Electronic block metal complexes, 105

Figure 1.8 Molecular orbital diagram for an octahedral d-block metal complex ML6. The vertical arrows indicate different types of electron transition that may be brought about by photon absorption...

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

In the case where light absorption leads to an electronic transition between orbitals which are largely metal in character, a d-d band will be observed for a d-block metal ion and an f-f transition for an f-block metal ion. In some cases, transitions between the f and n + l)d orbitals are also observed in the spectra of f-block metal complexes. Not all of the possible electronic transitions in a metal complex are allowed to occur and selection rules govern which electronic transitions may be observed (Box 7.1). 

The next two sections deal briefly with the consequences of symmetry on observed bands in infrared spectra and with the relationship between molecular S5anmetry and chirality. In Chapter 20, we consider the electronic spectra of octahedral and tetrahedral J-block metal complexes and discuss the effects that molecular S5anmetry has on electronic spectroscopic properties. 

Ni(H20)g] (d ) and [Zn(H20)g]" (d ) to vary as the electronic configuration of the metal ion changes. However, each of these species has an octahedral arrangement of ligands (19.1). Thus, it is clear that VSEPR theory is not applicable to rZ-block metal complexes. 

The Kepert model rationalizes the shapes of <5 -block metal complexes [ML ], [ML ] or [ML ] by considering the repulsions between the groups L. Lone pairs of electrons are ignored. For coordination numbers between 2 and 6, the following arrangements of donor atoms are predicted ... 

Bonding in d-block metal complexes crystal field Electronic spectra... 

Crystal field theory can bring together structures, magnetic properties and electronic properties, and we shall expand upon the last two topics later in the chapter. Trends in CFSEs provide some understanding of thermodynamic and kinetic aspects of J-block metal complexes (see Sections... 

Ligand field, like crystal field, theory is confined to the role of d orbitals, but unlike the crystal field model, the ligand field approach is not a purely electrostatic model. It is a freely parameterized model, and uses and Racah parameters (to which we return later) which are obtained from electronic spectroscopic (i.e. experimental) data. Most importantly, although (as we showed in the last section) it is possible to approach the bonding in d-block metal complexes by using molecular orbital theory, it is incorrect to state that ligand field theory is simply the application of MO theory. ... 

A characteristic feature of many d-block metal complexes is their colours, which arise because they absorb light in the visible region (see Figure 20.4). Studies of electronic spectra of metal complexes provide information about structure and bonding, although interpretation of the spectra is not always straightforward. Absorptions arise from transitions between electronic energy levels ... 

Since these selection rules must be strictly obeyed, why do many J-block metal complexes exhibit d-d" bands in their electronic spectra ... 

In this chapter, we discuss complexes of the -block metals and we consider bonding theories that rationalize experimental facts such as electronic spectra and magnetic properties. Most of our discussion centres on first row J-block metals, for which theories of bonding are most successful. The bonding in fif-block metal complexes is not fundamentally different from that in other compounds, and we shall show applications of valence bond theory, the electrostatic model and molecular orbital theory. 

On going from the first to the second edition of the text, we significantly modified the chapters on symmetry and molecular orbital theory. These changes have been well received, and as a result, we have developed the discussion of vibrational spectroscopy still further to include the use of character tables to determine the symmetry labels of the vibrational modes and which modes of vibration are IR and/or Raman active. We have also significantly altered the discussion of term symbols and microstates in the introduction to electronic spectroscopy of /-block metal complexes. 

Ligand-field theory is concerned only with the low-lying electronic states of a d- or y block metal complex which can be described by the rearrangements of electrons within the d- or y shell. These states describe the magnetic properties (paramagnetism, g-values) and the ""d-d (or ""f-f ) electronic spectra (color, linear, and circular dichroism). 

Further details and applications of electronic absorption spectra and the assignment of absorptions are given in Sections 17.4 (charge transfer complexes of the halogens), 19.5 (colours of d-block metal complexes), 20.7 (electronic spectra of d-block metal complexes) and 27.4 (electronic spectra of /-block metal complexes). 

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