Nitrido Clusters

The primary source of the nitrogen atoms in nitrido clusters is ultimately its monoxide, which is usually coordinated to a transition metal. Therefore, several of the methods used to form nitrido clusters are identical to methods used to form nitrosyl complexes. In this section the synthetic approaches are broken into three categories. The first involves the use of nitrosylating reagents such as NC, N02, or isolated metal nitrosyls themselves. The second category involves the conversion of a coordinated isocyanate into a nitrido cluster, and the last section discusses the interconversion of one nitrido cluster into another. Table III contains a list of the known nitrido clusters. 

The first syntheses of carbonyl clusters containing an interstitial nitrogen atom involved the use of NO [Eqs. (45) and (46)] (3). In the case of rhodium, the major product was suggested to be [Rli6(CO)i4(NO)], but no further data have been reported on this. The reaction of NO with the mixture of iron carbonyls, Fe(CO)s and [Fe2(CO)8] , yielded two clusters 

The reaction of the tetranuclear Ru and Os anionic clusters, [H3M4(CO)i2] , with NO has been reported to give both nitrosyl and nitrido clusters (22,38). With M = Ru only nitrido products are isolated in low yield [Eq. (47)]. The dodecacarbonyl cluster is similar in structure to 

Nitrosylation of [CoRu3(CO)i3] with NO produces the nitrido cluster CoRu3N(CO)i2 in low yield (81). The major products from this reaction [Ru3(CO),2 and Co(CO)3(NO)] arise from cluster fragmentation. 

The use of nitrite (NO ) as a source of metal nitrosyls is now well established as discussed in Section II,A3. One study of the reaction of PPN(N02) with carbonyl clusters of the Co triad revealed an alternative synthesis of [Co6N(CO)i5] and [Rh6N(CO)i5] (i5). As shown in Eqs. (48) and (49), both the tetranuclear and hexanuclear starting materials for Rh lead to the same nitrido cluster. The reasonable yields for Rh by either route suggest that the product is somewhat of a thermodynamic sink. Consistent with this idea, Eq. (50) shows the use of gaseous nitric oxide itself to form the Rh nitrido cluster directly (J). 

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The nitrido clusters [Fe4(CO)12N] and Fe4(CO)n(NO)N undergo substitution reactions yielding mono- and disubstituted products.359 The phosphines used (PPh3 and PMe2Ph) preferred to bond to the wingtip irons in the metal butterfly. 

The major site of protonation of [M4(CO)i2N] occurs at the M atoms, but a minor products shows vertex elimination and protonation at N [Eqs. (247)-(249)].145,155,357 Protonation of [FeRu3(CO)i2(NO)] induces the reduction of NO with formation of a nitrido cluster. The site of protonation in the final product is again at the metal centers. 

Reaction of the dimolybdenum nitrile complex 74 with Ru3(CO)i2 in refluxing toluene results in C=N bond cleavage to give the p4- and r5-nitrido clusters 75 and 76, respectively, in low yield. The metal geometry of 76 consists of a distorted bicapped square pyramid with a Ru atom spike, the nitrogen atom lying below the basal Mo2Ru2 plane.54... 

For nitrido clusters, open butterfly structures with a RU4 core are observed in (38) and the precursor molecules (35) or (37), where one nitride or isocyanate ligand bridges the open wings of the butterfly. RusN cores, exemplified by [RusN(CO)i4] (41), are of a similar square pyramidal arrangement as RusC clusters. The nitrido atom in the former is displaced out of the basal plane by as much as 0.21 A (i.e. about twice as much as the carbon atom in (30)) making the transition to a distorted octahedron. The decanuclear complex... 

The formation of nitrido clusters from coordinated NO will be discussed first, since it is likely that the other products (NH, etc.) form via coordinated N atoms. In this section we will consider only the reactions of well-characterized nitrosyl clusters. The general condensation of mononuclear nitrosyl complexes to yield nitrido clusters will be discussed in Section III,A,2. 

The conversion of [FeRu3(CO)i2(NO)] into HFeRu3N(CO)j2 via protonation (42) has been found to occur via the nitrido cluster [FeRu3N(CO)i2] (43). Equation (31), which is essentially quantitative,... 

The first observation of the formation of a nitrido ligand from a nitrosyl ligand involved the homonuclear Fe reaction shown in Eq. (51) (20, 42). There are numerous examples of reactions involving the conversion of a coordinated nitrosyl ligand into a nitrido cluster. In some cases [Eqs. (51) (20), (52) (91), and (53) (92, 93)], the NO is bound to a mononuclear... 

The observation of the conversion of a coordinated isocyanate into a nitrido cluster opens a new pathway to these species (97, 94). Equations (54) (94) and (55) (97) both proceed in high yield. 

Although the conversion of a particular carbido cluster into another with a different metal framework is an important synthetic method for carbides (1), the analogous process is essentially untapped for nitrido clusters. It has been used [Eq. (56)] to form the largest known nitrido cluster, [PtRhioN(CO)2i ] (29) (%), and it proved successful in adding a fifth vertex in the formation of the mixed-metal cluster [FeRu4N(CO)i4] [Eq. (57)] (43). 

The first measurements of the nitrogen NMR spectra of nitrido clusters were reported for [Co6N(CO)i6] and [Rh6N(CO)xs] and posed somewhat of an enigma (3). ITie chemical shifts were 196.2 and 107.6 ppm, respectively (downfield from NH3), which were far upheld from the anticipated position based on the related carbido clusters. As a hrst approximation, the chemical shifts for nitrogen (measured relative to are twice the values for carbon (measured relative to CH4) for isoelectronic molecules (61). This relationship arises from the comparison... 

More recent work with nitrido clusters within the iron triad revealed chemical shifts in the expected region. For instance, [Ru6N(CO),6] appears at 538 ppm (NH4 ) 91) which can be compared to the carbon resonance of [Ru6C(CO),6] which is at 461.2 ppm (CH4) 112). Although the value is not twice as large as it is at least larger than the carbon resonance. Of the approximately 15 compounds measured, a range from —450 to —620 ppm is observed. Because many of the compounds are closely related some useful trends have appeared. 

The scission of the N—C bond of coordinated isocyanates has been discussed as a useful method to prepare nitrido clusters. Under higher pressures of CO, Eq. (65) can be reversed. When [Ru6N(CO)i6] is placed... 

There also exist structural similarities between the MeN clusters and the class of solid-state compounds known as transition metal nitrides. In these refractory compounds the nitrogen exists in either an octahedral or a trigonal prismatic array of transition metals 125). The interest in using transition metal nitrides for superconducting thin films as well as chemically inert coatings (i25) should promote studies in which nitrido clusters could be used as starting materials for the synthesis of new refractory materials. 

Listed below are nitrosyl and nitrido clusters that have been recently reported. 

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