Bonding in Transition-Metal Complexes

Arier reading this chapter you should have an understanding of 

An equivalent value for Sb is best derived from the good linear correlation found for eQ values for isoelectronic, isostructural compounds of tin and antimony (Bancroft, Butler Libbey, 1972 Bertazzi, Gibb Greenwood, 1976) giving eQi2 19 22 = 3.40 and 3.19. An appropriate average value of cQiji (/j2= 1) is therefore —26 mm/s. 

These values are summarised in Table 2.31, and their use is illustrated below. 

4 Bonding in transition metal complexes 4.1 Deductions from partial isomer shift and partial quadruple 

While the p.i.s. and p.q.s. parameters yield valuable information about the degree of covalency in the metal-ligand bond, their value is enhanced when they are considered together. 

Both parameters are sensitive to the donor-acceptor characteristics of a ligand, but they respond differently. The partial isomer shift represents the contribution to s electron density at the nucleus, which is increased both by o-donation, and by Ti-withdrawal of d electron density. The partial quadrupole splitting represents charge placed in the d orbitals, and becomes more negative as a-donation increases, but more positive with increase in metal to ligand rr-bonding. This means that 

The importance of hybridization (see Addendum 3.11), between d-, s- and p-metal orbitals in chemical bonding is easily understood for the molecular organometallic transition-metal complexes. The bonding in the tetrahedral Ni(C0)4 complex, for instance, can best be understood by initially considering the 5d-atomic orbitals doubly occupied with 10 electrons. 

The 4s and three 2p orbitals of Ni are empty and can hybridize into four empty equivalent sp orbitals. Combination with the four doubly occupied COs orbitals leads to the formation of four bonding and four antibonding r-type orbitals. The corresponding molecular orbital scheme is shown schematically in Fig. 3.14. The bonding a as well as the non-bonding d-orbitals are occupied. The Ni(CO)4 complex is further stabilized by 

The Ni(CO)4 sp hybridization scheme is based on the formation of the four bonding sp orbitals directed towards the comers of a tetrahedron. The prototype example for such bonding is CH4. The two other hybridization schemes of importance are d sp , giving 6 directed orbitals for octahedral coordination, and dsp, given four directed orbitals for a square planar coordination. The latter is characteristic for complexes which contain 16 valence electrons, e.g. PtCl4. In PtCl4 two a electrons are counted for each Cl ion. 

The recombination of Co(CO)4 to Co2(CO)8 can also be understood within the octahedral d sp hybridization scheme. Let us put six of the nine Co d-electrons into the three nonbonding Co d-orbitals. Four of the six d sp orbitals form bonding orbitals with the four 5(7 CO orbitals, that each donate two electrons. Two lone-pair type d sp orbitals are left. One orbital can be doubly occupied whereas the other contains the final electron that is left. The latter orbital can combine with an equivalent orbital of another Co(CO)4 radical. A Co(CO)3 fragment contains three dangling d sp orbitals each of which is occupied with one electron. Hence it can combine with three other such fragments to form Co4(CO)i2 (see also Chapter 5,pages 226, 227). 

We note that in clusters as on the surfaces the contribution of metal d as well as s and p electrons to the formation of hgand or adsorbate bonds plays a very important role. Electron donation of the 5a-ligand orbitals can glue together cluster-atom fragments that otherwise would not be stable. Comparison of Fig. 3.17 and 3.6 indicates that there are many similarities between the interaction of CO on a transition-metal surface and a carbonyl complex. A main difference is the low CO/metal atom ratio at the surface as compared to that in the carbonyl complex. Hence binding of CO to a transition-metal surface tends to weaken metal atom bonds, whereas in carbonyl complexes the binding to CO is essential to the stability of the complex. 

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The detailed theory of bonding in transition metal complexes is beyond the scope of this book, but further references will be made to the effects of the energy splitting in the d orbitals in Chapter 13. 

A new look at structure and bonding in transition metal complexes. J. K. Burdett, Adv. Inorg. Chem. Radiochem., 1978, 21,113-146 (73). 

A New Look at Structure and Bonding in Transition Metal Complexes Jeremy K. Burdett... 

Metal-metal bonds in transition metal complexes 16... 

In this chapter, we survey the diversity of transition metals, beginning with an overview. Then we describe the stmcture and bonding in transition metal complexes. We describe metallurgy, the processes by which pure metals are extracted from mineral ores. The chapter ends with a presentation of some properties of transition metals and their biological roles. 

Moreno M, Aramburu JA, Barriuso MT (2003) Electronic Properties and Bonding in Transition Metal Complexes Influence of Pressure 106 127-152 Morita M, Buddhudu S, Rau D, Murakami S (2004) Photoluminescence and Excitation Energy Transfer of Rare Earth Ions in Nanoporous Xerogel and Sol-Gel SiC>2 Glasses 107 115-143... 

Indeed, the general tendency toward 3c/4e bonding in transition-metal complexes is so pronounced that such hypervalency should be considered the rule, rather than the exception, in transition-metal chemistry. 

Optical Spectra and Chemical Bonding in Transition Metal Complexes Special Volume II dedicated to Professor Jorgensen... 

As pointed out in Section 3.1, the particular nature of the Si-H bond in transition metal complexes may explain some features of hydrosilylation. 

The electrostatic theory of the preceding section is the starting point for a more complete treatment of the bonding in transition metal complexes, in which the covalency of the interactions is taken into account. 

Metal-metal bonding in transition metal complexes of low nuclearity (i.e., with only a few metal atoms) tends to be more directed and therefore stronger than the bonding in metals discussed in chapter 11. Accordingly, the metal-metal bonds in transition metal complexes are often localized and considerably shorter than those in most extended solids. Charge accumulations are frequently observed in metal-metal bonding regions of deformation density maps. 

The bonding in transition metal complexes has been elucidated in some detail by Albright et al. [58]. The reader is directed to that source for a thorough development... 

Complexes containing anions of the above formulation have attracted a large number of studies because of their alleged simplicity. This is illustrated by the central position such complexes have played in the evolution of crystal field, ligand field and molecular orbital models of bonding in transition metal complexes. 

The recent developments in generalized Valence Bond (GVB) theory have been reviewed by Goddard and co-workers,13 and also the use of natural orbitals in theoretical chemistry,14 15 and the accuracy of computed one-electron properties.18 The Xa method has been reviewed by Johnson,17 and Hurley has discussed high-accuracy calculations on small molecules.18 Several other reviews of interest have appeared in Advances in Quantum Chemistry.17 Localized orbital theory has been reviewed by England, Salmon, and Ruedenberg,19 and the bonding in transition-metal complexes discussed by Brown et a/.20 Finally, the recent developments in computational quantum chemistry have been reviewed by Hall.21... 

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