Transition-metal atoms, molecular systems

The study of molecular systems containing metal atoms, particularly transition metal atoms, is more challenging than first-row chemistry from both an experimental and theoretical point of view. Therefore, we have systematically studied (3-5) the computational requirements for obtaining accurate spectroscopic constants for diatomic and triatomic systems containing the first- and second-row transition metals. Our goal has been to understand the diversity of mechanisms by which transition metals bond and to aid in the interpretation of experimental observations. 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

Small molecular systems containing transition metal atoms are interesting both in themselves and as model systems (see e.g. the recent reviews by Harrison [7] and Noguera [8]). They are also useful as (quite difficult) trial systems for theoretical methods. 

Some general aspects related to the derivation, and interpretations of ELF analysis, as well as some representative applications have been briefly discussed. The ELF has emerged as a powerful tool to understand in a qualitative way the behaviour of the electrons in a nuclei system. It is possible to explain a great variety of bonding situations ranging from the most standard covalent bond to the metallic bond. The ELF is a well-defined function with a nice pragmatic characteristic. It does not depend neither on the method of calculation nor on the basis set used. Its application to understand new bond phenomenon is already well documented and it can be used safely. Its relationship with the Pauli exclusion principle has been carefully studied, and its consequence to understand the chemical concept of electron pair has also been discussed. A point to be further studied is its application to transition metal atoms with an open d-shell. The role of the nodes of the molecular orbitals and the meaning of ELF values below 0.5 should be clarified. 

This molecule was the first system containing a transition-metal atom to be studied with the CASSCF method . It is the simplest molecule containing a nickel atom, and is therefore a suitable prelude to studies of more complex nickel-containing systems. Some molecular parameters are also known experimentally, and can be used to judge the quality of the calculations. 

Although the theorem of DFT is in terms of the total electron density, in practical calculations the density is described in terms of orbitals, which are solved self-consistently as in HF theory. The wild popularity of DFT is due to its low computational expense (comparable to HF) and generally good accuracy, even for difficult systems such as radicals that contain transition metal atoms. For high precision, however, DFT is inappropriate, since convergent series are unavailable. Current functionals do not include the dispersion interaction, which is necessary for describing weakly bound molecular complexes [94]. Thus, DFT methods should be avoided wherever van der Waals interactions are important. 

As mentioned above, the PCI-X method was originally designed for the treatment of transition metal system. Therefore this procedure has to be tested for this kind of system too. The first test of the PQ-80 scheme on transition metal systems is performed on the atomic spectra, which is the only set of very accurate experimental data available. The s s° excitation was calculated for the neutral second row transition metal atoms and the first row transition metal cations, The s -> s excitation was calculated for both the first and the second row neutral transition metal atoms. In Table 3 we summarize the mean absolute deviation from experimental data on these excitations at the MCPF and the PCI-80 levels. A few technical comments on the results in Table 3 should be made. The first comment concerns the s states. As already noted above for first row atoms, valence excitations from s to p introduce additional problems. To avoid this problem for the s states of the transition metal atoms, the correlation energy associated with these two electrons is excluded from the extrapolation procedure. This somewhat awkward procedure was used to obtain the results in Table 3. but is entirely restricted to these atomic calculations. To avoid this problem for the molecular bond strengths, the calculated s asymptote should be used and experimental excitation energies should be used to obtain the final bond strengths. This has been done for most of the molecular results given below. Second, in... 

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

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