Bonding in Open-Shell Transition-Metal Complexes

Chemical Bonding in Open-Shell Transition-Metal Complexes 

The Chemical Bond Chemical Bonding Across the Periodic Table, First Edition. 

Stable multibonded transition metals followed [6] and led to the first experimental evidence of quintuple bonding in 2005 [7]. 

If a vector space representation of electronic states is chosen, that is, a basis-set expansion, two types of basis sets are needed. One for the many-electron states and one for the one-particle states. For the latter, two choices became popular, the molecular orbital (MO) [9] and valence bond (VB) [10] expansions. Both influenced the understanding and interpretation of the chemical bond. A bonding analysis can then be performed in terms of their basic quantities. Although both representations of the wave function can be transformed (at least partially) into each other [11,12], most commonly an MO analysis is employed in electronic structure calculations for practical reasons. Besides, a VB description is often limited to small atomic basis sets as (semi-)localized orbitals are required to generate the VB structures [13]. If, however, diffuse functions with large angular momenta are included in the atomic orbital basis, a VB analysis suffers from their delocalization tails. As a consequence, the application of VB methods can often be limited to organic molecules. 

In this chapter, we review quantum chemical theories developed to describe chemical bonding in open-shell (transition metal) compounds. We review some important electronic structure methods that provide us with the central ingredient for an analysis of the chemical bond, the electronic wave function. We then discuss how information from the electronic wave function is extracted for a qualitative interpretation of the electronic structure. For this purpose, different approaches are described to extract local quantities from quantum states. An example is the local spin concept, which can be employed to study spin-spin interactions in terms of a Heisenberg coupling model. Finally, the difficulty of describing electronic structures of open-shell molecules accurately is highlighted as an example. 

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I 8 Chemical Bonding in Open-Shell Transition-Metal Complexes... 

As an example of an open-shell transition-metal complex we discussed some of the pitfalls of present-day DFT and CASSCF calculations in determining accurate spin density distributions in open-shell transition-metal complexes. An accurate description of the spin density and of the electronic structure is mandatory for a subsequent qualitative analysis of the chemical bonding. This could only be accomplished by employing the DMRG algorithm to produce an accurate CASCI-type electronic wave function. 

This observation opens up a new possibility in the formation of C - O bonds for the already comphcated oxygenation reactions of organic substrates, i.e., the non-innocent behavior of the olefin in open-shell transition metal olefin complexes can allow a direct radical couphng of dioxygen with the coordinated olefin. 

DEALING WITH COMPLEXITY IN OPEN-SHELL TRANSITION METAL CHEMISTRY FROM A THEORETICAL PERSPECTIVE REACTION PATHWAYS, BONDING, SPECTROSCOPY, AND MAGNETIC PROPERTIES... 

Dealing with Complexity in Open-Shell Transition Metal Chemistry from a Theoretical Perspective Reaction Pathways, Bonding, Spectroscopy, and Magnetic Properties Frank Neese, William Ames, Gemma Christian, Mario Kampa, Dimitrios G. Liakos, Dimitrios A. Pantazis, Michael Roemelt, Panida Surawatanawong and Shengfaye... 

Studies of the bonding in some transition-metal complexes have identified a similarity between the He(I) photoelectron spectra of three-co-ordinate homoleptic bis(trimethylsilyl) amides of scandium and those of open-shell compounds. The two bands at ionization potential <9 eV were assigned to lone-pair orbitals. Bands corresponding to ionization from the metal 3d orbitals were considered to lie at ionization potentials >9eV or to be masked by the bands resulting from nitrogen lone-pair orbitals (> 8.1 eV). 

For an overall view of transition metal systems one has to confront a number of problems besides correlation, or sometimes as a part of it many electrons in open shells producing large number of close-lying electronic states, different spin multiplicities, magnetism, metal-metal bonds, core or " semicore electrons that occupy the same part of the space as the valence shell, fast electrons requiring relativistic corrections etc. In addition, many of transition metal systems of practical interest are highly complex and often incompletely characterized, so that careful modeling is to be added to the list of difficulties. 

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