Group 3, the Lanthanides, and Actinides

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Metals with a d f" configuration, group 3 metals, lanthanides, and actinides, are usually classified as f-elements. Because they are highly electropositive, they form polarized bonds with p-block elements, including carbon and nitrogen. So far, two reaction mechanisms have been established for d f metals cr-bond metathesis, a 2o—2o process, and 1,2-addition, a [2ct—2jt] process (2cr stands for the two electrons involved in the transition state that come from a tr bond and 2jt indicates the two electrons involved in the transition state that come from a Jt bond) (Scheme 1). " Oxidative addition, another type of reaction mecharusm that is common for late transition metals, is absent from the chemistry of rare-earth metals or actinides. This is partly because of the lack of valence electrons, i.e., a d electronic configuration however, even for uranium, which has multiple accessible oxidation states, no genuine oxidative addition reactivity has been reported. The subject of C—H bond activation mediated by f-elements has been dis-cussed by several recent reviews. ... 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

Between barium (Group 2, element 56) and lutetium (Group 3, element 71), the 4f orbitals fill with electrons, giving rise to the lanthanides, a set of 14 metals named for lanthanum, the first member of the series. The lanthanides are also called the rare earths, although except for promethium they are not particularly rare. Between radium (Group 2, element 88) and lawrenclum (Group 3, element 103), are the 14 actinides, named for the first member of the set, actinium. The lanthanides and actinides are also known as the inner transition metals. 

Transition metah—found in the groups located in the center of the periodic table, plus the lanthanide and actinide series. They are all solids, except mercury, and are the only elements whose shells other than their outer shells give up or share electrons in chemical reactions. Transition metals include the 38 elements from groups 3 through 12. They exhibit several oxidation states (oxidation numbers) and various levels of electronegativity, depending on their size and valence. 

Transition metals The elements of Groups 3 through 12 and Periods 4 through 7, excluding the lanthanides and actinides. 

Groups 13-18 have their last electrons in the P orbitals. Transition metals, groups 3-12, have their last electrons in the d orbitals. The lanthanide and actinide series have their last electrons in the f orbital. 

A majority of the halides of the pretransition metals in groups 1 and 2 of the periodic table, as well as those of the lanthanides and actinides at the valence states of -1-2 and -1-3, are usually ionic compounds, whereas the nonmetals and metals with valency states >3 are usually covalent halides. 

Catalysts based on high-valent compounds of the early transition series (groups 3-5) and the lanthanides and actinides generally form high polymers these electron-deficient see Electron Deficient Compound) metal complexes are poor n-bases and have little ability to bond either into the a orbital on the /3-C-H bond or into the rx of the bound alkene. The later transition metals (groups 9 10) are far more electron-rich ... 

FORMATTONOFR—M BONDS (M = GROUP in METAL AND THE LANTHANIDES AND ACTINIDES) 3... 

Structural Studies.—The structures of some compounds of scandium and yttrium, in addition to those of some of the lanthanides and actinides, have been reviewed. The crystal structure of scandium selenate pentahydrate, Sc(Se04)3,5H20, has been determined by single-crystal JT-ray diffraction. The scandium atoms were shown to be octahedrally co-ordinated by the oxygen atoms of the selenate groups and water molecules. The mean Sc—O distances in the scandium co-ordination polyhedra vary from 2.08 to 2.10 A and the mean Sc—O bond lengths in the selenate groups vary from 1.61 to 1.64 A. Two formula units were found in each cell. 

In this chapter, we learned about the transition metals, which are located in Groups 3 through 12 in the periodic table. Compared to other metals, the transition elements are more stable and are therefore more often found pure in nature. The transition metals also include the lanthanides and actinides, two groups that are often displayed separately in the periodic table. The actinides contain the three heaviest naturally occurring elements in the periodic table—thorium, protactinium, and uranium. 

The lanthanides and actinides are sometimes called /-block transition elements because they have incompletely filled/subsheUs. Figure 8.3 distinguishes the groups of elements discussed here. 

Numerical Hartree-Fock calculations, free from basis set artifacts, have been used to establish that the ground state momentum densities of all the atoms and their ions can be classihed into three types [84,85]. Type I and III momentum densities are found almost exclusively in metal atoms He, N, all atoms from groups 1-14 except Ge and Pd, and all the lanthanides and actinides. These momentum densities all have a global maximum at p = 0 and resemble the momentum density shown in Fig. 19.3 for the beryllium atom. The maximum atp = 0 comes mainly from the outermost s-subshell, 2s in this case. Type I and III densities dilfer in that the latter have a secondary maximum that is so small as to be invisible on a diagram such as Fig. 19.3. Type II densities are the norm for non-metallic atoms and are found in Ge, Pd and all atoms from groups 15-18 except He and... 

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