Comprehensive Inorganic Chemistry || SOLUTION CHEMISTRY
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University of Lund
J. O. L I U E N Z I N and J. RYDBERG
Chalmers University of Technology
1. GENERAL PROPERTIES OF ACTINIDE IONS 1.1. Electronic structure and oxidation states
The actinide solution chemistry reflects the peculiar electron configuration of the actinide elements. The 5/and 6d shells are of similar energy and the 5/electrons are, moreover, not so well shielded as the 4/electrons of the formally analogous lanthanide series. These features are especially marked at the beginning of the series. As a consequence, the redox chemistry of these elements is extremely complicated and some of them exhibit an extensive array of different oxidation states. The standard potentials of the various couples are often so close that appreciable concentrations of several oxidation states can coexist in the same solution. A survey of the known oxidation states of the actinides, and for comparison those of the lanthanides, is given in Table 1 where the most stable state is marked for each element.
TABLE 1. OXIDATION STATES OF THE ACTINIDES AND THE LANTHANIDES (The most stable oxidation states are in bold figures, those not known in solutions are within parentheses.)
(3) 4 5
3 4 5 6
60 Nd (2) 3
3 4 5 6 7
3 4 5 6 7
62 Sm 2 3
95 Am (2) 3 4 5 6
63 Eu 2 3
97 Bk (2) 3 4
101 Md 2 3
69 Tm (2) 3
102 No 2 3
70 Yb 2 3
At the very beginning of the actinide series, the 6d shell is still preferred before the 5/ shell. Actinium and thorium therefore behave chemically as true homologs of lanthanum and hafnium, respectively. Trivalent thorium may possibly exist in the solid state, in the very easily oxidized triiodide, but certainly not at all in aqueous solutions1. For gaseous atoms in their ground state, the 5/state is preferred from protactinium on, and this shell is therefore progressively filled. In the metallic state, on the other hand, the 6d shell is certainly preferred as far as uranium, or possibly neptunium, at least for the crystal modifications stable at ordinary temperature. This is evident from the variation of the metallic radius deduced from these structures (Table 2). Such a rapid contraction, as initially observed, is generally found when an outer electron shell is successively filled, owing to the increased attraction exerted
1 D. E. Scaife and A. W. Wylie, / . Chem. Soc. 1964, 5450. 465
466 THE ACTINIDES: S. AHRLAND, J. O. LIUENZIN AND J. RYDBERG
TABLE 2. METALLIC AND IONIC RADII* OF THE ACTINIDES, AND THE INTERATOMIC DISTANCES IN THE ACTINYL(V) AND A C T I N Y L ( V I ) IONS ()
Ac Th Pa U Np Pu Am Cm Bk
l-88b 1-80 1-63 1-56 1-55 1-60 1-74 1-75
1005 0-986 0-974 0-962 0-946 0-935
0-984d 0-944 0-929 0-913 0-896 0-888 0-886 0-870
0-90e 0-88 0-87 0-87 0-86
0-83e 0-82 0-81 0-80
l-98f 1-94 1-92
According to the usual conventions, the metallic radii refer to the coordination number 12, the ionic radii to the coordination number 6 (see text and refs. 18 and 27).
b Ref. 2. c J. R. Peterson and B. B. Cunningham, / . Inorg. Nucl. Chem. 30 (1968) 1775. d J. R. Peterson and B. B. Cunningham, Inorg. Nucl. Chem. Letters, 3 (1967) 327. e Ref. 27. f Ref. 24. See text.
on the electrons in outer shells by the increasing nuclear charge. Beyond neptunium, however, a sudden increase of the radius takes place; this is certainly due to a switch of electrons from the 6d to the 5/shell. Such a change of configuration would tend to increase the metallic radius for two reasons. First, it would bring about a shielding of the outer shells from the nuclear charge and hence prevent their contraction, and second it would decrease the number of electrons in these shells and hence the strength of the metallic bonds. The increase of radius results in a decrease of density from plutonium on, in spite of the increasing atomic weight. The decrease of bonding strength accompanying the change of configuration is also reflected in a lowering of the melting point along the series, with a minimum at neptunium and plutonium2. It must then be remembered that the melting points depend upon the bond strength of the high-temperature modifications. These may not have the same electron configuration as the modifications which are stable at room temperature and which have been used for the determination of the metallic radii of Table 2. Therefore, the variations found for the radii and for the melting points need not necessarily indicate a switch from 6d to 5/configuration at the same element in the series.
From protactinium to neptunium, the energy differences between the 6d and the 5/ shells are in fact not larger than ordinary bonding energies and the actual electron configuration will therefore depend upon the nature of the atoms bound to the actinide. Beyond neptunium, however, the electrons entering always seem to prefer the 5/ shell. The half-filled shell, of configuration / 7 , is therefore reached with element 96, curium, and the completely filled shell,/14, with element 103, lawrencium.
As pointed out, especially the first electrons entering the 5/shell are not as well shielded as the 4/electrons of the lanthanides. Moreover, as their energy is fairly high, they are prone to participate in bonding. Even in such cases where the 5/shell is preferred, the actinides are therefore able to exhibit high oxidation states, as is particularly evident for neptunium, plutonium and americium (Table 1).
2 L. B. Asprey and R. A. Penneman, Chem. Engin. News, 45 (1967) No. 32, 75.
SOLUTION CHEMISTRY 467
As the 5/shell is filled, the increasing nuclear charge will bring about a successive contraction, analogous to the lanthanide contraction. The shell will then be better shielded from the fields of neighboring atoms and simultaneously the energy differences between the 5/and the outer orbitals will grow. As a consequence, the participation of the 5/electrons in bonding becomes more difficult, resulting in a very marked stabilization of the trivalent oxidation state for the heavier actinides. Other oxidation states are prominent only when they represent either a half-filled or filled shell, i.e. the electron configurations/7 or / 1 4 . These are evidently preferred to such a degree, that an oxidation or reduction involving even the/shell readily takes place in order that they be reached.
The behavior in solution of each particular actinide will be a consequence of the conditions just described. The stabilities of the various oxidation states of the actinides in solution may be outlined as follows. Actinium and thorium exist only as Ac(III) and Th(IV), respectively. For protactinium, Pa(V) is the most stable state, though Pa(IV) can also be prepared and handled in aqueous solution3,4. The most stable oxidation state of uranium is U(VI). Its reduction in aqueous media generally leads to U(IV), the intermediate U(V) being prone to disproportionate5. The lowest state known, U(III), is a strong reducing agent which is slowly oxidized by water with evolution of hydrogen6.
For elements up to uranium the highest oxidation state is the most stable and as far as actinium and thorium are concerned, even the only one existing in solution. With neptunium, this is no longer the case. The highest oxidation state is Np(VII) which, like U(VI), Pa(V), Th(IV) and Ac(III), has the electron configuration/0. Contrary to these last states, however, Np(VII) is a strong oxidizing agent, especially in acid media89. Np(VI) is also fairly strongly oxidizing while Np(V) may be looked upon as the most stable state. Np(IV) is already a mild reductant and Np(III) a more powerful one, though less so than U(III). Thus Np(III) does not decompose water6.
With plutonium and americium, the stabilization of the lower oxidation states becomes increasingly marked67. To date, the/0 state, Pu(VIII), has never been prepared; the highest state so far observed8, Pu(VII), is a considerably stronger oxidant than Np(VII), and Pu(VI) is also easily reduced. In solution, reduction also occurs as a consequence of products formed by the radiolysis due to radioactive decay of the isotope of plutonium ordinarily used for chemical work, i.e. 239Pu, with ahalf-lifeof t1/2 = 24,300years. Solutions of these high oxidation states are therefore unstable when plutonium is present in the form of its most readily available isotope. The use of the longer-lived 242Pu, t1/2 = 3-8 x 105 years, recently10 isolated in kilogram quantities from heavily neutron-irradiated 239Pu, will certainly improve this situation in the future. Like U(V), but unlike Np(V), Pu(V) is prone to disproportionate and therefore difficult to prepare free from the adjacent oxidation states. In aqueous solution, Pu(IV) is also inclined to disproportionate when no stabilizing agents are present. Pu(III) may be classified as mildly reducing. The standard potentials of
3 Physico-chimie du protactinium, Centre National de la Recherche Scientifique, Paris 1966 (Colloque International No. 154, Orsay, 1965).
4 R. Guillaumont, G. Bouissires and R. Muxart, Actinides Rev. 1 (1968) 135. 5 J. Selbin and J. D. Ortego, Chem. Rev. 69 (1969) 657. 6 A. D. Jones and