Chapter 17
Stabilization of Unstable d-Metal Oxidation States by Complex Formation Κ. B . Yatsimirskii
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Institute of Physical Chemistry, Academy of Sciences of Ukraine, prospect Nauki 31, Kiev, 252039 Ukraine
Alfred Werner studied the "strengthening of the primary valence force by the saturation o f the secondary valence force," but he failed to explain fully this phenomenon. To elucidate the stabilization o f unstable oxidation states by complex formation two aspects should be taken into account: the thermodynamic stability of complexes and their kinetic redox lability. Such factors as ligand field stabilization energy for M and M and the geometry o f the donor atoms' spatial orientation also affects the stability o f a given oxidation state. Macrocyclic ligands are especially suitable for the stabilization o f unstable metal oxidation states, both from the thermodynamic and kinetic viewpoints. n
n-1
The stabilization o f unstable d-metal oxidation states by complex formation has been studied for many years as one of the important problems of coordination chemistry. Alfred Werner paid attention to this, writing, "as a very peculiar phenomenon o f the strengthening o f primary valence by means of secondary valence forces, saturation has been often observed. The essence of this phenomenon has not been clear until now" (7). He then gave some examples o f stabilization by formation o f oxide and chloride complexes in the cases o f Fe(VI), Mn(III), and Pb(IV). He pointed out that very unstable C o X salts can be stabilized by the coordination o f ammonia molecules. Similarly, silver(II) compounds may be isolated only as the tetrakis(pyridine) adduct [Ag(py) ]S 0 (7). Today inorganic chemists do not use such terms as "primary" (Haupt) and "secondary" (Neben) valence but instead speak o f "oxidation state" and "coordination number." One modern popular definition for the term "oxidation state" is: "The oxidation state o f a metal in a complex is 3
4
2
8
0097-6156/94/0565-0207$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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COORDINATION CHEMISTRY
simply the charge that the metal would have on the ionic model.... Once we have the oxidation state, we can immediately obtain the corresponding d configuration. This is simply the number of d electrons which would be present in the free metal ion that corresponds to the oxidation state we have assigned" (2). The d" configuration may be confirmed by experimental methods such as E S R , U V - V I S spectroscopy, etc. The primary characteristic of d-transition metals is their ability to assume several oxidation states with different stabilities. O f special interest is the stabilization of unstable oxidation states of transition metals, which is of great significance in explaining the essence of "strengthening of the main valence by means of saturation of secondary valence forces" (coordination number) (7). The coordination number is equal to the number of ligand atoms which are directly bonded to the central metal atom. In some cases (mainly in organometallic compounds) some difficulties exist in determining the coordination number, e.g., in ferrocene. It is clear that Werner's "strengthening of the main valence force" means the stabilization of unstable oxidation states by complex formation. This problem has at least three interconnected aspects: (1) structural (electronic and geometric), (2) thermodynamic, and (3) kinetic. The compatibility in donor-acceptor interaction is one of the most important aspects of the problem. Ligands can be classified according to their donor-acceptor ability into at least three groups: (i) molecules and ions possessing only one lone electron pair: N H , N R , C H " , etc., - σ group (single sigma donor group); (ii) molecules and ions incorporating donor atoms with two or more electron pairs. These ligands can act as σ- and π-donors. Many ligands fall into this group: F", OH", O H , RCOO", etc.; and (iii) molecules and ions with lone electron pairs and vacant π*- or dorbitals suitable for accepting electrons from metal atoms and π-back bond formation. These ligands can be denoted as the σ π π group. There are many, ligands that belong to this group. C O , CN", and N C R are examples of ligands with vacant π-orbitals, while S R , CI", Br", Γ, etc. are ligands with vacant d-orbitals. A similar classification can be proposed for d-metal ions, and thus many cases of electron structure compatibility in complexes can be explained. For instance, C o possesses a low-spin state and in an octahedral field has the (t ) electronic configuration. It is clear that this ion can act only as a σ-acceptor as π-orbitals are filled. This causes the very strong complex formation o f C o with a -ligands (log Κ for [Co(NH ) ] is 34.36). The neighboring ions ( M n and Fe ) have vacancies on the t orbitals and thus can interact with σ π ligands (OH", H 0 , etc.) which compete with ammonia ligands. The compatibility can be observed also in the case of phenanthroline and a , a'-dipyridyl complexes. These ligands form more stable Fe (d ) complexes than the 3
3
3
Δ
2
Δ
Δ
Α
2
3 +
6
2g
3 +
A
2+
3
3+
6
2g
Δ
Δ
2
2+
6
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
17.
2+
2+
analogous complexes with neighboring ions ( M n , C o , and even with F e , d -configuration). The ligand field stabilization energy (LFSE) plays an important role in the stabilization of some complexes with a given electronic structure of metal ions, i.e., with a given oxidation state. Most coordination compounds o f the d-metals have a coordination number o f 6 and an octahedral arrangement of ligand donor atoms in the coordination sphere. The L F S E has maximum values for ions with d ( V , C r ) , lowspin d ( M n , F e , C o , etc.) and d (Co , N i , C u ) electron configurations. For a different geometry of coordination polyhedron L F S E possesses a maximum for different electronic structures. Therefore the L F S E can be considered as a measure of metal-ligand compatibility. Ligand geometry plays an important role in the stabilization of unstable metal oxidation states whenever the possibility of forming one, two, or three metal-chelate rings exists. This circumstance plays an especially governing role in macrocyclic complexes where metal ions are encapsulated in the cavities of macrocyclic ligands. Each cavity has a definite dimension, and thus there must be a "correspondence" between the dimension of the macrocyclic hole and the metal ionic radius. The ionic diameter and the size of the macrocyclic ligand cavity must match each other. Here both thermodynamic and kinetic stabilization must be observed. The correspondence of electronic and geometric structures is realized by the definite structure of the coordination sphere and the corresponding electronic structure. A n examination of metal ions which are stable in aqueous solution will elucidate the importance of compatibility factors. The oxidation state 1+ can be observed only for ions with the stable n d electronic shell (second ionization potential is high): C u , A g , and A u . Other M ions with d-electrons reduce water to free hydrogen. The observed aqua ions with 2+ and 3+ oxidation states are: 3+
5
3
6
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209
Stabilization of Unstable d-Metal Oxidation States
YATSIMIRSKII
+
2+
3+
8
+
2 +
2 +
3+
3+
10
+
η 3 4 5
d'
3 4 5
Ti
d
2
d
4
d Cr* Mo
3
2+
v
2 +
5
+
2 +
2+
2+
2+
d Co Rh
+
2+
2 +
d Ni Pd Pt
2 +
2+
2+
3 +
V
—
—
-
-
3
Cr * Mo
3 +
3 +
-
3 +
Mn Fe Tc ? R u Os -
3+
3+
3+
Co Rh
3+
3+
— —
-
3+
10
9
8
7
6
d d Mn Fe Tc ? R u
+
— 3 +
Ag (Au ) 3+
d Cu Ag Au
2 +
2 +
2 +
-
d Zn Cd Hg Ga In Tl
2 +
2 +
2 +
3+
3+
3 +
The ions at the beginning of the d-series are strong reducing agents (e.g., V , C r , T i , and V ) , whereas the last metal ions in these series are oxidants. We assume that T c and T c may exist in aqueous solution, but they have not yet been observed. Some of the 4d- and 5d2 +
2+
3 +
3 +
2+
3+
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
210
COORDINATION CHEMISTRY
ions can form metal-metal bonds and thus may be identified in dimeric and cluster forms ( M o , R h , and possibly A u ) . The d-metal complexes in unstable oxidation states may possess high or low redox potential values. These values depend on the ionization potentials (I) and on the difference between the solvation free energy for n+ and (n-l)+ charged ions ( A G and A G . respectively): 4+
2
4+
4+
2
2
n
=
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En/n.i
n
l 5
I. - ( A G . - Δ Ο ^ ) - const
(1)
where const is a value dependent on the reference electrode and in the case of the normal hydrogen electrode is equal to 4.55 eV (439 kJ) (5). The En/n-i values can be regulated by complex formation and for the M L / M " L pair can be expressed by the following equation: n
n
!
E V i
=
+ (RT/F) (ln K n - i - ln K n ) M
L
M
0
(2),
L
n
n
n _ 1
where E ^ is the redox potential for the pair M L / M L , which depends on the difference in the logarithms of the stability constants or on the ratio K n - i / K n = η , which can be more or less than 1. In most cases this ratio (η) exceeds 1, but for the ligands with soft donor atoms (thioether group, phosphine, etc.) η < 1. The ratio η depends on the nature of the metal and ligand in the complex. For instance, the rather "hard ligand N H forms coordination compounds that are more stable with M than with M , but there are exceptions (e.g., log K 2+ > !og KCUÎNHV)- o r the hard bases (F", OH", RCOO", etc.) the ratio η > 1 in all cases. The carbon monoxide molecule (CO, with the dominating resonance form C"sO ) and the cyanide ion (CN" with the strong donor ability of the highest occupied σ-orbitals and lowest unoccupied π*orbitals) form σ L —> M and π M —> L bonds. Such compounds as K [ M n ( C N ) ] and K [Re(CN) ] illustrate the stabilization of M n and R e owing to the synergetic effect (the electronic structures of the central atoms are 3d and 5d , respectively). The stabilization of the 1+ oxidation state may be realized by the encapsulation of metal ions into the cavity of macrocyclic ligands containing azomethine or thioether groups. Only one F e macrocyclic complex has been described in the literature (5). Since 1972 no reports concerning the stabilization of F e by macrocyclic complex formation have appeared in the literature. The macrocyclic complexes [M ([9]aneS ) ] (where [9]aneS = 1,4,7-trithiacyclononane) have been obtained for C o (3d ) and R h (4d ). These ions have electronic structures which best fit the O symmetry of the arrangement of S-donor atoms. Much more information is available for C o tetraazamacrocyclic complexes. The cobalt(I) corrin systems (compounds modeling vitamin B ) have been most thoroughly studied. M
L
M
L
11
3
n +
( n - 1 > f
Cu(NHj)2
p
+
+
5
6
5
6
+
6
6
+
+
I
+
3
2
3
+
8
+
h
+
12
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
8
17.
Stabilization of Unstable d-Metal Oxidation States
YATSIMIRSKII
I
+
7
211
9
Macrocyclic complexes [M ([9]aneS ) ] with d and d electronic configurations are not yet known, although many Ni(I) macrocyclic complexes are more stable than the corresponding Co(I) complexes (4). Many macrocyclic complexes of N i ^ d ) and P d ^ d ) are known. Polyazamacrocyclic ligands with methylated N-donor atoms form rather stable complexes with these metals. There are many examples of the stabilization of unstable 3+ oxidation states by complex formation with macrocyclic ligands. N i and C u are stabilized by complex formation with tetraazamacrocyclic ligands. The most universal ligand for the stabilization of unstable oxidation states is [9]aneS (L), which forms coordination compounds of general type M L with many unstable ions, e.g., A g , P d , and Pt . The stabilization of H g in the tetraazamacrocyclic complex [Hg(Cyclam)] was established in 1976 (6). Thus the stabilization by complex formation of such ions that do not exist in aqueous solution ( M n , Fe , C o , N i , R h , P d and N i , C u , P d , Pt , H g ) has been well established. However, there are still no data about the stabilization by complex formation of metals in the oxidation state 1+: Os , Ir , and Pt . In all cases the stabilization of unstable metal oxidation states depends on the ratio of stability constants K n - i / K n , which regulates the redox potential in accordance with equation 2 (thermodynamic stabilization). Finally, the kinetic aspect of this problem exists. The essence of kinetic stabilization is the screening of the central atom from attack by different active molecules ( H 0 , 0 , etc.). For this purpose bulky groups in ligands are desirable. For instance, norbornyl anions can stabilize cobalt in the 5+ oxidation state (Co(Nor) ), i.e., a e ^ electronic structure (in the field of T symmetry) (7). Nickel(I) complexes are stable when alkylated nitrogen donor atoms are incorporated into the macro ring. Redox-unstable complexes may be stabilized by the coordination of additional ligands. For instance, many unstable nickel(I) and cobalt(I) tetraazamacrocylic complexes can be stabilized by additional coordination, forming mixed ligand complexes containing macrocyclic ligands and such π-acceptors as C O and P R . Thus the essence of the "strengthening of primary valence by means of saturation of secondary valence forces" mentioned by Werner or the "stabilization of unstable oxidation states by complex formation" can be explained by three different reasons: (i) the electronic and geometric structure compatibility (correspondence) for the central ions and ligands; (ii) the thermodynamic characteristics - redox-potential, which depends on the ratio of the stability constants of the complexes with the same 3
2
9
9
3 +
3 +
3
n +
2 +
3+
3+
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2
3 +
3+
+
3+
3+
+
+
+
+
+
3 +
3+
1
1
1
M
2
+
L
M
L
2
4
0
4
d
3
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
3+
212
COORDINATION CHEMISTRY
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composition but different oxidation states; and (iii) the kinetic characteristics (inertness of the complex), which depend on the screening of central metal atoms by different bulky ligands. Literature Cited 1. Werner, A. Neuere Anschauungen auf dem Gebiete der anorganischen Chemie, 5th ed.; F. Vieweg und Sohn: Braunschweig, 1923. 2. Crabtree, R. H. Organometallic Chemistry of the Transition Metals; Wiley: New York, 1988; p 30. 3. Yatsimirskiĭ, Κ. B. Theor. Exper. Chem. 1986, 25, 280 (in Russian). 4. Yatsimirskiĭ, Κ. B. Russ. J. Inorg. Chem. 1991, 30, 2010 (in Russian). 5. Rillema, D. P.; Endicott, J. F.; Papaconstantinou, E. Inorg. Chem. 1971, 10, 1739. 6. Deming, R. L.; Allred, A. L.; Dahl, A. R.; Herlinger, A. W.; Kestner, M. O. J. Am. Chem. Soc. 1976, 98, 4132. 7. Byrne, Ε. K.; Theopold, Κ. H. J. Am. Chem. Soc. 1987, 109, 1282. RECEIVED April 6,
1994
In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
