Chromium hexacarbonyl structure

In Chapter 3, we optimized the structure of chromium hexacarbonyl using two... 

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. 

The rearrangement could be corroborated via addition and cycloaddition reactions and also by the synthesis of the (OC)5 V=SnRR, complex97. The analogous chromium complex is formed via a reaction of chromium hexacarbonyl in THF (Scheme 16)98. The molecular structure is shown in Figure 42. The molybdenum complex is formed in the same way (Scheme 16, Figure 43)98. 

Chromium hexacarbonyl reacts with Ph2AsCH2AsPh2 yielding a compound of stoichiometry Cr(CO)2(Ph2AsCH2AsPh2) 357) and with triarylphosphines yielding complexes of the type [Cr(CO)2L]2 [L = PPhg, P( i-tolyl)3, and P(p-tolyl)3] (31). The arsine and triphenyl-phosphine compounds have been shown by X-ray analysis to possess the structures (VII) and (VIII), respectively, in which a phenyl group is... 

Optimize the structure of chromium hexacarbonyl at the Hartree-Fock level, using the STO-3G or 3-21G basis set. Include SCF=NoVqrAcc in the route section of your job (this option says to use full convergence criteria throughout the SCF computation, and it aids in convergence for this calculation). 

In Chapter 3, we optimized the structure of chromium hexacarbonyl using two different basis sets. Now we will investigate the structures of M(CO)6> where M is III chromium, molybdenum and mngsten-... 

The electron configurations for the transition metals discussed here and in Appendix B are for individual metal atoms in the gas phase. Most chemists work with the transition metals either in the metallic state or as coordination compounds (see Chapter 25). A solid transition metal has a band structure of overlapping d and s orbital levels (see Section 13-7). When transition metal atoms have other types of atoms or molecules bonded to them, however, the electronic configuration usually becomes simpler in that the d orbitals fill first, followed by the next higher s orbital. This is illustrated by Cr, which has a 4s 3d electronic configuration as a free atom in the gas phase. But in the compound Cr(CO)5, chromium hexacarbonyl, which contains a central Cr atom surrounded by six neutral carbon monoxide (or carbonyl) groups, the chromium atom has a 3d electronic configuration. 

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. 

A. Jost, B. Rees, and W. B. Yelon, Acta Crystallogr., Sect. B, 31, 2649 (1975). Electronic Structure of Chromium Hexacarbonyl at 78 K. 1. Neutron Diffraction Study. 

Synthesis of tricarbonyl[t -(methylsulfenyl)benzene]chromium(0) (Structure 2). Complexation of (methylsuifenyl)benzene (Structure 1) by thermolysis with chromium hexacarbonyl (Scheme 6.5)... 

Synthesis of tricarbonyl[/ /,/V-diniethyl-a(f )-phenylethylamine] chromium(O) (Structure 12). Complexation of chiral complex 11 by thermolysis with chromium hexacarbonyl... 

The structural studies have suggested an octahedral geometry for chromium hexacarbonyl. The Ci C distance is found to be 1.92 A, while the C—O bond length is 1.16 A. The molecule is also found to be diamagnetic. 

Treatment of chromium hexacarbonyl with cyclo-octa-l,3,5-triene gives the deep red complex CsHioCr(CO)3 [95]. X-Ray analysis shows the structure, 5.11 [96] in which six of the carbons are within 2-24-2-28 A of the chromium atom and may be assumed to bond to it, the remaining two carbon atoms are further away (/ 3-l A). The bond lengths of the six-bonded carbons show distinct alternate single and double bond character. 

Chromium hexacarbonyl reacts with 1,6-methanocyclodecapentaene forming the complex, 5.12h [97h], whose crystal structure has been determined by X-ray diffraction [97c]. In this molecule the chromium lies below one side of the clerical hat configuration of the olefin. In order that the chromium achieves the 18-electron configuration it is necessary to postulate that it is bonded, albeit weakly to the C, and C, carbons. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

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. 

Chromium forms a white solid, hexacarbonyl, Cr(CO)6, with the chromium in formal oxidation state 0 the structure is octahedral, and if each CO molecule donates two electrons, the chromium attains the noble gas structure. Many complexes are known where one or more of the carbon monoxide ligands are replaced by other groups of ions, for example [Cr(CO)5I] . 

R. N. Perutz, and J. J. Turner, Infrared Spectra and Structures of 13CO-Enriched Hexacarbonyls and Pentacarbonyls of Chromium, Molybdenum, and Tungsten, Inorg. Chem. 14, 262-270 (1975) (Part II). 

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