CO force constants

The infrared evidence bearing on this conclusion is reviewed by Malatesta and Bonati (90), and need not be presented here in detail. It is observed only that comparisons are made between the values of vco (or CO force constants) for series of analogous compounds such as Ni(CO)4 j5Lj or M(CO) (M = Cr, Mo, W L=CNR, CNAr, phos, etc.) also comparisons are often made between the values of for the free ligand and for the ligand bonded in a metal complex. Much of this work dates back over... 

C8 Angelici, R. J., Blacik, L. J. Inorg. Chem. 11, 1754 (1972) Proposed relationship between v(CO) force constants and reactivity... 

The reviewer is led to a melancholy conclusion. If the theory used to correct for anharmonicity is questionable, and the data are never sufficient to supply overproof, then anharmonicity remains a major source of uncertainty. Indeed, since corrections due to anharmonicity are as large as the errors caused by neglecting the distinction between CO force constants and parameters, there seems little point, as far as the CO vibrations are concerned, in attempting the fuller force field analysis at all. 

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

Proposed relationship between t>(CO) force constants and reactivity... 

However, the Cotton-Kraihanzel force field and the entire philosophy of energy factoring was sharply criticized by Jones who contended that the CKFF yielded CO force constants, which showed some consistency of pattern but which had no theoretical foundation except in gross approximation . The validity of the EFFF was further called into question following... 

The CO force constants decrease with decreasing number of CO ligands, while the CN force constants increase with decreasing number of MeNC ligands. The variations, however, are very small indeed. This explains why all four CO molecules of Ni(CO)4 can be easily replaced... 

A correlation between the CO bond properties in the closed-shell molecules (single and double bonds) was proposed to estimate the bond lengths and stretching frequencies of open-shell phenoxyl radicals. Nevertheless, while it is possible to estimate the CO force constants using the Badger-type relations, it is difficult to relate them to the experimental frequencies that do not represent the stretching of a single bond. 

The negative deviation for the methoxy derivative is also seen in a correlation of log Ki with the vibrational CO force constants obtained from IR measurements the more negative A5° values are further testimony to the deviant behavior of the methoxy derivative. Even though not stated in these terms by Fischer, the most plausible source of the deviations with Z = MeO is the strong 7r-donor effect of the methoxy group (63a) which leads to a substantial stabilization of the carbene complex. The reduced CO vibrational force constants for the methoxy derivative are consistent with this explanation. 

The effort required to obtain results of the kind just discussed is so great that there have been many attempts to devise some simple method of calculating useful force constants from limited data and by simple means. The most widely used of these simple methods is that commonly called the Cotton-Kraihanzel (CK) method,19 although very similar approximations have been suggested by others.20 The most important approximation is that CO force constants can be calculated from the CO stretching frequencies alone because these are at very much higher frequencies (> 1850 cm 2) than all other vibrations (< 700 cm-1) in simple metal carbonyls as well as most substituted carbonyls. The main effect of this approximation is that the force constants so obtained are not absolute but in a series of related molecules the shift from absolute values will probably be essentially constant. Hence, relative values, and, thus the differences between force constants for different but similar molecules should be rather reliably given. 

Table 11.1. N2 and CO Force Constant and Stretching Frequency Data as a Predictive Tool for the Stability of Dihydrogen Complexes (Reference 13)...

To calculate the A (CO) force constants, the Cotton-Kraihanzel model is often applied, which assumes that CO force constants can be calculated from the CO stretching frequencies alone because these are at much higher frequencies (>1800cm ) than all other vibrations (<700cm ) in simple metal carbonyls and in many substituted carbonyls. Furthermore, this model assumes that the vibrations are truly harmonic and that the interaction constants between cis- and trans-CO groups are related in a simple manner. It is assumed that the interaction of two trans-CO groups is twice as large as... 

An antisymmetric combination of ns orbitals on two alkali atoms can couple to the CO In orbital. Formally this perturbation lifts the n degeneracy since the combined system has C2 symmetry. The energy denominator should be small for such a perturbation since both the alkali ns and the CO In orbitals are located close to Ep. If the spacial overlap between these orbitals is significant i.e. if the species occupy adjacent sites on the adsorbent a considerably enhanced occupation of the In level should follow [1]. In another local approach Tomanek and Bennemann show that a K atom, as a substitutional impurity in the surface layer of Ni(lll), will decrease the activation barrier for CO dissociation if the molecule is bound parallel to the surface [9]. They represent the clean surface by a closely packed cluster of four atoms and the substituted surface by NiaK. In a competing approach Ray and Andersson simulate a nonlocal effect of K adsorbed onto Pt(lll) by a varying Pt valence state ionization potential [7]. They predict an altered most favourable adsorption site and weakened CO force constants for all sites as results of a lowered ionization potential, i.e. an enhanced K coverage. Their predictions are in accordance with experimental results for Pt(lll)/K/CO (2]. 

---