Carbon monoxide symmetry

Many molecules, such as carbon monoxide, have unique dipole moments. Molecules with a center of inversion, such as carbon dioxide, will have a dipole moment that is zero by symmetry and a unique quadrupole moment. Molecules of Td symmetry, such as methane, have a zero dipole and quadrupole moment and a unique octupole moment. Likewise, molecules of octahedral symmetry will have a unique hexadecapole moment. 

In SCF problems, there are some cases where the wave function must have a lower symmetry than the molecule. This is due to the way that the wave function is constructed from orbitals and basis functions. For example, the carbon monoxide molecule might be computed with a wave function of 41 symmetry even though the molecule has a C-xt symmetry. This is because the orbitals obey C41 constraints. 

The process of constructing the MOs of ethylene is similar to that used for carbon monoxide, but the total number of AOs is greater, 12 instead of 8, because of the additional AOs from hydrogen. We must first define the symmetry of ethylene. Ethylene is known from experiment to be a planar molecule. 

Table I shows that there is a general tendency for the C—0 force constant to fall as the constant on the other side of the carbon increases. In all these compounds (except ketene) the symmetry is such that there must be two equivalent ir-bonds between carbon and oxygen decrease of the force constant is a measure of the weakening of these bonds, as compared to free carbon monoxide, by the presence of a third atom which can also form ir-bonds. Table II shows that the bonds are also lengthened slightly by the process. The position of nickel carbonyl in the series shows that the back-...

Further crystallographic evidence for metal-carbonyl ir bonding is found in phosphine and phosphite derivatives of hexacarfconylchromium- Substitution of R3P for CO in Cr(CO)ft creates a complex of C4 symmetry in which one CO group lies trans to the phosphorus ligand (Fig. 11.29). The two trans ligands will compete for the same ir orbital, but carbon monoxide is a better v acid (ir acceptor) than the phosphine (Fig. 11.30). As a result, the Cr—CO r bond should be shorter relative to Cr—CO and to Cr—CO in CrtCO). The data in Table 11.12 show that these predictions are borne out. in keeping with the substantial ir character in the metal-carbonyl bond. 

Finally a very important fact arising from our results is the occurrence of the asymmetric hydroformylation in olefins having C2v symmetry like d -butene. In this case the two faces of the olefin are identical, and the catalyst is evidently able to dissymmetrize the double bond favoring the attack of hydrogen to the re carbon atom and of carbon monoxide to the si carbon atom. A very simple way of looking at this dissymmetriza-tion of the double bond is to assume that the face of the olefin attacked... 

In the reaction of carbon monoxide on a cobalt crystal which could exist in two crystal structures depending on the temperature, the pattern of deposits followed the symmetry of the structure existing at the temperature of the reaction. 

We now begin to understand how the 7t-interaction can explain the observed spectro-chemical series. The anomalous strong field ligands, such as carbon monoxide, pyridine, 2,2 -bipyridine and CN, all possess vacant orbitals of 7t-symmetry of similar energy to the metal valence shell orbitals (Fig. 1-15). 

The tendency of the finite clusters toward close packing of metals is clearly evident in the structure of the anions [Rhu (CO) 2)H5 n]n, (n = 2, 3), reported in Fig. 11 (IS). The central metal atom is dodecacoordi-nated, whereas the 12 rhodium atoms on the surface are pentacoordinated toward the other metal atoms and tricoordinated toward carbon monoxide. The average Rh—Rh distance is 2.81 A without significant differences between internal and surface bonds. The high symmetry of the metal skeleton, Dsh, decreases to apparent C in the molecule it has not been possible so far to determine the positions of the hydrogen atoms. 

Electronic Properties. Ligands with the electron-poor penta-fluorophenyl groups have good jr-acceptor properties and electronically bridge the gap between phosphites and carbon monoxide. Other diols, with or without C2-symmetry, have been used as ligand backbones. Pentafluorophenyl can also be replaced by other aromatic electron-withdrawing groups. ... 

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