Pi back-bonding

Electron correlation is often very important as well. The presence of multiple bonding interactions, such as pi back bonding, makes coordination compounds more sensitive to correlation than organic compounds. In some cases, the HF wave function does not provide even a qualitatively correct description of the compound. If the weight of the reference determinant in a single-reference CISD calculation is less than about 0.9, then the HF wave function is not qualitatively correct. In such cases, multiple-determinant, MSCSF, CASPT2, or MRCI calculations tend to be the most efficient methods. The alternative is... 

The work on cobalt compounds raised two questions which could best be answered by studying a smaller model compound. One was the role of dispersion forces in metal ligand binding. The other was the elusive role of pi back bonding which is supposed to account for a large fraction of the metal carbonyl bond strength. 

The pi back-bonding model of Dewar (29), Chatt, and Duncanson (30) has been widely invoked as an explanation of a variety of features of transition metal complexes. While extended Huckel theory clearly shows such mixing of orbitals, ab initio calculations have found them more elusive (31,32). 

The energetic consequences of pi back-bonding are also evident in this case. Among the s d states at 2.2 X, the dr is 9.8 kcal/mol below the d, presumably entirely due to pi back-bonding. The da is 15.5 kcal/mol above d because of the repulsion between the da electron and the CO lone pair. 

Some of the most convincing evidence in support of pi backbonding comes from the IR spectra of the metal carbonyls. For the isolectronic series of compounds listed in Table 16.6, the v(CO) stretching frequency decreases as the electron density on the metal accumulates. The more electropositive the metal, the stronger the pi back-bonding, and the weaker the CO bond because of population of the ti CO) MO. 

Beyond sigma bonding transition-metal hyperbonding and pi back/frontbonding... 

A further distinction must be made between ligands of both types, namely the ability to receive electrons by pi back donation from the metal. This ability has a number of important consequences, beside the obvious one of increased thermodynamic stability of the bond. The energy of the nonbonded valence orbitals, chiefly of the dxy, dxz, or dyz type, is lowered by the in-phase interaction. 

Pacchioni and Koutecky calculated the interaction of CO on Pd clusters of up to 4 atoms, and found that here too the sigma u interaction is essentially repulsive, with bonding due primarily to pi back-donation. 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

In this Nb6 cluster, each edge is involved in bonding with a pi-CI ligand, so the edges do not correspond to 2c-2e Nb-Nb bonds. Each face of the Nb6 cluster forms a 3c-2e NbNbNb bond, and has bond number 2. The sum of bond number, 16, is just equal to the bond valence of 16, as shown in the front and back views of the Nb cluster in Fig. 19.1.2(b). 

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