Carbonyl binary mononuclear

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

Electrochemical reduction of mononuclear binary carbonyls on the preparative timescale generally leads to CO loss and the formation of dinuclear dianionic products. For example, M(CO)6 (M = Cr, Mo, or W) undergoes irreversible electrochemical reduction near —2.7 V versus SCE in tetrahydrofuran (THF) containing [NBU4][BP4]. The product of the reductions are the dinuclear dianions [M2(CO)io] [14, 21], although under some conditions... 

In the unusually effective reducing medium of sodium-hexamethylphos-phoramide (HMPA), [Mn(CO)5] underwent facile reduction to form a golden yellow to yellow brown solution. On the basis mainly of derivative chemistry and infrared spectra, the major soluble component was formulated as Na3[Mn(CO)4] (14). Once this materia] was in hand it was evident that similar binary carbonyl trianions should be possible for other rf-block transition elements of odd atomic number. Since 1976, syntheses of mononuclear binary carbonyl trianions of V, Nb, Ta, Re, Co, Rh, and Ir have also been reported from this laboratory (vide infra). Because these substances contain transition metals in their lowest known oxidation states, they have often been referred to as superreduced species (15). 

Several mononuclear carbonyl complexes may be synthesized with iridium(I), but most intriguing of those are the binary carbonyls. [Ir(CO)4] (46) may be synthesized by the reduction of Ir4(CO)i2 (45) with sodium under an atmosphere of carbon monoxide. More vigorous reduction of (45) with sodium can also yield the very reactive trianion, [Ir(CO)3] (47) (equation 13). These anions may then be used for the synthesis of other mono and polynuclear (see Polynuclear Complexes) carbonyl compounds of iridium. 

Mononuclear allenylidene complexes have been prepared by either (i) reaction of a deprotonated propargyl alcohol with a binary metal carbonyl or (ii) dehydration of the acetylenic alcohol HC=CCR20H with an appro-... 

In addition to mononuclear carbonylate anions, a variety of polynuclear species have been obtained. The iron carbonylate ions, which have been much studied, are obtained by the action of aqueous alkali or Lewis bases on binary carbonyls or in other ways, for example,... 

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