Ir-back-bonding

Silicon-transition metal chemistry is a relatively new area. The work of Hein and his associates (1941) on Sn—Co derivatives established the possibility of forming bonds between a Group IVB metal and a transition element 139), but it was another fifteen years before CpFe(CO)2SiMej 203), the first of many silyl derivatives, was synthesized. The interest in these compounds derives from (1) comparison with the corresponding alkyl- and Ge-, Sn-, and Pb- transition metal (M) complexes, including the role of ir-back-bonding from filled d orbitals of M into empty d orbitals on Si (or other Group IVB metal), and (2) expectation of useful catalytic properties from such heteronuclear derivatives. 

The unexpectedly small shifts in the 3dj electron binding energies of XeF6 and XeOF4 have been reported by Carroll et al. s these workers suggest that this provides evidence for extensive F-to-Xe ir-back-bonding and for orbital independence in the Xe—F bonding. 

Coordination of a ligand to a metal changes its properties by polarization of electron density by the positive charge on the metal ion but also can affect properties by more subtle effects of particular bonding type, such as ir back-bonding. Coordinated metals often stabilize molecules, as strikingly illustrated in the oxidation of a coordinated thiol to the corresponding coordinated sulfenic acid ... 

Substituent groups on the olefin with a +1 effect (e.g. Me) will tend to strengthen the M- -ol o-bond and weaken the 7r-bond while -I substituents should have the opposite effect. The tendency for the thermodynamic stability of Ag+—ol complexes to increase as the +1 nature of substituents increases supports the claim that o-bonding is more important than ir-back-bonding. However, there is also some evidence that the jr-acceptor properties of a ligand are more important than its o-donor properties 75). 

Table 111-10 lists main contributors to their potential energy distribution for the above six vibrations which account for their sensitivities to the oxidation and spin states in iron porphyrins. Changes in the oxidation state cause alterations in ir-back-bonding to the porphyrin ring. Thus, P3, r, and p,o which contain PtC Cp). p(C C ) and/or KC N) are sensitive to the oxidation state. The sensitivity of to the change in the central metalcan also be accounted for on the same basis. 

A glance at Table 2.1 will show how the first-row carbonyls mostly follow the 18e rule. Each metal contributes the same number of electrons as its group number, and each CO contributes 2e for its lone pair -ir back bonding makes no difference to the electron count for the metal. In the free atom, it had one atomic orbital (a.o.) for each pair of electrons it uses for back bonding in the complex it still has one filled molecular orbital (m.o.), now delocalized over metal and ligands. 

The poorer ir-back bonding centers will have the highest reactivity Pd > Pt cation > neutral phosphite > phosphine. 

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