Carbonyl CO Complexes

The overall effect is synergistic. CO can donate electron density via a a orbital to a metal atom the greater the electron density on the metal, the more effectively it can 

The energy necessary to stretch a bond is proportional to, , where k = force constant 

Both y donation (which donates electron density from a bonding orbital on CO) and tt acceptance (which places electron density in C—O antibonding orbitals) would be expected to weaken the C—O bond and to decrease the energy necessary to stretch that bond. 

Additional evidence is provided by X-ray crystallography. In carbon monoxide, the C—O distance has been measured at 112.8 pm. Weakening of the C—O bond by the factors described above would be expected to cause this distance to increase. Such an increase in bond length is found in complexes containing CO, with C—O distances approximately 115 pm for many carbonyls. Although such measurements provide definitive measures of bond distances, in practice it is far more convenient to use infrared spectra to obtain data on the strength of C—O bonds. 

The charge on a carbonyl complex is also reflected in its infrared spectrum. Five isoelectronic hexacarbonyls have the following C—O stretching bands (compare with v(CO) = 2143 cm i for free CO)  

FIGURE 13.13 Sigma and Pi Interactions between CO and a Metal Atom. 

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Technetium is stabilized at low oxidation states by suitable ligands such as phosphines, isonitriles, carbon monoxide, and thiourea (Gorski and Koch 1970). Organometallic carbonyl (CO) complexes are interesting precursors for a new class of Tc(I) radiopharmaceuticals (Alberto et al. 2001 Schibli et al. 2000). 

Tc chelates suitable for conjugate formation with biomolecules are derived from Tc in oxidation states V, III, and I. Organometallic carbonyl (CO) complexes serve as precursors for the synthesis of Tc(I) pharmaceuticals (Sect. 2.2). 

Succinic acid diesters are also obtained by one-step hydrogenation (over Pd on charcoal) and esterification of maleic anhydride dissolved in alcohols (40) carbonylation of acrylates in the presence of alcohols and Co complex catalysts (41—43) carbonylation of ethylene in alcohol in the presence of Pd or Pd—Cu catalysts (44—50) hydroformylation of acetylene with Mo and W complexes in the presence of butanol (51) and a biochemical process from dextrose/com steep Hquor, using Jinaerobiumspirillum succiniciproducens as a bacterium (52). 

With respect to CO complexes, the luminescence spectra of a series of Group VI metal carbonyls and substituted carbonyls were obtained in frozen gas matrices at 12K. In addition, the IR spectra of HCo(CO>4 and HCo(CO)3 (proposed as an intermediate in hydroformylation) were observed in an argon matrix. ... 

Fig. 13. Proposed reaction mechanism for ACS. The reaction involves the sequential assembly of acetyl-CoA from a carbonyl, methyl, and CoA. We favor a Ni(l) nucleophile to form a catEdytically competent paramagnetic M-CO complex, but see text for discussion of Em alternative mechanism.

A key step proposed in the radical chain mechanism for the formation of the formyl complex is the coordination of CO to the Rh(OEP)- monomer, to give an intermediate carbonyl complex, Rh(OEP)(CO)- which then abstracts hydride from Rh(OEP)H to give the formyl product.This mechanism was proposed without direct evidence for the CO complex, and since then, again from the research group of Wayland, various Rh(fl) porphyrin CO complexes, Rh(Por)(CO), have been observed spectroscopically along with further reaction products which include bridging carbonyl and diketonate complexes. 

The NIS investigation of heme complexes includes various forms of porphyrins (deuteroporphyrin IX, mesoporphyrin IX, protoporphyrin IX, tetraphenylpor-phyrin, octaethylporphyrin, and picket fence porphyrin) and their nitrosyl (NO) and carbonyl (CO) derivatives, and they have been the subject of a review provided by Scheidt et al. [109]. 

The field of alkyl and aryl Co complexes in low formal oxidation states has been extensively studied, and is frequently associated with cluster chemistry.92 Alkyl and aryl ligands, with or without additional functionality, are often co-ligands with tt acceptors such as carbonyls and/or phosphines, e.g., (MeOCOCH2)Co(CO)3(PPh3).93 A simple example of a cluster is (RC)Co3(CO)9, where the triangular Co(CO)3 3 moiety is capped by the 73-alkyl fragment, which occupies the apex of a distorted Co3C tetrahedron. 

High-valent Co complexes incorporating carbonyl ligands are rare, and those extant are outside the scope of this review. The companion series Comprehensive Organometallic Chemistry covers this area. 

The IR and XH-NMR spectral data for the various titanocene mono-carbonyl-phosphine complexes are compiled in Table III. Examination of the carbonyl stretching frequencies (Table III) nicely demonstrates the enhanced 7r-backbonding of the titanium center to CO as the -accepting ability of the phosphine ligand decreases. 

Evidence was shown for migration of an alkyl group in carbonyl insertion, and deinsertion steps between the methyl carbonyl rhodium complex [ r/Vi/ -Indenyl-l -(CH2)3PPh2 Rh(CO)-Me](BF4) and the acetyl rhodium complex [ r/5 r/l-(Indenyl-l -(Cl I2)3PPh2) RhI(COMe)] by crystallography as well as by 1H NMR spectroscopy.28... 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

DF calculations were carried out on CO complexes of small neutral, cationic, and anionic gold clusters Au with n= 1-6. The -coordination mode (terminal C-coordination) was found to be the most favorable one irrespective of the charge of the cluster, and cluster planarity is more stable for the bare clusters and their carbonyls. As expected, adsorption energies are greatest for the cationic clusters, and decrease with size. Instead, the adsorption energies of... 

Two hydrogen bridges are present in some carbonyl hydride complexes. This is illustrated by the structure of the [H2W2(CO)8]2 anion,... 

Table 4.25. The NBO descriptors o/W(CO) complexes (n = 3-6), showing carbonyl net charge (geo) and tungsten hybridization (hw) for a- and uo-bonded carbonyl ligands (see Fig. 4.36)...

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

Figure 4.101 displays the subtle variations in metal-carbonyl bond lengths in the group 6 M(CO) complexes. In each case one can clearly distinguish the coordinate omc bonds (solid lines) from the hypervalent toMc prebonds (dashed lines). The latter are about 0.1A longer, but exhibit a similar vertical variation within the group. 

Figure 4.103 Incremental CO binding energies, Eq. (4.129), for M(CO) complexes of group 6 metals Cr (circles), Mo (squares), and W (triangles). For n = 3, the quantity plotted is the average energy of dissociating all three carbonyls from M(CO)3.

Apart from Cr(0) carbonyl complexes, similar Mo, W and Co complexes also catalyze 1,4-cis-hydrogenation of dienes, though the selectivity of these catalysts is relatively low [63]. 

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