Carbonyl compounds force constants

In the series of compounds [W(CO)sL] [L = P(OPh)3, P(OBu)3, PPh(OBu)2, PPh2(OBu), NH2CgHjj, or APhj, where A = P, As, Sb, or Bi] a linear relationship between the Cotton-Kraihanzel carbonyl stretching force constant and the corresponding chemical shift has been established. This suggests that,... 

The 13C-NMR spectra of 4-7, 9-11 show a close similarity to the spectral data of analogous carbene complexes. The shift differences between the metal carbonyls of the silylene complexes and the related carbon compounds are only small. These results underline the close analogy between the silicon compounds 4-7, 9-11 and Fischer carbene complexes. This view is also supported by the IR spectral data. On the basis of an analysis of the force constants of the vco stretching vibration,... 

The most optimistic response to this situation is to claim that the force constant — t-bond order relationship is still valid, but that the reference points need to be changed V(CO)e itself is then a possible reference compound (76). The relationship can then only be quantified by using calculated orbital populations for the reference species, and can only be tested by more extended comparisons between calculated bond order and observed force constant. Precisely this test has been apphed to a whole group of substituted and unsubstituted octahedral carbonyls of groups VI and VII, the substituents in every case being hahde (77). The data used in fact were not force constants, but Cotton-Krainhanzel parameters this does not actually matter, since no reference molecules were used at all. Excellent agreement was found with an expression. 

Nickel carbonyl is by far the best-known of these compounds. All the data support a tetrahedral structure with four equivalent, linear Ni—C—0 arrangements. The Raman (3, 62, 61) and infrared spectra (8, 53, 135, 136) have been investigated and various assignments of the fundamental frequencies have been put forward on the basis of Td symmetry (53, 136, 208). Several approximate coordinate analyses have been made using these assignments, but the spectra of isotopically substituted species have not been measured, so exact evaluations of force constants have not yet proved possible. The Ni—C and C—0 stretching force constants are particularly important and the values obtained by workers using various approximations are included in Table I. 

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-...

In summary, it can be seen that the structural chemistry of the zero-valent complexes has to date been dominated by investigations on nickel carbonyl. Preliminary studies on other compounds indicate that the tetrahedral structure may not be the only possibility, and thorough investigation of the unsaturatcd species is likely to develop. Back-donation may be examined by study of force constants, in favorable cases merely of observed frequencies, and it would be interesting to see more cases of competition for back-donation, as in (MeNC)3Ni(CO). 

While the lowering of the carbonyl stretching frequency reflects a reduction in the C—O force constant and bond order, the resultant carbonyl ligand can hardly be construed as activated. In general the terminal M—CO unit is unreactive to most reagents. One of the reasons for this lies in the fact that the bond order in a terminally bonded CO is not greatly reduced, i.e., the CO triple bond has not been reduced beyond a bond order of 2, with the exact value depending on the particular compound. 

Force constant calculations have been especially valuable in the important field of carbonyl complexes M tL>(CO)z, where M is one of the d block elements and L represents some other ligand(s). It has already been noted that the C-O stretches around 2000 cm-1 are very useful in characterising such compounds. The C-O stretching force constants, if obtainable, should provide additional information. The nature of the bond between M and CO is discussed in more detail in Chapter 8 for the moment, we may say that the stronger the M-C bond, the weaker will be the C-O bond, if the conventional description is valid. Furthermore, the theory predicts the relative strengths of C-O bonds in carbonyls where the CO ligands are not all equivalent. For example, in complexes such as these ... 

With hindered aldehydes and with most ketones, the equilibrium constants favor the carbonyl compounds rather than the acetals. To enhance these reactions, the alcohol is often used as the solvent to assure a large excess. The water formed as a by-product is removed by distillation to force the equilibrium toward the right. 

The variation in reactivity of N-acetylimidazoles (and other azohdes) in nucleophilic reactions involving the carbonyl group is paralleled by the marked shift in the carbonyl bands (toward higher frequencies for the more reactive compounds).212 This shift, i.e., increase in the C-0 force constant, can also be attributed to increased electron attraction by the heterocyclic rings.213... 

Fig. 1. A plot of the chemical shift, 8( CO), against the force constant for the carbonyl group in some tungsten carbonyl compounds., [W(CO)6-nLn] > [W(CO)3-CMe(SMe)] , [(arene)WCO)3] or [(C7He)W(CO)3l and T. [(C5H5)W(CO)uMe]. [Reproduced with permission from J. Chem. Soc., Dalton Trans. 2012(1973).]...

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