Tetranuclear anions

Many of the methods used for the preparation of mononuclear hydrides may be applied to the polynuclear systems. Base attack on metal carbonyls, which furnished one of the first methods for the production of carbonyl hydride species, is applicable to a wide range of carbonyls. Borohydride reduction leads to a variety of products, depending upon the reaction conditions, Os3(CO)12 reacting with NaBH in di-oxane under reflux to give, after 4 hours, a mixture of the anions [H30s4(C0)12] and [H2Os4(CO)i2]2 (79). The related reaction in tetra-hydrofuran for 1 hour yields the anion [HOs3(CO)n]- as the main product with minor amounts of the two tetranuclear anions. 

In the chromium triad, the only claimed tetranuclear compound is the anion [Mo4(CO)u ]2 which was mentioned together with [Mo3(CO)14]2, in a study of the reaction of sodium tetrahydroborate and hexacarbonyl-molybdenum in liquid ammonia14). Later work failed to confirm the existence of the trinuclear anions, [M3(CO)i4]2 (M = Cr, Mo, W)109 1S0,169 and, therefore, the real existence of the tetranuclear anion also seems dubious. 

The stability of the tetranuclear anions increases on descending the sub-group we have already shown in Eq. (21) that the orange carbomethoxy species,... 

My co-worker J. Sedlmeier then held the view that the amine-containing iron carbonyl complexes were also ionic compounds (VII, 14, 21). Hence the compound Fe2(CO)4(en)3 (en= 1,2-ethylenediamine) was formulated as [Fe(en)3]2+[Fe(CO)4]2. Systematic investigations revealed that reactions of the iron carbonyls with other nitrogen and oxygen donors likewise involved valency disproportionation of the metal with concomitant formation of mono- and polynuclear carbonylferrates, viz., [Fe(CO)4]2, [Fe2(CO)8]2-, [Fe3(CO)n]2-. R. Werner (VII, 15, 17, 19, 20) even discovered and characterized compounds containing the tetranuclear anion [Fe4(CO)13]2, the first being that from pyridine and iron carbonyl, viz.,... 

Figure 79 Representation of the tetranuclear anion [Bi4Cli6]" (reproduced by permission of the Royal Society of Chemistry from J. Chem. Soc., Dalton Trans. 1991, 961-965).

This [Ni2(CO)6] anion may be precipitated in basic solution by such large cations as [M(o-phen)3] + 33). However, if aqueous solutions of [Ni2(CO)6] are acidified with weak acid the tetranuclear anion [HNi4 ( 0)9] is said to be formed 33). This is also reported to form precipitates with the tetraethylammonium cation and the [M(o-phen)3] + cations (M = Fe or Ni). The [Ni4(CO)9] anion is also formed by treatment of nickel tetracarbonyl with a mixture of ethylenediamine and pyridine or, as a... 

While the tetranuclear anion [17] always contains tar-trato gronps of the same enantiomeric form, formation of the dinuclear complexes is stereospecific [16]. Continuous effort to understand the stereospecific aspecfs of vanadium(V) tartrato complexes leads us to take a closer look at their spectral properties. Hence, this work is aimed to complement the aforementioned studies and provide deeper understanding of the experimental observations. 

---