Complexes of first row transition metals

Table 8-2. Thermodynamic terms for the formation of 1 1 complexes of first row transition metals with 1,2-diaminoethane.

From the discussion earlier in this chapter, we know that a value of 16,000 cm-1 for A0 is typical of most complexes of a +3 first-row transition metal ion. For V3+, the free-ion B value is 860 cm-1, so the value 600cm-1 found for V3+ in the complex indicates a value of 0.70 for the nephelauxetic ratio, ft. All of these values are typical of complexes of first-row transition metal ions. Therefore, even though the identity of the bands may be uncertain, performing the analysis will lead to B and Dq values that will be reasonable only when the correct assignment of the bands has been made. 

Octahedral and distorted octahedral complexes of first-row transition metal atoms were subjected to X-ray charge density analysis in the pioneering studies of Iwata and Saito in the early 1970s (Iwata and Saito 1973). 

This redox behavior is typical of complexes of first row transition metal ions chelated by two thiometalates and Table 2 shows data for many of these species. The following conclusions can be drawn on the basis of these data. 

Similarly, the bonding in tetrahedral complexes of first row transition-metal ions is considered in terms of four equivalent sp3 hybrid orbitals (which are constructed from the 4s and 4p orbitals of the metal) oriented towards the vertices of a tetrahedron (Fig. 1-10). For a further discussion of the application of the valence bond method to transition-metal complexes, the reader is referred to publications by Pauling.4) The essential feature is that the bonding consists of localised, two-centre two-electron bonds. 

It is becoming increasingly common these days to find H atoms directly using X-ray diffraction methods. Success is not always guaranteed, but one s chances are generally better for hydride complexes of first row transition metals, as opposed to those of the heavier transition elements. 

The hybrid orbital type d2sp3 refers to a case in which the d orbitals have a smaller principal quantum number than that of the s and p orbitals (e.g., 3d combined with 4s and 4p orbitals). The sp3d2 hybrid orbital type indicates a case where the s, p, and d orbitals all have the same principal quantum number (e.g., 4s, 4p, and 4d orbitals) in accord with the natural order of filling atomic orbitals having a given principle quantum number. Some of the possible hybrid orbital combinations will now be illustrated for complexes of first-row transition metals. 

Electron delocalization between the metal atom and a polypyridine ligand and between polypyridine ligands within the coordination sphere is weak. (Complexes of first-row transition metals in intermediate oxidation states and complexes of a few unusual polypyridines (abpy) are the only exceptions.)... 

The Bp ligand is the simplest of all the heteroscorpionate ligands. It is basically bidentate, forming the typical boat-shaped H2B( x-pz)2M ring, in which the pseudoaxial B-H may form an agostic bond with the metal.4,5 [M(Bp)2] complexes of first row transition metals are usually square planar or tetrahedral.6,7 Some octahedral anionic [M(Bp)3]-species of low stability can be also isolated.8,9 Five-coordinate anionic species [MX(Bp)2]- (M = Cr, Mn X = halides or pseudohalides) have been also known.10,11... 

Luminescence is observed from coordination complexes of first row transition metals less often than from complexes of second or third row metals. This is a result of the presence of low-energy ligand field (LF) states into which the excitation is funneled that can rapidly deactivate through either non-radiative pathways to the ground state or photo-reaction pathways to products. Recent reports relating to the luminescence of titanium," chromium", and zinc complexes provide examples of new emissive first row coordination compounds. 

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