Transitions hexacarbonyl complexes

It is, however, for the transition metals themselves that DFT has proven to be a tremendous improvement over HF and post-HF methods, particularly for cases where tlie metal atom is coordinatively unsaturated. The narrow separation between filled and empty d-block orbitals typically leads to enormous non-dynamical correlation problems with an HF treatment, and DFT is much less prone to analogous problems. Even in cases of a saturated coordination sphere, DFT methods typically significantly ouqierform HF or MP2. Jonas and Thiel (1995) used the BP86 functional to compute geometries for the neutral hexacarbonyl complexes of Cr, Mo, and W, the pentacarbonyl complexes of Fe, Ru, and Os, and the tetracarbonyl... 

Transition metal complexes are used as catalysts and as reagents in the synthesis of imides. Molybdenum hexacarbonyl activates strained aziridines and allows the nucleophilic attack of caibanions. Intramolecular rearrangement and a final oxidation yields imides completely stereospecifically (equation 59).38> Dicobalt octacarbonyl catalyzes the conversion of 3,7-unsaturated amides to imides in the presence of carbon monoxide (equation 60). ... 

The reactions of boronates 2.68 (Re = Rz = H, R = i-Pr) are somewhat less selective [698], However, by using the arenechromium tricarbonyl complex of benzaldehyde or dicobalt hexacarbonyl complexes of a-alkynylaldehydes, homoal-lyl alcohols are obtained with a high selectivity after decomplexation [722, 1203] (Figure 6.44). These selectivities are interpreted by distorted chair transition states (Figure 6.44). In the reactions of allylboranes, the approach of the aldehyde minimizes both the steric interactions with the boron substituents and the eclipsing 1,3-interactions of the aldehyde C-R bond with the B-C bond. In the case of boronates 2.68, repulsive interactions between the oxygen lone purs are also avoided [698,1204] (Figure 6.44). 

An alternative to the construction of multiplet states is the use of time-dependent DFT (TDDFT, see Chapter 2.40). TDDFT not only offers the potential for computing accurate transition energies but also provides the transition moments so that the intensity of the transition can be assessed. For example, hexacarbonyl complexes of Cr, Mo, and W have been analyzed using relativistic TDDFT. In contradiction of the original interpretation of the lowest... 

The heat of formation of [MolCOlg] has been determined as -960 + 12 kJ mol by measuring its heat of decomposition. The Mossbauer parameters for the 100 keV transition of in [W(CO)g] and some tungsten(vi) complexes have been measured and discussed in terms of known bonding and structure. Secondary ions [M (CO) ] (M = Mo, m = I or 2 M = W, m = 1—4 n = 0—14) formed by ion-molecule reactions have been observed in the mass spectra of the hexacarbonyls. A mixt u re of [Cr(CO) ] and [MolCO) ] vapours affords [CrMo(CO) ] ( = 5—7). [MofCOl ] and [WICO) ] catalyse the condensation of isocyanates with aldehydes to give imines in high yields. ... 

While the first transition metal carbene complex was reported in 1964134, the first cyclopropylcarbene complex salt [(CO)5Cr=C(chromium hexacarbonyl, followed by tetramethylammonium bromide135. Subsequent reaction with trimethyloxonium fluoro-borate gave methoxycarbene complex (CO)5Cr=C(OMe)(c-Pr) (equation 62)136. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

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