Ligand field effects

Transition-metal complexes span an enormous range of stabilities. One of the principal aims of this chapter is to attempt to understand some of the factors which control these, and to determine the importance of ligand-field effects. Very extensive compilations of stability constants are available. 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

In the lanthanide and actinide series, arguments like these are greatly eased by the very small ligand field effects. Consider the reaction... 

The 15 trivalent lanthanide, or/ -block, ions La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, and Lu3+, which may be collectively denoted Ln3+, represent the most extended series of chemically similar metal ions. The progressive filling of the 4/orbitals from La3 + to Lu3 + is accompanied by a smooth decrease in rM with increase in atomic number as a consequence of the increasingly strong nuclear attraction for the electrons in the diffuse / orbitals (the lanthanide contraction). Thus, the nine-coordinate rM decrease from 121.6 to 103.2 pm from La3+ to Lu3+, and the eight-coordinate ionic radii decrease from 116.0 to 97.7 pm from La3+ to Lu3+ (2). Ligand field effects are small by comparison with those observed for the first-... 

Woodley M, Battle PD, Catlow CRA, Gale JD (2001) Development of a new interatomic potential for the modeling of ligand field effects. J Phys Chem B 105 6824... 

In Fig. 9.2 we present results of a first-of-a-kind study of the hydration of the first-transition-row metals within the quasichemical framework. The biphasic behavior of the actual hydration free energy is consistent with features inferred experimentally. Removing the ligand field effects reveals the linear decrease [12]. The results shown in Fig. 9.2 are largely outside the purview of extant simulation techniques, but are treated simply in the quasichemical framework developed below. 

On the basis of ligand field effects, would it be easier to form a trigonal bipyramid transition state from a square planar or a tetrahedral starting complex ... 

For trivalent 5 f ions, the ligand field effect and the additional stabilization (de-stabilization) due to the nephelauxetic effect are probably slight (5) and of the same magnitude. The accuracy of the / > determinations must therefore be high so as to distinguish one element from another. 

While the piecewise linear regions and energy maximum around +4 valence of Figures 9—13 are consistent with ligand-field effects, it is important to bear in mind that LFSE cannot by itself predict the energy difference between octahedral and tetrahedral Mn. 

Since the results of the previous sections indicate that the site preference and tendency toward migration of Mn or Co is strongly affected by the electron occupancy of the d levels split by ligand-field effects, it is possible that this may be the case for all of the 3d TM ions. 

Bithiazole, 4,4 -btz = (97), is not a diimine its iron(II) complex [Fe(4,4 -btz)3] is only weakly colored but its magnetic properties and Fe—N bond distances 1.970-1.973A suggest that the ligand field effect of 4,4 -btz is not enormously less than that of bipy. ... 

II Ligand field effects (from unpaired electrons in transition metal ions and complexes) Phosphorescence, lasers... 

Ligand field effects e.g. iron oxide reds and yellows) Charge transfer e.g. chromates, ultramarines)... 

This analysis of the chemical shifts of octahedrally coordinated Co(III) in terms of ligand field effects should point the way to many future studies... 

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