Complexes back-bonding

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

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 silver(I) carbonyl complexes [Ag(CO)]+, [Ag(CO)2]+, and the stable [Ag HB[3,5-(CF3)2pz]3 (CO)] (where pz = pyrazolyl) have been characterized spectroscopically and crystal-lographically, they indicate little or no Ag to CO 7r-back bonding. 26... 

Although the [AuCl(CO)] complex has been known since 1925, the linear dicarbonyl cation [Au(CO)2]+ has been obtained as thermally stable IJF62 or Sb2Fn2- salts only recently in these compounds back-bonding to CO is almost entirely absent.40,41... 

Tertiary phosphines - and the analogous arsines - are able to stabilize transition metals in a variety of oxidation states and coordination geometries. Investigations of complexation with P-ligands were promoted by the high stabilization of metal by P ligands, which is mainly due to n-back bonding. 

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