Metal carbonyls polynuclear

Structure. The CO molecule coordinates in the ways shown diagrammaticaHy in Figure 1. Terminal carbonyls are the most common. Bridging carbonyls are common in most polynuclear metal carbonyls. As depicted, metal—metal bonds also play an important role in polynuclear metal carbonyls. The metal atoms in carbonyl complexes show a strong tendency to use ak their valence orbitals in forming bonds. These include the n + 1)5 and the n + l)p orbitals. As a result, use of the 18-electron rule is successflil in predicting the stmcture of most metal carbonyls. 

Synthesis of Heteronuclear and Polynuclear Metal Carbonyls. Heteronuclear metal carbonyls are usually synthesized either by metathesis (27) or addition (28,40). 

The most important fragmentation reactions of sulfur diimides occur with polynuclear metal carbonyls, e.g., M3(CO)i2 (M = Fe, in... 

A further point of interest is their ready interconversion in solution, which we have studied by variable temperature n.m.r. measurements (8). Reaction of [(n-C5H5)(0C)2W =CR] with polynuclear metal carbonyl species is likely to afford many new heteronuclear cluster compounds containing tungsten. 

The principal assumption of transferability (Section 1.3.) seems to be acceptable on the basis of the data accumulated so far, but clearly requires further testing where organic ligands are concerned. No alternative assumption suggests itself in relation to M-CO bond enthalpies, but the resulting values of T and M could be improved when measurements are made on the more exotic polynuclear metal carbonyls that have been prepared in the last few years. 

In this chapter we have examined examples of polynuclear metal carbonyl complexes as well as simple metal carbonyl hydrides. Consider now the polynuclear carbonyl hydride complex, H,Os,(CO)i2- Rationalize the formulation of this species. From your application of the 18-electron rule, what can you say about the structure of this molecule1 How is it similar to or different from the complex Os COln shown in Figure 5.9 (See Churchill M. R. Wasserman, H. J. Iticrg, Chen. 1980, 19, 2391-2395.)... 

These complexes ure chemically like polynuclear metal carbonyl complexes (class I) but are included here instead of in Chapter 15 because they do not possess metal-carbon bonds. A rich chemistry has been developed in which alkoxides Func-... 

A second feature of metal halide cluster chemistry is that the early transition metals are more prone to form metal -metal bonds than are the later noble metals and coinage metals. Again the polynuclear metal carbonyls differ in this facet of metal-metal bond behavior, and, in fact, metal carbonyl clusters become more common on going from the left to the right of the Periodic Table. 

In summary, this procedure will be most effective with an anion of high nucleophilicity, an iodosilane of low steric requirements, and a nonpolar solvent. Reactions usually begin at low temperatures, although long shaking at room temperature may be needed for completion. The method can be capricious, and occasionally a familiar system may yield no product at all, with no obvious explanation. Nevertheless, it has considerable extra potential, particularly for the synthesis of silicon derivatives of polynuclear metal carbonyls. 

There is no doubt that most of the mono- and polynuclear metal carbonyls react with liquid NH3 via nonisolable carbonyl carbamoyl intermediate complexes in which the acid amide group —CONH2 is directly bound to the transition metal. For the reaction of Mn2(CO)10 with liquid NH3, we have been able to show that, at -78°C, quantitative formation of ciJ-Mn(CO)4(NH3)—CONH2 and HMn(CO)5 occurs (105) according to the following reaction ... 

C. Mass Spectra of Polynuclear Metal Carbonyl Derivatives. 95... 

The facile loss of carbonyl groups in the mass spectra of metal carbonyls permits the generation of novel bare metal cluster ions Mj in the mass spectra of polynuclear metal carbonyls of the type Mx(CO)y. Thus in the mass spectra of Mn2(CO)io and Co2(C0)s all carbonyl groups are lost before rupture of the metal-metal bond resulting in the production of the bimetallic ions Mn and Co 2, respectively 14>. 

In some polynuclear metal carbonyls of the first row (3d) transition metals the metal-metal bonds are too weak to survive complete loss of carbonyl groups. Thus in the mass spectrum of Fe3(CO)i2 stepwise loss of carbonyl groups occurs only as far as the tricarbonyltriiron ion Fe3(C0)s 2fl>. The rupture of the iron-iron bonds competes with the stepwise loss of carbonyl groups givingions such as Fe2(CO)4 and Fe(CO)4. The mass spectrum of Fe2(CO)9 exhibits the ion Fe(CO) which may represent pentacarbonyliron formed by the following pyrolysis process25) ... 

Special success has followed the use of alkali metal amalgams in tetra-hydrofuran or other ethers (VII, 11, 44) as reducing agents in the syntheses of mono- and polynuclear metal carbonyls, e.g.,... 

In the last 5 years at least eight reviews concerning polynuclear metal carbonyls have been published (16,18, 84, 41, 86, 87, 91, 108), and it may appear unlikely that any new material could be added at this time. Nevertheless, we have willingly accepted the invitation of the Editors to present a comprehensive review which, beside the material published up to June 1974, also summarizes the most relevant of our recent results, as yet only published as preliminary notes. 

In the last three decades of the twentieth century, following Walter Hieber s retirement, four aspects of the research on mono- and polynuclear metal carbonyl complexes found particular attention. These were the preparation of highly reduced carbonyl metallate anions, the generation of stable metal carbonyl cations, the matrix isolation of uncharged metal carbonyls obeying or not the 18-electron rule and, last but not least, the giant metal carbonyl clusters. 

For practical purposes the field of metal-metal bonds and metal atom clusters can be divided into two broad areas. (1) Those compounds with the metal atoms in formal oxidation states of zero or close to it, including negative ones. For the most part these are polynuclear metal carbonyls, or very similar compounds. In these compounds the M-M bonds are usually long, weak and of order one. (2) Compounds with the metal atoms in low to medium positive oxidation states, and ligands of the same kinds normally found in classical Werner complexes, e.g., halide, sulfate, phosphate, carboxylate or thiocyanate ions, water, amines and phosphines. Compounds of this type include metal-metal bonds of orders ranging from about 1/2 to 4.0. 

The thermochemical data obtained, for the most part, by high temperature microcalorimetry of thermal decomposition or of halo-genation of polynuclear metal carbonyls have been summarized recently (12). The disruption enthalpy, AHD referring to the process... 

Metal-Metal bond enthalpy contributions, M/kJ mol in polynuclear metal carbonyls derived from a) two-centre electron pair bond description, b) bond enthalpy/bond length relation. 

Tetrahedral cages or clusters have long been known for the P4, As4, and Sb4 molecules and in more recent years have been found in polynuclear metal carbonyls such as Co4(CO)i2, Ir4(CO)12, [Tf-QHsFelCO)], RSiCo3(CO)9, Fe4(CO) 3, Re4(CO)12H4, and a number of others B4C14 is another well-known example and doubtless many more will be encountered. 

Di- and polynuclear metal carbonyl compounds have a general tendency to engage in a type of fiuxional behavior called carbonyl scrambling. This type of behavior arises because of some of the inherent properties of metal to CO bonding. As shown in Fig. 1-13, the energy of a binuclear system consisting of two metal atoms... 

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