ERVIN COLTON University of Illinois, Urbana, Illinois
ONE
of the most unusual oxidation states is the serovalent state found in metal carbonyls, in complex cyanides, and recently in interesting ammine and isonitrile complexes. The carbonyls represent a large and fascinating branch of inorganic chemistry, but they \ d l not be discussed here as they have been reviewed adequately elsewhere (1, 2). Rather, examples of the other classes of compounds containing transition elements in the zero oxidation state will be descrihed. The reduction of the complex cyanides of certain transition metals by potassium metal in liquid ammonia leads to interesting products in which the central atom of the complex ion exhibits an effective atomic number equal to the atomic number of the inert gas at the end of the period in which the element is placed in the periodic classification (3). Eastes and Burgess (4) obtained a bulky yellow precipitate by reaction of an excess of potassium with potassium tetracyanonickelate(II), KpNi(CN)4, in liquid ammonia. When freed of ammonia the product was copper colored and extremely unstable in air and in oxygen, turning black on exposure. A deep red solution resulted upon dissolving the reduction product in water. The red aqueous solution exhibited all the properties of a solution of K2Ni(CN)%. Analvsis of the
unusual product, taking nickel to be unity, showed the ~onstituents,K, Ni, and CN to he present in the ratio of 3.977: 1.000:3.863. Thus the empirical formula K I N (CN)4was assigned. K,Ni(CN),
+ 2K
-
K,Ni(CN),
This complex can be regarded as an analogue of the mrbonyl Ni(CO)4, in which the neutral carbon monoxide groups have been replaced by four monovalent ryanogen ions. Since studies by means of electron diffraction (5) and by means of infrared (6) have shown that Ni(CO), is best described as a tetrahedral structure, it seems reasonable to believe that the [Xi(CN)I]4- ion also would exhibit this same configuration. ~d
xi atom
.(a
4n
00000 0 000
A similar zero state has been observed with palladium. Burbage and Fernelius (7) reacted potassium metal and potassium tetracyanopalladate(I1).
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K?Pd(CN)4, in liquid ammonia and observed the formation of a moderately soluble white precipitate which was stable for a t least six hours in liquid ammonia a t -33', and for approximately two hours in an ammonia atmosphere a t room temperature. Total analysis and reactine ratios of ~otassiumto startine material indicate the reaction product t o be potassium tetracyanopalladate(O), I
