Electron metal carbonyls

We synthesized cationic y2-acetyl compounds 28,25 by combining iron acetyl complexes CpFe(C0)L(C0CH3) (g7) [L=C0,PPh3] with a coordinatively unsaturated (16-electron) metal carbonyl salt CpM(CO)n+[M=Fe,n=2 M=Mo,n=3], as indicated in Scheme 5. Thus... 

Solution. Boron trihydride has six valence electrons, two short of a filled octet. Thus, we are looking for a 16-electron metal carbonyl. The only possibility is [Cr(CO)s]. 

Numerous metal carbonyl triangles formed by trimer-ization of 16-electron metal carbonyl fragments have been prepared. These include the homometallic trimetal dodecacarbonyls M3(CO)12 = Pe, Ru, and Os),... 

The main mechanistic exception to the above generalizations is V(CO)g, which has an I, mechanism for PRj substitution reactions. This compound is unique in that it is the only 17-electron metal carbonyl and also is by far the most labile. Some kinetic results for substitution on V(CO)g in hexane are given in Table 5.1. 

Brown has shown that metal-carbonyl-hydride complexes such as [Re(H)(CO)s] can nndergo CO substitntion by a phosphine (PBus) according to the H-atom-transfer-chain mechanism A classic type of initiation to introduce the radical species into the chain is to photolyze the metal-carbonyl dimer, which generates the reactive 17-electron metal-carbonyl monomer ... 

The high-nuclearity metal-carbonyl clusters [M6(CO)i6] and beyond often escape the 18-electron rule to obey Wade s rules (see Chap. 2). Thus, mononuclear neutral metal carbonyls are known for the metals that have an even number of d electrons, whereas neutral dimers with a single metal-metal bond are known for metals that have an odd number of d electrons. Metal-carbonyl clusters are essentially known in... 

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. 

Most metal carbonyls are synthesized in nonaqueous media. Reactive metals, such as sodium (85), magnesium (105), zinc (106), and aluminum (107,108), are usually used as reducing agents. Solvents that stabilize low oxidation states of metals and act as electron-transfer agents are commonly employed. These include diethyl ether, tetrahydrofiiran, and 2-methoxyethyl ether (diglyme). 

On the basis of the 18-electron rule, the d s configuration is expected to lead to carbonyls of formula [M(CO)4] and this is found for nickel. [Ni(CO)4], the first metal carbonyl to be discovered, is an extremely toxic, colourless liquid (mp —19.3°, bp 42.2°) which is tetrahedral in the vapour and in the solid (Ni-C 184pm, C-O 115 pm). Its importance in the Mond process for manufacturing nickel metal has already been mentioned as has the absence of stable analogues of Pd and Pt. It may be germane to add that the introduction of halides (which are a-bonded) reverses the situation [NiX(CO)3] (X = Cl, Br, I) are very unstable, the yellow [Pd"(CO)Cl2]n is somewhat less so, whereas the colourless [Pt (CO)2Cl2] and [PtX3(CO)] are quite stable. 

The most satisfactory route to the synthesis of the ri -borole complexes is the reaction of dihydroboroles (2-borolenes and 3-borolenes) with metal carbonyls. An alternative method of synthesis includes formation of the borole adducts with ammonia, 320 (R = Me, Ph) [87JOM(336)29]. Thermal reaction of 320 (R = Me, Ph) with M(C0)6 (M = Cr, Mo, W) gives 321 (M = Cr, R = Me, Ph M = Mo, W, R = Ph). There are data in favor of the Tr-electron delocalization over the borole... 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

The metal carbonyls Ni(CO)4, Fe(CO)s, and Cr(CO)6 are observed to be diamagnetic. This follows from the theoretical discussion if it is assumed that an electron-pair bond is formed with each carbonyl for the nine eigenfunctions available (3d64s4p3) are completely filled by the n bonds and 2(9 — n) additional electrons attached to the metal atom (n = 4, 5, 6). The theory also explains the observed composition of these unusual sub-... 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

Susac et al. [33] showed that the cobalt-selenium (Co-Se) system prepared by sputtering and chemical methods was catalytically active toward the ORR in an acidic medium. Lee et al. [34] synthesized ternary non-noble selenides based on W and Co by the reaction of the metal carbonyls and elemental Se in xylenes. These W-Co-Se systems showed catalytic activity toward ORR in acidic media, albeit lower than with Pt/C and seemingly proceeding as a two-electron process. It was pointed out that non-noble metals too can serve as active sites for catalysis, in fact generating sufficient activity to be comparable to that of a noble metal, provided that electronic effects have been induced by the chalcogen modification. 

In the following we shall see that a more detailed picture of the structural properties of small metal deposits and their electronic interaction with the substrate can be obtained from the infrared spectra of metal carbonyls created in situ by reaction with CO from the gas phase. 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

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