Metal ligand back donation

For the heavier actinides the situation is little bit different as the energy of the 6d-orbitals drops across the series and hence the metal-ligand interactions become weaker. This is consistent with the report that plutonium forms less robust adducts compared to its lower actinide analogs. The com- 

All the isocyanides have 1 1 stoichiometry with the exception of MeC5H4 and R = 2,6-Me2CeH3 for which both the 1 1 and 1 2 adducts were obtained. Table 6 lists the isocyanide complexes and the vcnr frequencies in the infrared spectrum. The IR spectra showed that vcn increased shghtly for the alkyl iso cyanide complexes and decreased slightly for the aryl isocyanide complexes relative to vcn for the free ligands. The substituents on the cyclo- 

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CS, N2, NO+, CN-, NC-, HCCH, CCH2, CH2, CF2, H2), which showed that the metal—>ligand back-donation correlates well with the change of the M-COtrans bond length, while the ligand metal donation does not [88]. 

It is generally assumed that force constant is a measure of valence bond order. If so, this would imply that the force constant of a coordinated CO group would be a direct measure of the degree of metal-ligand back-donation. 

Of central importance in the chemistry of nitrogen fixation is the concept of activation (4f). This involves a transfer of electronic charge from the transition metal to the coordinated N2 by metal — ligand back donation, making this very inert molecule susceptible... 

Figure 1. Ligand-metal donation and metal-ligand back-donation models and their extremes, where ligands become redox noninnocent .

Figure la shows a schematic representation of the Dewar-Chatt-Duncanson (DCD) model. The pivotal idea is that the olefin serves as a donor and an acceptor at the same time. There is ligand metal donation and metal -> ligand back-donation. The former interaction involves a donor orbital of the ligand which has n symmetry in the free ligand but cr symmetry in the complex. The metal acceptor orbital is mainly the d 2 orbital of the metal. Quantum chemical calculations have shown that the valence s orbital of the metal is less important as an acceptor orbital than the d 2 orbital. The metal ligand back-donation takes place via a d( r) orbital of the metal and the n orbital of the olefin. 

Table 1 Theoretically predictedpyramidalisation angles 6 (degrees) and C—C bond lengths ( 4) of the platinum complexes 1-5 and the free olefins. Calculated metal-olefin binding energies (kcal moC ). Results of the CD A calculations of ligand metal donation d, metal ligand back-donation b and ratio d/b ...

The DCD bonding model of TM olefin complexes is supported by accurate quantum chemical calculations which quantify the amount of ligand -> metal donation and metal ligand back-donation. The geometries and bond energies... 

The difference in the ligand conformation between the U(V) and the U(IV) ddt derivatives was explored extensively by DFT, which confirmed that the oxidation state was responsible for the fference in the conformation of the ligand [101]. The calculations also confirm a significant U-(C=C) interaction between the metal center and the C=C bond of the endo dithiolene ligand in the lU anium(V) complex, which does not exist in the dianionic ura-niiun(IV) species. A metal / ligand back-donation occurs in both complexes from the partially occupied manium 5/ orbitals toward the vacant n (C=C) antibonding MO of the dithiolene ligand, which becomes partially occupied after interaction. 

The metal-donor bonds are predominantly ionic and become more labile for calcium, strontium, and barium compared to beryllium and magnesium. The solubility and stability of the complexes decrease from calcium to barium. The 1 1 adducts of NHCs with BH3 or BF3 (28 and 29) are thermally stable and can be sublimed without decomposition. This is in sharp contrast to the properties of conventional carbenes, which rely on a pronounced metal-to-ligand back donation and are, thus, not suited to forming adducts with electron-poor fragments such as... 

Only in homoleptic M(L)2 (L = 1,3-dimesitylimidazolin-2-ylidene) of zero-valent nickel and platinum significantly shorter metal-carbon bonds for NHCs and, thus, metal-to-ligand back donation can be observed. The Ni-C bond length is about 0.15 A shorter than in [Ni(CO)2(L)2] (L = 1,3-dimesitylimidazolin-2-ylidene) which cannot be explained exclusively by the change of the coordination number. 

The presence of electrons in d orbitals, which may be involved in back donation, is not a prerequisite for the stabilisation of an imine by co-ordination some imines are stabilised by co-ordination to lead(n). The many factors involved (charge on metal, charge on ligand, back-donation, configuration of ligand, stabilisation of products, etc.) are interdependent and finely balanced. The formation of a chelated imine complex is an important factor, but once again examples are known in which chelated ligands are either activated or deactivated towards hydrolysis. 

Donation from ligand Back donation from metal... 

We have already discussed the case of ir-acceptor ligands (CO, NO, etc. in Section 16-3) in which there is at least fractional ir-character based on metal to ligand back donation. In this section we are concerned with bonds in which there are full double and triple bonds. Typical of these are the following ... 

Figure 3.3. The synergistic stabilizing effect of metal carbonyl complexes. Shown is (a) ligand-to-metal O donation from the carbon lone pair to the metal d 2 orbital and (b) metal-to-ligand back-donation from the d 2 y2 orbital to the empty TZ orbital on CO. This weakens the C-O bond, while concomitantly strengthening the M-C interaction.

MuUiken charges reveals that the corresponding molecular orbitals (7a j and llb ) have some admixture of metal 4p states. The coupling with 4p states causes a transfer of the ligand s charge to originally unoccupied 4p orbitals of the metal. This "back donation" mechanism was first discovered in SW-Xa calculations (29) and is confirmed by the present LCGTO-LSDF study. 

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