Chromium atom hexacarbonyl

Cuadrado and co-workers have reported the synthesis of chromium-containing organosilicon dendrimers.334 These dendrimers, 282, have chromium tricarbonyl units incorporated pendent to the terminal aromatic rings.334 Synthesized via the reaction of the silane dendrimer precursor with chromium hexacarbonyl, complete complexation was not possible due to steric hindrance at higher generations. Electrochemical studies showed that the oxidation of the chromium atoms occurred reversibly in the absence of a nucleophilic species and that the chromium tricarbonyl units behaved as isolated redox centers. 

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. 

Organosilicon dendrimers containing chromium tricarbonyl moieties pendent to peripheral aromatic rings (101) have been synthesized by Cuadrado. These organometallic dendrimers were isolated following reaction of a precursor silane dendrimer with chromiiun hexacarbonyl. It was not possible to effect complexation of every aromatic ring due to steric hindrance. Cyclic voltammetry of these materials indicated that oxidation of the chromium atoms occurred reversibly in the absence of nucleophilic species and that the chromium tricarbonyl imits behaved as isolated redox centers. 

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. 

CBS extrapolation 155, 278 CBS methods 10, 96, 155 cost vs. G2 methods 159 CBS-4 method 155 CBS-Q method 155 CCSD keywords 114 CH bond dissociation 186 charge xxxv, xlii, 15, 286 predicted atomic li charge distribution 20 Cheeseman 53 chlorine (atomic) 137, 159 chlorobenzene 165 chromium hexacarbonyl 52 Cioslowski 198 CIS keyword... 

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

The binary metal carbonyls are named by giving the name of the metal followed by the name carbonyl, with the number of carbonyl groups indicated by the appropriate prefix. For example, Ni(CO)4 is nickel tetracarbonyl, whereas Cr(CO)6 is chromium hexacarbonyl. If more than one metal atom is present, the number is indicated by a prefix. Thus, Co2(CO)8 is dicobalt octacarbonyl, and Fe2(CO)9 has the name diiron nonacarbonyl. 

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. 

A special highlight of this chemistry is the possibility to construct compounds with two adjacent - C-labeled carbon atoms [17], which are of great interest for the investigation of biological systems by means of H NMR spectroscopic methods. For this purpose, the starting complexes (i.e. 36) are prepared from C-labelled chromium hexacarbonyl (Scheme 11). 

Complexation of dihydronaphthalene with chromium tricarbonyl facilitates the attack of a nucleophilic amine and stereoselectively directs the protonation "anti" to the metal atom. (+)-(l S,2 R)-, 2-Dihydro-7-methoxy-1,4-dimethyl-1 - 2 -[methyl(trifiuoroacetyl)amino]propyl -naphthalene [(+)-7] was treated with chromium hexacarbonyl to give a mixture of a- and /(-chromium tricarbonyl complexes in a 10 1 ratio ( H NMR). The complexes were separated and the a-isomer cyclized by base treatment and sonication. Dccomplexation gave the ben-zomorphan tricycle 8 in low yield [40%, based on recovered (+)-7]18. The trifluoroacetyl protection of the amino group was necessary to achieve both stereoselective complexation with chromium tricarbonyl and successful cyclization. 

Enzymatically active NADH has been selectively produced by visible light photoreduction of NAD using [Ru(bipy)3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers and triethanolamine as electron donor (Wienkamp and Steckhan). There is continuing interest in the photogeneration of co-ordinatedly unsaturated species from metal carbonyls etc. which can act as or give rise to catalysts, e.g., for cis-trans isomerization and hydrogenation of alkenes. Ger-rity et al. have used chromium hexacarbonyl to make the first quantitative measurements of the distribution of atomic excited states produced by multiphoton dissociation of a metal carbonyl. The distribution of states turns out to be statistical rather than spin- or polarity-difierentiated. The use of perfluoromethylcyclohexane as solvent has enabled Simon and Peters to observe naked Cr(CO)s as a transient from Cr(CO)6. 

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. 

Cr(CO)sI is isoelectronic with V(CO)6 and like the latter exhibits the expected paramagnetism for a species containing one unpaired electron. Chromium pentacarbonyl iodide appears to be more stable to oxidation but less stable to thermal decomposition than vanadium hexacarbonyl. This higher oxidative stability of Cr(CO)5l as compared with that of V(CO)j may be due to the higher formal oxidation state of the central metal atom of Cr(CO)5l. 

Hydridocarbenes 8.19 are not available by this route. They can be prepared from formamides by reaction with the highly nucleophilic pentacarbonylchromium dianion 8.15, itself prepared by reduction of chromium hexacarbonyl with potassium/graphite (Scheme 8.4). The oxygen atom of the adduct 8.16 is removed using two equivalents of trimethylsilyl chloride. 

Over a series of papers we have investigated several factors which are important for a successful comparison with experiment. To deal with the most serious problem — the MT approximation — we have, with colleagues, successfully implemented a full-potential version of the real-space, MSW method. The modifications to include non-sphericity of the atomic sphere potentials and the varying interstitial potential (the central improvements of the non-MT MSW theory) were found to be important in systems such as chromium hexacarbonyl, although quantitative agreement with experiment had to await the inclusion of a final state (including a core hole) self-consistent-field (SCF) potential. These latest calculations, some of which we present below, appear to have been the first to combine the full-potential treatment and SCF electron densities within the MSW theoretical firamework. Furthermore, they include improvements to the treatment of exchange-correlation, which we discuss. 

Various compounds containing a labile H-atom can be smoothly added to carbon-carbon double bonds in the presence of Fe-pentacarbonyl.—E Ethyl 5-mercaptovalerate, methyl acrylate, and Fe-pentacarbonyl charged in an autoclave after replacement of the air by Ng, and heated 4 hrs. at 130-150° 4-carbethoxybutyl 2-carbomethoxyethyl sulfide. Y 85%.—In place of Fe-carbonyl, colloidal iron prepared from the former may be used. F. e. and reactions, also use of chromium hexacarbonyl, s. A. N. Nesmeyanov et al.. Tetrahedron 17, 61 (1962) s. a. R. G. Petrova and R. K. Freidlina, Izvest. 1962, 59 G. A. 57, 12303d. 

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