Carbon cluster dianions

The cluster dianions [Co8C(CO)ig]- (Fig. 8a) and [NigC(CO)i6] provide rare examples of a first-row main-group atom occupying a square antiprismatic cluster cavity,and also illustrate the capacity of carbon to become eight-coordinate. However, on progressing to the heavier main-group elements such as Si, P, S and As we find a propensity for clusters based on the square-antiprism. 

Watts JD, Bartlett RJ (1992) A theoretical study of Unear carbon cluster monoanions, C-n, and dianions, C2-n (n = 2-10). J Chem Phys 97(5) 3445-3457... 

C)(CO)i6] and [Pd(NCMe)4][BF4]2 was found to depend on the reaction conditions employed equimolar reaction in refluxing CH2CI2 in air gave [PPN]2-[Rui2Pd2(ii6-C)2(CO)32] (14) whereas the same reaction in THF at room temperature under Ar afforded [PPN]2[Rui2Pd4()i6-C)2(CO)32] (15). The cluster dianion 14 consists of two Ru6 octahedra linked via two Pd atoms the Ru-Ru bond between the equatorial ruthenium atoms which interact with the palladium atoms is broken, and each octahedron contains an interstitial carbon atom. Complex 15 contains two additional Pd atoms forming a bent square the two Ru6 octahedra are now twisted by almost 90°. " ... 

With 13 carbonyls, the metal atoms can adopt the usual tetrahedral arrangement although considerable steric crowding occurs with the smallest metal atoms, as shown in the short contacts present in the dianion [Fe4(CO)i3]2 (6). (The shortest van der Waals contacts between the carbon atoms have been found experimentally to depend strongly on the relative inclination of the carbonyls parallel carbonyls present a minimum distance of 3.0-3.1 A, while this distance decreases to 2.5-2.6 A at the relative angle of 90°-l 10° this effect is clearly related to the expected oval shape of the ligand (I).) Cluster opening occurs by... 

Protonation of 21 yields H2Os,0C(CO)24 (5 7). The crystal structure of the dihydride has not been determined, but analysis of the vibrational spectrum of the cluster in the region associated with motion of the interstitial carbon atom has led to the conclusion that the symmetry of 21 is reduced on protonation, probably by protonation of the central Os6 octahedron (see Section VI,A) (57). Cluster 21 also reacts with iodine to yield sequentially [Osl0C(CO)24I]", 22, and Os.oCfCO), 23, the result of electrophilic attack by 1+ on the dianion [Eq. (16)] (55). 

Neutral Rh6(CO)i6 is perfectly stable under a carbon monoxide atmosphere at 25°C, and the dianion [Rh6(CO)i5]2 is still stable at 25°C (although degraded at — 70°C), but the tetraanion []Rhe((X)) h]4- is readily demolished under the same conditions. The interpretation of this behavior is that the term 1) m-co is very dependent on the MA/NC ratio, due to the related change in back donation. Probably this is a very general effect and most of the HNCC are expected to become increasingly unstable toward carbon monoxide on increasing the reduction of the cluster. 

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

The reduction of neutral carbonyls with alkali metal hydroxides or carbonates is an important route to HNCC dianions. In a few instances reduction of rhodium clusters has been reported to proceed further with oxidation of another CO. Examples are the formation of [Rh6(CO)i4] from [Rh6(CO)i5] 170) (Scheme 16) and the reduction of the dicarbide [Rhi2(C)2(CO)24] to give [Rhi2(C)2C023] 213). In the latter case, the unstable paramagnetic intermediate [Rh 12(0)2(00)23] has also been isolated upon slight alteration of the reaction conditions [Eqs. (32) and (33)]. 

A separate series of closo-carboranes, isoelectronic with the BnH2 dianions and the neutral QB 2H compounds, consists of the monocarbon anions CB iH (Table 5-3). The preparative routes to these species vary widely, but a commonly used method for incorporating a single carbon atom into a borane cage involves the conversion of cyanoboranes to amino derivatives the cyano carbon atom satisfies its valence requirements via absorption into the cluster framework. 

At present, there is no firm experimental evidence for dipole-bound dianions. However, it is instructive to consider the results of multiply charged anions of fullerene derivative clusters discussed in Section VI. Tuinman and Compton reported the formation of doubly charged anions of C6o(CN)2 dimers, [C6o(CN)2]J, in electrospray ionization. The dipole moment of the C6o(CN)2 molecule in which the two CN radicals are bonded to two adjacent carbon atoms in Cgo is expected to be large... 

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