Metal carbonyls, transition metals

Covalent. Formed by most of the non-metals and transition metals. This class includes such diverse compounds as methane, CH4 and iron carbonyl hydride, H2Fe(CO)4. In many compounds the hydrogen atoms act as bridges. Where there are more than one hydride sites there is often hydrogen exchange between the sites. Hydrogens may be inside metal clusters. 

Liicke et al. have prepared other phosphinated POSS compounds Tg[(CH2)2-PMe2]8 and Tg[(CH2)3-PMe2]8 by treating T8[CH = CH2]8 or T8[CH2-CH = CH2]8 with H-PMe2 under UV irradiation. The former compound has shown to have good coordination properties to carbonyl transition metal complexes such as CpMn(CO)3 (Table 15). 

Further examples of coordinate bonds are found in metal carbonyl complexes. Metal carbon (carbon monoxide) bond distances in a selection of (first-row) transition-metal carbonyls and transition-metal organometallics are examined in Table 5-11. As expected, Hartree-Fock models do not perform well. The 6-3IG model is clearly superior to the STO-3G and 3-2IG models (both of which lead to completely unreasonable geometries for several compounds), but still exhibits unacceptable errors. For example, the model shows markedly different lengths for the axial and equatorial bonds in iron pentacarbonyl, in contrast to experiment where they are nearly the same. Hartree-Fock models cannot be recommended. 

Structural Comparison of Corresponding /l-Methylene and -Carbonyl Transition Metal Complexes... 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

It was shown in Sec. 11.3.4.3 that Aubke, Willner and colleagues [47] recently carbonylated transition metal fluorosulfates and other compounds in neat SbF5 to generate naked transition metal carbonyl cations in association with the very weakly basic anion Sb2Ff,. 

Carbonyl precursors of the type [R3MM (CO)5] (M = group IV metal M = transition metal) react readily with PF3 either thermally or under the influence of UV irradiation (method D), and PF5 has also been utilized in the case of silyl-substituted metal carbonyl complexes (method E). 

Question 4.16 Lanthanide alkoxide clusters do not contain metal-metal bonds. Transition metal carbonyl clusters frequently do. Comment. 

Trialkylborohydride reducing agents differ from borohydride in their ability to transfer hydride directly to a carbonyl ligand without prior substitution in the coordination sphere. They are used to synthesize formyl complexes " . When formyl complexes lose CO and undergo hydride migration from the formyl ligand to the metal, a transition-metal hydride results. The process is formally similar to nucleophilic attack by [OH] on a carbonyl ligand, followed by loss of CO and formation of a transition-metal hydride. Examples of hydride syntheses via formyl complexes are ... 

A large fraction of the binuclear mixed chalcogen/carbonyl transition metal complexes of iron and manganese contain the M2E (CO)6 core with the butterfly-type structure (n = 2) or substructure (n — 3, see Section 1.10.3). As an example of a complex with thiolate ligands, the structure of [Fe2(SC3H7)2(CO)6] is shown in... 

The answers to these questions and other questions about the cyanide and carbonyl transition-metal complexes can be derived from the idea that the cyanide and carbonyl groups form double bonds with the transition metal atom. 

The Dewar-Chatt-Duncanson model that takes into account this bonding mode, including the n backbonding with the CO and C2H4 ligands, is represented below for classic metal-carbonyl and metal-ethylene bonds. Of course, the other unsaturated hydrocarbons bind the transition-metals according to the same n backbonding model. 

Numerous 71-cyclopentadienyl and carbonyl transition metal aryl and alkyl complexes are known which also contain tertiary phosphine ligands. These complexes are discussed as 71-cyclopentadienyl- or carbonyl-a-organo complexes, to which they are more closely related. 

TT-Allylpalladium chloride (36) reacts with the nucleophiles, generating Pd(0). whereas tr-allylnickel chloride (37) and allylmagnesium bromide (38) reacts with electrophiles (carbonyl), generating Ni(II) and Mg(II). Therefore, it is understandable that the Grignard reaction cannot be carried out with a catalytic amount of Mg, whereas the catalytic reaction is possible with the regeneration of an active Pd(0) catalyst, Pd is a noble metal and Pd(0) is more stable than Pd(II). The carbon-metal bonds of some transition metals such as Ni and Co react with nucleophiles and their reactions can be carried out catalytic ally, but not always. In this respect, Pd is very unique. 

Not all ligands use just two electrons to bond to transition metals Chromium has the electron configuration [Ar]4s 3rf (6 valence electrons) and needs 12 more to satisfy the 18 electron rule In the compound (benzene)tricarbonylchromium 6 of these 12 are the tt elec Irons of the benzene ring the remammg 6 are from the three carbonyl ligands... 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

A second process is the direct carbonylation of dimethylamine [124-40-3] ia the presence of a basic catalyst or a transition metal. This carbonylation is often mn ia the presence of methanol ia order to help solubilize the catalyst (7), and presumably proceeds through methyl formate as an iatermediate. 

A modification of the direct process has recentiy been reported usiag a ckculating reactor of the Buss Loop design (11). In addition to employing lower temperatures, this process is claimed to have lower steam and electricity utihty requirements than a more traditional reactor (12) for the direct carbonylation, although cooling water requirements are higher. The reaction can also be performed ia the presence of an amidine catalyst (13). Related processes have been reported that utilize a mixture of methylamines as the feed, but require transition-metal catalysts (14). 

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