Chromium carbonyl structure

Chlorophyll, and photosynthenc reaction center. 917-919 Chromium carbonyl complexes, bond lengths in, 427 Circular dichrotsm ICD). 496-499 Claasen, H. H., 70 Clathrate compounds, 304-306 Claihro-chelates, 530 Clays. 750 Clementi, E., 31, 32 Closo structures, 798-800. 807 Clostridium pasieurianttm, 934 Clusters, 738, 807-819 Coenzyme, 919 Coenzymes, vitamin B,-,... 

Chromium-based reagent systems, for pinacol coupling, 11, 63 Chromium carbenes, in ene-yne metathesis, 11, 272 Chromium carbonyl compounds with bridging hydrides, 5, 206 computational studies and spectroscopy, 5, 203 experimentally determined structures, 5, 204 nitro and nitroso compounds, 5, 205 silatropic migrations, 5, 249 with very weakly bonded ligands, 5, 205 Chromium carbonyl hydrides, preparation and characteristics,... 

The nonastannide cluster is slightly distorted to accommodate the chromium carbonyl fragment at the open face. Figure 14.6.3 shows the structure of [M9Cr(CO)3]4 (M = Sn, Pb). 

The diverse chemistry of chromium carbonyl complexes has its origin in the discovery of Cr(CO)6 in 1926 by Job and Cassal. Early developments in this area included the preparation of the (jj -arene)Cr(CO)3 family of organometallics by Nicholls and Whiting in 1959, the synthesis of the first structurally characterized carbene complex by Fischer and Maasbdl (1965), and a... 

Crystal-structure determinations performed on chromium carbonyl carbynes indicate an average Cr C bond length of 1.70 A, ca. 0.2 A shorter than the average Cr C(carbonyl) distance. An exception is seen in X(CO)4Cr=C-NR2, in which the Cr-C bond is somewhat longer and the C N bond somewhat shorter than expected. These findings are consistent with the presence of some double bond character. The Cr C-R bond angle is customarily very close to linear, but structures bent up to 12° are known. Chromium carbonyl carbynes are frequently employed as starting materials for the synthesis of carbenes inaccessible... 

The photochemistry of chromium carbonyl intermediates was also investigated by Graham and co-workers via IR spectroscopy [24,25] and looked to disprove the possibility that Cr(CO)5 forms a structure of D h symmetry that is more stable than one of 4 symmetry, but they make mention that there could be a rapid... 

Regarding the structural diversity of the primary alcohols, which have been successfully resolved, there seems to be almost no limitation, as demonstrated by the axial-chiral diols 112-114, the ferrocene alcohols 116-122 and the chiral chromium carbonyl complex 123, and most remarkably the helicenediol 115 (Table 11.1-19). 

The chromium crystal structure is characterized by a cubic lattice with valence 6 and coordination number 8. Therefore, each chromium surface atom is able to attach k <3 carbonyl CO groups without destruction of the lattice (Darken Gurry, 1960 Atkins,... 

Such aspects of metal carbonyl structure may be explained by consideration of the coordination number of the central metal atom as an important factor in determining the stability of metal carbonyls. As is the case with other transition metal derivatives such as the ammines, octahedral hexa-coordinate metal carbonyl derivatives seem to be especially favored. Thus, hexacoordinate chromium hexacarbonyl is obviously more stable and less reactive than pentacoordinate iron pentacarbonyl or tetracoordinate nickel tetracarbonyl. Moreover, hexacoordinate methylmanganese pentacarbonyl is indefinitely stable at room temperature (93) whereas pentacoordinate methylcobalt tetracarbonyl (55) rapidly decomposes at room temperature and heptacoordinate methylvanadium hexacarbonyl has never been reported, despite the availability of obvious starting materials for its preparation. 

Espelid and B0rve [100] have recently explored the structure, stabihty, and vibrational properties of carbonyls formed at low-valent chromium boimd to sibca by means of simple cluster models and density fimctional theory (DFT) [101]. These models, although reasonable, do not take into consideration the structural situations discussed before but they are a useful basis for discussion. They foimd that the pseudo-tetrahedral mononuclear Cr(II) site is characterized by the highest coordination energy toward CO. 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

The mononuclear metal carbonyls contain only one metal atom, and they have comparatively simple structures. For example, nickel tetracarbonyl is tetrahedral. The pentacarbonyls of iron, ruthenium, and osmium are trigonal bipyramidal, whereas the hexacarbonyls of vanadium, chromium, molybdenum, and tungsten are octahedral. These structures are shown in Figure 21.1. 

FIGURE 21.1 The structures of the mononuclear carbonyls of nickel, iron, and chromium. 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

Some important reactions of chromium hexacarbonyl involve partial or total replacements of CO ligands by organic moieties. For example, with pyridine (py) and other organic bases, in the presence of UV hght or heat, it forms various pyridine-carbonyl complexes, such as (py)Cr(CO)5, (py)2Cr(CO)4, (py)3Cr(CO)3, etc. With aromatics (ar), it forms complexes of the type, (ar)Cr(CO)3. Reaction with potassium iodide in diglyme produces a potassium diglyme salt of chromium tetracarbonyl iodide anion. The probable structure of this salt is [K(diglyme)3][Cr(CO)4lj. 

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

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