Bond Strengths in Transition Metal Complexes

39 For a guide to a detailed understanding of the subject, the reader is encouraged to study the somewhat dated but highly enlightening contribution of Carter and Goddard, 1988. 

Method SctT Till1 VH+ Crll1 MntC Fetf CoET NiH+ Cull1 MADe 

During the last years, more and more researchers have applied density functional theory to small transition-metal complexes and benchmarked the results against either high level wave function based methods or experimental data. A particular set of systems for which reasonably accurate benchmark data are available are the cationic M -X complexes, where X is H, CH3 or CH2. Let us start our discussion with the cationic hydrides of the 3d transition-metals. 

Method scch TiCH vch CrCH MnCH FeCH coch NiCH cuch MADd 

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Blomberg, M. R. A., Siegbahn, P. E. M., 1998, Calculating Bond Strengths for Transition Metal Complexes in Computational Thermochemistry, Irikura, K. K., Frurip, D. J. (eds.), American Chemical Society Symposium Series 677, Washington, DC. 

Blomberg, M. R. A., Siegbahn, P. E. M., 1998, Calculating Bond Strengths for Transition Metal Complexes in... 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

In this contribution we shall present several applications of the new method, which we shall refer to as LSD/NL, to the calculation of bond energies of transition metal complexes. We shall focus on trends along a transition period and/or down a transition triad. The following subjects will be discussed a) metal-metal bonds in dimers of the group 6 transition metals b) metal-ligand bonds in early and late transition metal complexes c) the relative strength of metal-hydrogen and metal-methyl bond in transition metal complexes d) the metal-carbonyl bond in hexa- penta-and tetra-carbonyl complexes. 

The Relative Strengths of the Metal-Hydrogen and the Metal-Methyl Bonds in Transition Metal Complexes 14,21,22). 

Given the approximate nature of these calculations, the frequency factor for bimolecu-lar electron transfers in transition-metal complexes can be taken as v = 10 M sec With this pre-exponential factor and the barrier calculated above for the Fe(Cp)2/Fe(Cp)2 exchange, AG = 26.5 kJ mol with n = 1.25, we obtain kjjM = 2.1X10 M sec . The experimental rate in acetonitrile, extrapolated to zero ionic strength, is 9.3x10 M sec [21]. It could be anticipated that the calculated rate would be an upper limit for the experimental values in solution because the bonding character of the t2 a/ orbitals was overemphasised. With = 1, a lower limit for this parameter, the calculated rate is = 5.0X10 M sec . ... 

Calculations of the electronic structure of the complexes of this type were first carried out for [Tc2C18]3, using the semi-empirical EHT method [146], It was shown that a strong quadruple M-M bond is formed in both the d4-d4 binuclear transition-metal complexes and d4-d5 technetium complexes. The addition of an excess electron does not decrease the Tc-Tc bond strength,... 

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