Tricarbonylmetallates 3- of Cobalt, Rhodium, and Iridium

Approximately 25 years after Mond and co-workers had prepared the first carbonyl of cobalt, Co2(CO)8 (78), Hieber obtained HCo(CO)4 by the acidification of salts containing [Co(CO)4] (79), one of the first established carbonylmetallates (80). In 1941, Behrens, a student of Hieber, initiated his important and extensive investigations on the reduction of metal carbonyls and their derivatives by alkali and alkaline earth metals in liquid ammonia (81). At this time he established that various salts of [Co(CO)4]-, including Na[Co(CO)4] and K[Co(CO)4], could be obtained from the neutralization of HCo(CO)4 or the reduction of Cd[Co(CO)4]2 by sodium or potassium in liquid ammonia (4). On the basis of Behrens pioneering studies, we were 

Several years ago preliminary results in this study were reported and provided good evidence for the reduction of [Co(CO)4] by sodium in liquid ammonia (82). Although the nature of the reduction product remained obscure, it was formulated to contain [Co(CO)3]3- on the basis of infrared spectra and derivative studies. More recently, the first isolation of a satisfactorily pure sample of Na3[Co(CO)3] was accomplished. Formally, the trisodium salt contains [Co(CO)3]3-, an exceedingly rare example of a three-coordinate, 18-electron metal ion. To our knowledge the only previously well-characterized example of such a compound is [Ni(C2H4)3]2, which is formally present in [Li(TMEDA)]2[Ni(C2H4)3] (83). 

Of the homoleptic carbonylmetallates(l -) we have attempted to reduce, [Co(CO)4] appears to be the most difficult. Although the sodium salts of [M(CO)6] (M = V, Nb, and Ta) were quickly reduced in liquid ammonia by sodium metal to provide the corresponding trianions, [M(CO)5]3 (vide supra), it seems unlikely that we have ever effected complete reduction of Na[Co(CO)4] to Na3[Co(CO)3]. Even after 2 days of refluxing (at — 33°C) anhydrous ammonia solutions of Na[Co(CO)4] with excess Na, considerable amounts of the tetracarbonylcobaltate(l —) remained. Low yields of a heterogeneous-appearing brown to olive-brown insoluble solid were isolated this solid has been shown to contain Na3[Co(CO)3] (vide infra). As in the case of [Re(CO)s], we found that solutions of potassium in liquid ammonia were far more effective at reducing [Co(CO)4]-. However, unlike [Re(CO)s], [Rh(CO)4], or [Ir(CO)4] (vide infra), there was no evidence that [Co(CO)4] was reduced by sodium or potassium metal in hexa-methylphosphoric triamide. We observed that excess sodium naphthalenide slowly (over a period of 40-50 hr at room temperature) converted Na[Co(CO)4] in THF to an impure and insoluble brown powder that contained Na3[Co(CO)3], but this synthesis appeared to be of little or no utility. 

Reduction of [Co(CO)4] by sodium or potassium in liquid ammonia proceeded according to the Eq. (26) (shown for the potassium reaction). 

The presence of K2C202 in the reaction product was established by comparison of the mineral mull IR spectrum of bonaflde K2C202, prepared from the reaction of free CO and potassium metal in liquid ammonia (84), and 

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