Metal lons in Unusual Valency States Robin M. Sellers Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gioucestershire, GL13 9PB United Kingdom Dorfman ( I ) and Schwarz ( 2 ) have described how ionizing radiation interacts with water to form highly reactive intermediates such as the hydrated electron, the hydrogen atom, and the hydroxyl radical. In the absence of dissolved solutes these disappear by reacting with one another to give water, hydrogen, and hydrogen peroxide as stable products. If a solute is nresent. other reactions mav occur leading to chemical changes in that solute. In the case of a metal ion these may involve a change in valency state in some cases to give metal ions in unstable valency states. This paper is concerned with the formation of these novel valency states, their characterization and chemical hehavior. Reacllvily of Metal lons with the Primary Products of Water Radiolysis The reactivity of the primary products of water radiolysis (eaq-, H, OH) is of prime importance in this work, and it is important, therefore, to understand what factors influence their reactions and how they occur. This is dealt with in the following paragraphs. Other species such as Hz02 and H+ are formed in the radiolvsis of water.. and.. thoneh thev react with a number of metal ions to form novel complexes, etc., they find no use in the eeneration of unstable valence states and will not be considere& further here. ~
The Hydrated Electron, e,The hydrated electron is the reducing agentpar excellence. Its low redox potential (Eo = -2.77V) (3) means that it is capable of reducing a large number of solutes, including many that are not normally thought of as being reducible. e . q
- + M"+ + Min-l)+
(1)
It can convert Zn2+ into Zn+. for e x a m ~ l eManv . of these reactions are rapid, with rates at, or apprbaching, the diffusion controlled limit and ~ r o b a b l voccur via a tunnelling mechanism ( 4 ) . ~ o m p l t . x ; ~ t ~I,yo nl&ndsother ~ h a nwateican p r w duce a dramarc reduction i n mte. The Ni'+ aauo ion, for in. stance, is reduced with a rate constant of 2.2 x 10IoM-Is-', while its EDTA complex has a rate constant of 1.0 X lo8 M-Is-', and its tris-ethylenediamine complex 5 2 X lo7 M-Is-' (5).The initial product of the reaction is in either a vibrationally or electronically excited state because of the Frank-Condon restriction. The vihrationally excited state is de-excited too raoidlv . , (t on(,cl:e, the redudon nf Ru C S I , t r r t. an electronicallv e n ~ t c di)rodu~thas been detected. :I n u n ~ i ) ~ r of other claims for long-lived excited products have been made, but these are now known to be incorrect, (7). The Hydroxyl Radical, OH The hydroxyl radical has a redox potential of E D= 2.8V (8), and not surprisingly it is capahle of oxidizing many metal ions. The mechanisms of very few of these reactions are known with certaintv. Manv metal aano ions react with rate constants of 1 3 X 108M-Is-', and thia hns I~eeninterpreted (9)as indicatine that the first s t w iv II-:~tmnabstraction from water in the inner coordination sphere, eqn. (2). 114
Journal of Chemical Education
O H t M(H20i,"+'(HzO),,-, Mn+OH
+
(HzO),-, Mn+OH H 2 0 ( H 2 0 i m - , Mi"+"+ O H -
-
(2)
(3) I t is envisaged that this would he followed hy rapid intramolecular electron transfer as shown in reaction (3). If the ligand exchange rate is sufficiently rapid an inner sphere substitution seems a more likely route, and this is prohahly huw Sn2+and TI+ react. Alternativelv. exchange rate is slow. .. if the lieand " addition with a change in coordination may be involved. The square planar complex PtCla2- is thought to he oxidized in this way (10). An outer sphere mechanism seems to he ruled out on energetic grounds, for the conversion of OH into OH.,involves a large reorganization energy due to the high hydration energy of OH- (--460 kJ mol-'). In many cases, however, it is convenient to think of the reaction as occurring by an outer sphere path, since the products observed in conventional pulse radio1;sis experiments (time resolution typically -1 psec) are those expected from such a reaction.
The Hydrown Atom, H 'I'he hydrogen atom has proved ratlwr leis versatile than the hvdroxvl radical and the hydnrwd e l e r ~ r min the production of metal ions in unusual valency states. It is a strong reductant ( E L -23.3) ( 8 ) hut tends to react much less rapidly than does the hydrated electron. With strongly reducing metal ions such as Cr2+,Fez+, and Ti3+, it acts as an oxidant giving the metal in the next higher valency state and molecular hydrogen via an inner sphere substitution reaction (7). In none of the cases where this occurs, however, are novel valence states formed. Hyper-Reduced Metal lons What sort of properties do the products of pa,-, H and OH attack on metal ions have, and how does the radiation chemist set about characterizing the hyper-reduced and hyper-oxidized species formed? T o answer these questions it is instructive to consider what haonens in the nulse radiolvsis of Cd2+,Co2+,Ni2+, and Zn2+. ~ L e s were e among the firstmetal ions to he investigated, and much is now known about their radiation chemistry. If one takes a dilute, de-aerated aqueous solution of one of these ions, and Duke irradiatesit, what does one see? If the solution is ~"fficikntlydilute (say k 5 p ~an) intense absorption centered on 720 nm appears immediately after the pulse. This decays away rapidly (psec) by a firstorder process, linearly dependent on the metal ion concentration, and concomitant with this decay, a new absorption builds up in the uv region. This absorption is not indefinitely stable hut decays slowly, typically over a period of milliseconds, by an approximately (but not exactly) second order Drocess. after which nracticallv no ahsor~tionremains. How ahsorption are thrse findings to be interlmttd" l ' l w 1111t1al is clearlv that of the hvdrared electron. u t d he nature of i t s decay ekahlishes that it is reacting with the metal ion as shown in reaction (4). e.,-
+ M2+
-
M+
(4)
The uv absorption is that of the product, which one might not
unreasonablv- exnectto . be the reduced metal ion M+. Addition of a variety of solutes such as oxidizing agents reduces the half-life of the product, and from an analysis of the kinetics the specific second-order rate constants for the reactions can he determined. Table 1 summarizes some of the measurements that have been made to date and shows M + to he a powerful reductant. T o confirm that the product of eqn. (4) is M+ a useful method has been to investigate the effect of ionic strength on the reaction with a charged oxidant (nitrite and hromate have commonly been used). According to the Debye-Hiickel theory, the rate constant for a reaction between charged species varies with ionic strength according to eqn. (51, log k = log ko + 1.02 ZAZR1112 (5) where ko is the rate constant a t zero ionic strength, Zn and Zn are the charges on the rewtants, nnrl I i5 the ionic strength. found fm rr:lrticm with monovalent anion.; Values of 2 n Z ~ are E-1, indicating that the reduced metal ion does indeed have a charge - of +l.Conductivity measurements confirm this (11). By taking readings a t a series of wavelengths the absorption spectra of the M+ ion can be determined. Cd+ and Zn+ have intense absorptions centered on 300 nm, Ni+ on 300 nm with a shoulder a t =370nm, and Co+ twin peaksat 310 and 370 nm (12). The nature of these absorptions has been investigated in some eleeant exneriments hv Walker et al. (13) usine a combinatiolof pulse radiolysis and flash photolysis. They fi'nd that the absorptions a t ~ 3 0 nm 0 are due in the main to a charge-transfer-to-solvent process but that other transitions are responsible for the Co+ and Ni+ absorptions a t lonaer wavelengths. These hyper-reduced metal ions are unstable and their decay in the absence of oxidizing solutes has proved to be extremely complicated. In solutions containing only the divalent metal salt (usually the sulfate or perchlor&), the decay of M+ is, as noted above, approximately second-order. If a scavenger of OH radicals, such as an aliphatic alcohol, is added Table 1.
Speciflc Rates 01 Reaction of Cd*, Co*, Mif and Znf with Some Oxidants. (From ref. (35)) . Oxidant 10-ek/Mb-~
Br0.-
IDsNO2NOsS20s2Cr20?Cu2+ H202 01 N20 Co(NHs)sSf Ru(NH&*+ bemophenone 1.4-benzoquinone
riboflavin
CdC
CO+
Nit
Zn+
0.125 2.3 2.0 0.35 2.4 16 0.12 2.2 3.6 0.0035 0.17 2.2 1.0 4.1 5.1
4.8
50.0084 0.22 0.15 50.0014 0.15
2.1 3.6 2.2 2.1 1.3 16 0.25 2.3 2.4 0.037 0.84 2.2 2.5 3.0
4.3
1.8 2.8
-
-
0.41 1.6 6.0 1.0
50.024 0.032 2.2 0.0091
