Block f : Inner Transition Metals (Lanthanoids and Actinoids)

However it is not entirely time to despair since at least one family of metals show a remarkable consistency in their chemistry. This occurs in the oft-ignored f block of the Periodic Table. The first row of that block is the lanthanoids. The lanthanoids (also called the lanthanides), were once called the 'rare earths', but are not particularly rare elements. They exist as the first row of the 14-wide f-block transition elements, where filling of the seven f orbitals is the key to their chemistry. All but promethium (for which the most stable isotope has a half-life of only 2.6 yrs) occur naturally. The scarcest naturally occurring element (thulium) is as common as bismuth and more common than arsenic, cadmium, mercury and selenium. They are named after lanthanum (La) electronic configuration [Xe]6s25d', a d-block element. Immediately after this element the 4f orbitals lie slightly lower in energy than the 5d and so fill first (with 14 electrons) up to Lu ([Xe]4f46s25d1) before returning to filling of the d shell. This means the f block breaks up the d block of the Periodic Table after the first column, but it is more traditional (and convenient, in the context of the shape of most printed pages) to represent the f block essentially as a subscript to the main Periodic Table in what is sometimes referred to as the short form of the Table. Because for bitals occupy quite different spatial positions, the shielding of one f electron by others from the effect of the nuclear charge is weak. Thus with increasing atomic number (and nuclear charge) the effective nuclear charge experienced by each f electron increases causing a shrinkage in the radii of the atoms or ions from La (1.06 Å) across to Lu (0.85 Å) called the lanthanide contraction.

As splitting of the degenerate f-orbital set in crystal fields is small, so crystal field sta- bilization issues are only of minor importance. Preferences between different coordination numbers and geometries are controlled dominantly by metal ion size and ligand steric effects. Coordination numbers greater than six are usual (Figure 6.2) with seven eight and nine all important. Examples with coordination numbers up to twelve exist. Coordination numbers of greater than nine are usually restricted to the f block, and rarely found in the d block. Some low coordination numbers (<6) are known but are rare and only exist when stabilized with particular ligands, such as aryloxy (-OR) or amido (NR2) ligands. Domi- nantly the lanthanoids exist in only one oxidation state, M(III) through loss of nominally the 6s25d' outer electrons. Certain metals form M(II) or M(IV) ions; their occurrence is related to the formation of especially stable empty filled or half-filled f shells, but these are readily and/or rapidly oxidized or reduced to the M(III) ion. Because the 4f electrons are essentially inner electrons, due to effective shielding, their spectroscopic properties are little affected by the surrounding ligands. Their coordination chemistry tends to be very similar for the whole family of elements, and as hard Lewis acids they have a common

Figure 6.2 O High coordination numbers are common in the f block. The didentate chelate ligand 2.4-dioxopentan 3-ido (acac−) forms six-coordinate octahedral complexes with first-row d-block metal ions (like Cr(III) shown at left) but eight or nine coordinate complexes with f-block lanthanide ions (like Eu(III) shown at right).

preference for O-donors though they can accommodate other donors even including forming some metal-carbon bonded species. All form [M(OH₂)]³⁺ (where n > 6), such as [Nd(OH2)₉]³⁺ although these readily hydrolyse; this tendency increases from La to Lu (as the ionic radius decreases). Chelating ligands give the most stable complexes, in line with expectations developed for d-block metals in Chapter 5; basically, they follow the normal rules of complexation developed in detail for d-block elements.

The row below the lanthanoids is the actinoids (also called the actinides), which are mainly synthetic elements. Those that are found on Earth naturally are isotopically long- lived thorium and uranium, but all are radioactive. After their d-block parent actinium, in principle the f orbitals are then filled for the following elements. However, energies of 5f and 6d are so close that elements immediately following Ac (and their ions) may have electrons in both 5f and 6d orbitals, at least until 4 or 5 electrons have been entered, when 5f alone seems to be more stable. This means that the early actinoid elements tend to show more d-block character (variable oxidation states and associated chemistry). Consequently, resemblance of the series to the parent is less marked than with the lanthanoids, at least until americium. Only after americium (about half-way along the series) are the elements similar and lanthanoid-like in chemistry, with only the M(III) oxidation state stable. Earlier elements, such as uranium, display oxidation states of up to M(VI); in fact, U(VI) is the most common oxidation state for that element. High coordination numbers (up to fourteen) are characteristic; for example [Th(NO₃)₆] is twelve-coordinate as each nitrate ion acts as an O.O-chelate ligand. Their solution chemistry is often complicated; hydrolysis in water (even to oxo species) is common. Elements above fermium are short-lived and isolable only in trace amounts; others can be prepared in gram or even kilogram amounts. Being radioactive and in most cases rare synthetic elements, they are not met by most scientists let alone in everyday life although they sometimes find an application.

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مواضيع ذات صلة

Block d: Transition Metals

Block p: Main Group Metals

= Block s : Alkali and Alkaline Earth Metals

A New Face - Oxidation-Reduction

A New Body - Stereochemical Change

Lability and Inertness in Octahedral Complexes

Kinetic Rate Constants

Thermodynamic and Kinetic Stability

Undergoing Change– Kinetic Stability

Overall Stability Constants

Factors Influencing Stability of Metal Complexes

Bedded Down– The rmodynamic Stability