analytical scientists towards 'hot topics' and issues which are still open for accessing enantiomers of ferrocenes featured by planar chirality.

A combined theoretical and experimental study of chelidamate cadmium (II) complex, [Cd2(dpa)2(chel)2]⋅2[Cd(dpa)(chel)]⋅6H2O

A new chelidamate complex of Cd (II) ion, [Cd2(dpa)2(chel)2]⋅2[Cd(dpa)(chel)]⋅6H2O [(chel: chelidamate or 4-hydroxypyridine-2,6-dicarboxylate, dpa: di (2-picolyl)amine)] was synthesized and characterized by spectroscopic (UV-Vis and FT-IR spectroscopy) and structural (single-crystal X-ray diffraction) methods. Quantum chemical calculations were carried out by using Hartree Fock (HF) and Density Functional Theory (DFT)/B3LYP methods Stuttgart/Dresden (SDD) basis set. The asymmetric unit of the title compound contains two symmetry unrelated monomeric units, one dimeric unit and six water molecules of crystallization. The geometries around the Cd (II) metal centers in the units can be described as distorted octahedral for the monomeric units and distorted monocapped trigonal prism for the dimeric unit. The electronic structure of the complex was calculated using time dependent DFT (TD-DFT) method with polarizable continuum model (PCM). Molecular stability and bond strength were investigated by applying natural bond orbital analysis (NBO). The computed frequencies were compared with experimental frequencies.

A uniform approach to the description of multicenter bonding

A novel method for investigating the multicenter bonding patterns in molecular systems by means of the so-called Electron Density of Delocalized Bonds (EDDB) is introduced and discussed.

Ab initio study of structural and magnetic properties of TM(n)(ferrocene)(n+1) (TM = Sc, Ti, V, Mn) sandwich clusters and nanowires (n = infinity)

Structural and magnetic properties of multidecker sandwich clusters TM(n)(ferrocene)(n+1) [TM = V, Ti, Sc, Mn, ferrocene=FeCp(2), n = 1-3] and corresponding one-dimensional sandwich nanowires (n = infinity) are studied by means of gradient-corrected density functional theory. The TM(n)(FeCp(2))(n+1) clusters are highly stable polyferrocene-like sandwich structures due to strong Fe-Cp interaction. The total magnetic moment of TM(n)(FeCp(2))(n+1) (TM = V, Ti, Mn) increases linearly with the size n. More strikingly, Ti(n)(FeCp(2))(n+1) and V(n)(FeCp(2))(n+1) (n = 1-3) exhibit high magnetic moments 4, 8, 12 mu(B) and 1, 6, 11 mu(B), respectively. In contrast, Sc(n)(FeCp(2))(n+1) clusters are paramagnetic. The [TM(FeCp(2))](infinity) sandwich nanowires are ferromagnetic semiconductors whose band gap is 0.361, 0.506, 0.51, and 1.310 eV, respectively, for TM = Ti, Sc, V, and Mn. Among the four sandwich nanowires, [V(FeCp(2))](infinity) nanowire possesses the highest magnetic moment (5 mu(B)) per unit cell.

Substitution effect on the geometry and electronic structure of the ferrocene

The substitution effects on the geometry and the electronic structure of the ferrocene are systematically and comparatively studied using the density functional theory. It is found that -NH(2) and -OH substituents exert different influence on the geometry from -CH(3), -SiH(3), -PH(2), and -SH substituents. The topological analysis shows that all the C-C bonds in a-g are typical opened-shell interactions while the Fe-C bonds are typical closed-shell interactions. NBO analysis indicates that the cooperated interaction of d --> pi* and feedback pi --> d + 4s enhances the Fe-ligand interaction. The energy partitioning analysis demonstrates that the substituents with the second row elements lead to stronger iron-ligand interactions than those with the third row elements. The molecular electrostatic potential predicts that the electrophiles are expected to attack preferably the N, O, P, or S atoms in Fer-NH(2), Fer-OH, Fer-PH(2), and Fer-SH, and attack the ring C atoms in Fer-SiH(3) and Fer-CH(3). In turn, the nucleophiles are supposed to interact predominantly by attacking the hydrogen atoms. The simulated theoretical excitation spectra show that the maximum absorption peaks are red-shifted when the substituents going from second row elements to the third row elements.

The Equilibrium Structure of Ferrocene

The molecular structures of ferrocene in the eclipsed (equilibrium) and staggered (saddle-point) conformations have been determined by full geometry optimizations at the levels of second-order Møller-Plesset (MP2) theory, coupled-cluster singles-and-doubles (CCSD) theory, and CCSD theory with a perturbative triples correction [CCSD(T)] in a TZV2P+f basis set. Existing experimental results are reviewed. The agreement between the CCSD(T) results and experiment is in all cases excellent; the calculated structure parameters and the barrier to internal rotation of the ligand rings differ from the most accurate experimental values by less than two estimated standard deviations. The CCSD(T) calculations for single-configuration-dominated transition metal complexes such as ferrocene thus appear to have an accuracy comparable to that observed for molecules containing only first- and second-row atoms, and to be of a quality similar to that obtained experimentally. A comparison with previous DFT results indicates that the B3LYP model gives overall the best DFT results, with a deviation of around 2 pm for the metal-carbon distance and smaller errors for the cyclopentadienyl rings.

The molecular structure of [Sn(P2C2But2)] using gas-phase electron diffraction and DFT calculations

The molecular structure of 2,4-di-tert-butyl-eta4-1,3-diphosphacyclobutadiene tin has been determined in the gas phase by electron diffraction using both the DYNAMITE and SARACEN methods. The suitability of many different theoretical methods for the calculation of structures of half-sandwich main-group metal complexes has been investigated, and, by comparison of the results with the experimental structures, suggestions have been made as to the most suitable methods for this class of compound.

Topological studies of the charge density of some group 2 metallocenes M(η5-C5H5)2 (M = Mg or Ca)

Ab initio quantum chemical calculations using both the Hartree–Fock and the B3LYP density functional theory methods have been performed for the group 2 metallocenes M (η5-C5-C5H5)2 (M = Mg or Ca). The topology of the calculated charge density (ρ) and its Laplacian [Formula: see text] have been analysed using a new critical point search algorithm in order to understand why Mg(η5-C5-H5)2 is linear while Ca(η-C5H5)2 is bent. The Laplacian of the core of the Ca atom in Ca(η5-C5H5)2 is perturbed by the polarizing field of the cyclopentadienyl ligands and the bent geometry is a consequence of the interactions between the distorted core and the ligand atoms. In the case of Mg(η5-C5H5)2, charge concentration maxima in the Mg core occur along the vectors connecting the metal to the centroids of the cyclopentadienyl ligands irrespective of whether or not the molecule is linear, and the preferred geometry is linear as expected. The results of these calculations demonstrate that the geometries of the group 2 metallocenes can be understood in terms of the repulsive interactions between the ligands and between the ligands and the distorted core of the metal atom. Key words: atoms in molecules, group 2 metallocenes, VSEPR, charge density, Laplacian of ρ.