Metal-ligand n interactions

So far we have only considered a bonding in an octahedral complex. What of 7T bonding It appears to be generally true in chemistry that n is rather weaker than a bonding. Thus, there is no compound known in which it has been established that, in the ground state, there is a tt bond but no g bond. On the other hand, the vast majority of molecules have at least one a bond with, apparently, no associated n bond. Consequently, for our purposes, it can reasonably be assumed that the effects of n bonding will be to modify, but probably not drastically alter. Fig. 6.11. 

As there are two such orbitals on each ligand, there is a total of 12 in an octahedral MLg complex. Again, group theory can be applied to obtain the 

In this case, illustrated in Figs. 6.14 and 6.15(c), it follows that the symmetry ligand n orbitals are also occupied. Interaction with the fi (l) molecular orbitals of Fig. 6.11 will raise or lower this latter set depending on whether its energy is higher or lower than that of the ligand 7r(Ti ) set. As long as we retain two occupied sets the orbital occupancy is unaffected 

Appendix 4 in Symmetry and Structure, 2nd edn., by S. F. A. Kettle, Wiley (Chichester and New York) 1995. Note that the atom labelling pattern used in this text differs from that in the present. 

We are left with the ligand n set of symmetry. This set may interact with the metal t2g orbitals (d, dy, d ) which, so far, have been non-bonding because there is no ligand cr combination of T2g symmetry. The consequences 

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For [CoII(2,3,17,18-TDC)]+, the association constant Kt was found to increase in the range 102-104M 1 as the pKa of 4-substituted pyridines increased.254,265 The Kx is linearly correlated to the pATa in the [Co11 (ODC)]+-N-donor system (equation 3V).269 Though the for L = Im fits the equations, aliphatic amines show lower Kx values than expected from their pK values, suggesting some metal-axial ligand n interaction. The steric hindrance of the 2-methyl group lowers K by a factor of 103. 

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Fnrther docnmenting the breadth of metal hgand n-interactions is an open calcocene derived from an acyhc pentadienyl ligand. With the addition of trimethylsilyl... 

In the case of the octahedral metal carbonyls Cr(CO)6 and Mo(CO)6, pairs of intense bands at 35,800 and 44,500 cm-1 for the former and 35,000 and 43,000 cm-1 for the latter have been plausibly assigned to transitions from the bonding (mainly metal) to the antibonding (mainly ligand) components of the metal—ligand n bonding interactions.21... 

The order of shielding obtained can be interpreted in terms of the metal-locyclic model for metal-olefin/acetylene bonding and its required rehybridization olefin sp -> sp (upfield shift), acetylene sp sp (downfield shift). An alternative interpretation was based on the following considerations the metal to ligand n interaction (i) increases the total electron density at the olefinic/acetylenic carbons and (ii) reduces the carbon-carbon n bond order and hence the n character of these carbons. Ligand n to metal dsp bonding (iii) decreases the n character and (iv) decreases the electron density at the olefinic/acetylenic carbons. ... 

Symmetry prevents interaction between the ai(t2g) and a2(ir) orbitals. The metal-ligand n acceptor interaction will thus stabilize the e-component of the t2g shell, while leaving the a i-orbital in place, as shown in the simple orbital-energy diagram in the left panel of Fig. 6.6. We can now calculate the transfer term for the e and U2 orbital transitions. In each case only one component needs to be calculated. The interaction element in this case is obtained from Iiq. (6.96) and the transfer fraction reads ... 

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