Metal carbonyl complexes bonding

J. R. Wells, and E. Weitz, Rare Gas-Metal Carbonyl Complexes Bonding of Rare Gas Atoms to the Group VI Pentacarbonyls, J. Am. Chem. Soc. 114, 2783-2787 (1992). 

The nature of the bonding, particularly in CO, has excited much attention because of the unusual coordination number (1) and oxidation state (-f2) of carbon it is discussed on p. 926 in connection with the formation of metal-carbonyl complexes. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

N.m.r. studies are reported on the triethylphosphine and trisdi-methylaminophosphine complexes with boron halides, and triethylphos-phine complexes with aluminium chloride. A correlation of Sp with the number of phosphorus ligands in metal carbonyl complexes has led to a qualitative rationalization of 8p in terms of a- and 7r-bonding. ... 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

Figure 17 Some examples of metal carbonyl complexes with M-Ag bonds.

Therefore bonding in PdSiO may be compared to that in typical transition metal carbonyl complexes. 

While the majority of group 4B metal carbonyl complexes contain 7r-bonded hydrocarbon ligands, most notably 17-cyclopentadienyl, recent studies by Wreford and co-workers have led to the identification and isolation of three novel phosphine-stabilized titanium carbonyl complexes (12,13). 

The very large perturbing influence of C and 0 bonding on the CO bond order led us to explore the influence of Lewis acid and proton acid promoted reactions of metal carbonyl complexes. 

Pensak and McKinney (28) [PM], using this method, have recently reported a systematic study of first-row transition metal carbonyl complexes for which experimental bond distances and angles were reliably reproduced, along with key bond dissociation energies. 

Metal bromides, 4 322-330 Metal can food packaging, 18 37-39 Metal-carbene complexes, 26 926 Metal-carbon compounds, 4 648, 650 Metal-carbon eutectic fixed points, 24 454 Metal carbonyl catalysts, supported, 16 75 Metal carbonyl complexes, 16 73 Metal carbonyls, 15 570 16 58-78 bonding and structure of, 16 59-64 from carbon monoxide, 5 12 in catalysis, 16 72-75 economic aspects of, 16 71 health and safety aspects of, 16 71 heteronuclear, 16 69-71 high nuclearity, 16 66-69 high nuclearity carbonyl clusters, 16 64-66... 

Table 24. Metal-metal bond enthalpies (kJ mol 1) in binuclear metal carbonyl complexes determined by electron impact and reaction kinetic methods...

The tetra-cA-cycIononatetracne 241 is unstable and easily rearranges at 23 °C (t /2 50 min) to the isomeric d.v-8,9-dihydroindcne 242 (equation 77)89. It is interesting, however, that the iron(III) tricarbonyl complex of tetraene 241 is stable for many days at room temperature and isomerizes to the Fe-complex of 242 only upon heating in octane at 101 °C89. The principle of stabilization of the reactive multiple bonds with metal carbonyl complexes is well-known in modem organic synthesis (e.g. see the acylation of enynes90). 

The force constant of carbon monoxide can be associated with a bond order of three that of an organic carbonyl (such as formaldehyde) can be associated with a bond order of two, and that of CO in metal carbonyl complexes can be found by intrapolation. Thus (to the extent that the uncertainties of Sections II.l—II.8 can be ignored) we have a facile probe for d-electron delocalisation within a molecule. 

The facile formation of metal carbonyl complexes makes rhodium a very useful catalyst for both the hydroformylation of multiple bonds and the decarbonylation of the aldehydes. Two groups have independently utilized the metal carbonyl complex obtained from decarbonylation of aldehydes in the PK reaction (Scheme 11.11) [24]. 

Further examples of coordinate bonds are found in metal carbonyl complexes. Metal carbon (carbon monoxide) bond distances in a selection of (first-row) transition-metal carbonyls and transition-metal organometallics are examined in Table 5-11. As expected, Hartree-Fock models do not perform well. The 6-3IG model is clearly superior to the STO-3G and 3-2IG models (both of which lead to completely unreasonable geometries for several compounds), but still exhibits unacceptable errors. For example, the model shows markedly different lengths for the axial and equatorial bonds in iron pentacarbonyl, in contrast to experiment where they are nearly the same. Hartree-Fock models cannot be recommended. 

Anomalous behavior is exhibited by the tricarbonylruthenium complex of 1-methoxycar-bonyl-1//-azepine in that at room temperature with TCNE, perfluoropropanone or l,l-dicyano-2,2-bis(trifluoromethyl)ethylene, exo-2,5-adducts, e.g. (143), are obtained. Deuteration studies reveal that the addition is non-concerted and involves an initial electrophilic attack by the 2u--system at the uncoordinated double bond of the azepine-metal carbonyl complex <77JCS(D)204). 

The facility of 1H-azepines to form transition metal carbonyl complexes was realized soon after they were first synthesized. Variable temperature HNMR studies on the tricarbonyliron complex formed either by photolysis of 1-ethoxycarbonyl-l//-azepine with tricarbonyliron in THF, or by heating the azepine with nonacarbonyldiiron in hexane, demonstrated that it undergoes rapid reversible valence tautomerism and that there is considerable restricted rotation about the N—CO bond (B-69MI51600). The molecular geometry of the complex has been determined by X-ray analysis (see Section 5.16.2.2). 

Jahn, H. A. Teller, E. Proc. Roy. Soc. bond. A 1937, 161, 220-235. Mckinlay, R. G. Paterson, M. J. The Jahn-Teller Effect in Binary Transition Metal Carbonyl Complexes. In The Jahn-Teller Effect Fundametals and Implications for Physics and Chemistry , Eds. Koppel, H. Yarkony,... 

Processes for two-electron reductions, two sequential one-electron reductions with a radical anion intermediate, and reactions of dianions with unreduced parents to give radical anions were observed. Structural reorganization is occasionally observed, particularly in the case of Fe(CO)2 and Fe(CO)3 complexes (26). There appears to be little correlation between structure and electrochemical behavior. In general, the presence of metal-metal bonds in the substrate appears to correlate well with the ability to yield a stable radical anion on reduction. The lack of a metal-metal bond correlates, although poorly, with the ability to form radical cations (25). At present, the predictability of results from reduction in metal-carbonyl complexes is very low. The area remains one in which a great deal more work is needed. 

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