Pi-backbonding

Whether of nM 7tL or n- -OL (off-axis) type, pi-backbonding interactions correspond to partial inclusion of ionic resonance structures featuring a metal-ligand double bond, as illustrated schematically in (4.79) for phosphine ligands ... 

Tables 4.26 and 4.27 clearly reveal that the strength of metal-to-ligand pi-backbonding in MHmXY complexes (expressed in terms of either A M /2)...

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

The inverse relationship to pi-backbonding interactions as shown in (4.79) is apparent. 

Figure 4.102 A comparison of coordinative ctmc hybrids (left) and nM 7rCo+ pi-backbonding interactions (right) for M(CO)3 compounds of group 6.

Since alkyl amines have no low energy orbitals for pi-backbonding interactions, the iodine-amine enthalpies can be represented by the following equations ... 

These interactions are synergicin increading the M-CO bond strength. In fact, the second interaction, as known as pi backbonding increases the available electron density on the CO. 

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. 

A similar effect is observed for the v(CO) bands in the IR spectra of the W(CO)5L compounds listed in Table 16.7. The stronger the W-L pi backbonding, the more it will compete with the W-CO pi backbonding of the trans carbonyl... 

As expected, ligands having the largest values of fare those that can pi backbond and therefore lie at the top of the spectrochemical series (CN , bpy) those in the middle (en, NH3, NCS , HjO, 204 ) are tr-donor ligands and those having the smallest values of f (F , d , Br ) are both tr-donor and r-donor ligands. 

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