Metal carbonyls tetranuclear

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. 

We are pleased to accept the invitation of the editors to present a review on metal carbonyl clusters which affords us the opportunity to discuss the tetranuclear clusters and to complete the previous discussion of 3-dimensional clusters60. ... 

Table 6. Bond distances (A) in tetranuclear metal carbonyl closed clusters... 

The number of 13C NMR studies on metal carbonyls has greately increased over the last few years as shown by the presently available data on tetranuclear carbonyls (Table 11) compared with that available in 1974 when the last review232 on this subject appeared and contained only two references to tetranuclear clusters. 

CYCLOPENTADIENYLSODIUM AND SOME MONO-, TRI-, AND TETRANUCLEAR METAL CARBONYL DERIVATIVES AND CLUSTER COMPLEXES... 

The reaction expressed in Eq. (88) is the reverse of the hydrogen addition [Eq. (84)] and constitutes one of the few demonstrated cases of reductive elimination of H2 from an intact cluster. The tetranuclear clusters that have been studied appear to fragment consistently in the presence of CO, to produce a metal carbonyl trimer and a monomeric fragment in highly specific fashions. 

Studies of thermal reactions of other tetranuclear metal carbonyl clusters seem to be rather rare. Reports on some interesting reactions of complexes such as H Riii, (C0)i2 (68, 69) have recently appeared as have descriptions of fragmentation kinetics of complexes such as H2RU4(CO)13 (70). 

Several other tetranuclear complexes containing different metals have been obtained from the reaction between metal carbonyl and metal trifluorophosphine complexes (method E) or by intermolecular ligand-exchange reactions (method F) between tetranuclear complexes. The following structures have been proposed on the basis of 19F NMR and mass spectroscopic studies. 

For the mononuclear and dinuclear compounds, except V(CO)6, the mnnber of valence electrons per metal atom is 18. The EAN (effective atomic number) of the metal is the number of electrons of the d" configuration added to twice the number of CO groups, one electron being further added in the case of metal-metal bonded dinuclear systems. For trinuclear, tetranuclear, and hexanuclear compoimds (metal-carbonyl clusters), the cluster valence electron counting is 48, 60, and 86, respectively. As the [K(cryptand 2.2.2)] derivative. As the tetraalkylammonium derivative. As the tetrachloroaluminate. 

Carbon monoxide insertions in metal carbonyl clusters have been reported. One example is the reaction of the tetranuclear iridium cluster Ir4Me(CO)g(/r4- 7 -... 

Although in a few tetranuclear complexes fragmentation is the primary result of photolysis, and others, e.g., (h -C5H5) Fe (CO), are inert with respect to both photo-induced M—M cleavage and carbonyl loss , substitution without fragmentation is the dominant photoreaction of tetranuclear metal carbonyls, especially of those containing second and third-row metals. Both Ir (CO),2 and H Os/CO),2 " tetranuclear... 

Although in a few tetranuclear complexes fragmentation is the primary result of photolysis , and others, e.g., (h -CjHp Fe fCO), are inert with respect to both photoinduced M—M cleavage and carbonyl loss , substitution without fragmentation is the dominant photoreaction of tetranuclear metal carbonyls, especially of those containing second and third-row metals. Both Ir (CO),2 and H Os (CO),2 " lead to tetranuclear olefin-substituted products when irradiated with alkenes, both with some rearrangement of the Ir moiety in the former case, and a hydride abstraction in the latter. Irradiation of H Ru/CO),j in the presence of P(OMe)j or PPhj leads to stepwise formation of the substituted clusters H Ru/CO), (n = 1-4) with low quantum efficiency . Carbonyl substitution also dominates the photochemistry of HjM CCO) (M = Ru, FeRuj, FeOsj) . 

Metal carbonyl clusters are also numerous. In addition to dinuclear carbonyls such as M2(CO)io (M = Mn, Tc, Re), many hydrido clusters and ionic species exist which contain three or more metal atoms [M3H3(CO)i2] (M = Mn, Tc, Re), Mn3(CO)i (linear compound, 1)4 ), and Re3(/i-H)(CO)i4 (the Re —Re —Re angle is 90°). Rhenium forms a number of tetranuclear compounds. Some of them have a butterfly structure while others adopt a triangle of rhenium atoms with one Re attached to it Re4(/i3-H)4(CO)i2, [(Re4( -H)4(CO)i3] , [Re4( -H)6(CO)i4], and [Re4(CO)i6]. Rhenium also forms mixed metal carbonyls possessing several metal atoms (Table 3.14). There are numerous clusters of the group 7 metals containing cyclopentadienyl groups, ... 

A quantitative evidence for the cr-antiaromatic nature of saturated tetranuclear metal-carbonyl clusters [M4(CO)i2] (M = Fe, Ru, Os), which are similar metal-carbonyl analogs of cyclobutane was recently provided by Corminboeuf et al. [124], The isotropic NICS of the C2v [M4(CO)i2] (M = Fe, Ru, Os) clusters are about one-third less negative than those of their trinuclear congeners. Some differences in the magnetic response between cyclobutane and transition metal four-membered rings were identified and analyzed in detail using CMO-NICS (Fig. 37). 

Important pioneer work in the synthesis of polynuclear metal carbonyls was made by reduction of simple metal salts in the presence of carbon monoxide or similar ligands. The best results are obtained in general for the most stable clusters such as triruthenium, triosmium, tetrarhodium, and tetrairidium dodecacarbonyls. Thus, heterogeneous pyrolysis reactions of metal halides with carbon monoxide in the presence of halide acceptors such as Cu or Zn under relative drastic conditions of temperature and pressure can be used for preparing these types of three and tetranuclear carbonyl clusters ... 

Mention must be made of three reviews which appeared in 1977 but which were published too late for inclusion in Volume 7 of this series. The three topics reviewed together were (i) tetranuclear carbonyl clusters, (ii) thermochemical studies on metal carbonyls, and (Hi) the vibrational spectra of metal carbonyls. Two new volumes of Inorganic Syntheses have appeared and contain recipes for preparing the following metal carbonyl species ... 

Corminboeuf, C. Schleyer, P. v. R. King, R. B. Aromaticity of tii- and tetranuclear metal-carbonyl clusters based on magnetic criteria. Chem. Eur. J. 2006,12, 978-984. 

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