Cationic metal carbonyls disproportionation

In the sections above there has been frequent reference to base-induced disproportionation of transition metal cations in low oxidation states (Sec. 11.3.4.2), of carbonyl cations of transition metals (Sec. 11.3.4.3) and of polyatomic cations of non-metals in fractional oxidation states (Sec. 11.3.4.4). In the last-named section, disproportionation of polyiodine cations was presented and discussed in considerable detail but a similar rationale can be applied to stability of all cations — metallic or non-metallic — in unusually low or fractional oxidation states, whether produced in non-aqueous ionising media or as naked cations. 

The preparation of metal carbonyl anions from Lewis bases and certain neutral metal carbonyl derivatives was described above. In general, such reactions involve disproportionation of the zero-valent metal atom in the metal carbonyl to a cation, with coordination of the base to the metal atom, and to the metal carbonyl anion. A different t3rpe of reaction between Lewis bases and metal carbonyls involves displacement of carbonyl groups in the metal carbonyl by the Lewis base without change in the oxidation state of the central metal atom, e.g.,... 

Formally, in each of these cases the disproportionation produces a positive metal ion and a metal ion in a negative oxidation state. The carbonyl ligands will be bound to the softer metal species, the anion the nitrogen donor ligands (hard Lewis bases) will be bound to the harder metal species, the cation. These disproportionation reactions are quite useful in the preparation of a variety of carbonylate complexes. For example, the [Ni2(CO)6]2 ion can be prepared by the reaction... 

In analogy to hydroformylation, alkenes react with SO2 and H2 to give a so-called hydrosulftnation product, sulfinic acids [116]. Cationic Pd(II) and Pt(II) complexes bearing bidentate phosphine ligands are effective catalyst precursors. A plausible mechanism for the hydrosulfination involves formation of alkyl intermediates by olefin insertion into metal hydrides, subsequent insertion of SO2, and reformation of the hydrides with the release of sulfinic acids (Scheme 7.19). However, ahphatic sulfinic acids readily undergo disproportionation to give thiosulfinic acid esters, sulfonic acids, and water at the reaction temperature. The unstable sulfinic acids can be conveniently converted into y-oxo sulfones by addition of a,-unsaturated carbonyl compounds as Michael acceptors to the reaction mixtine (Eq. 7.23) [117]. 

Photolysis of Mn2(CO)8(phen) in the presence of CCU, however, results in the formation of a mixture of MnCl(CO)5 and MnCl(CO)3phen. The reaction, which involves the initial photohomoloysis of the Mn-Mn bond followed by chlorine atom abstraction from CCU by the resulting Mn radicals, is discussed in more detail in a later section of this chapter. In other cases, however, photolysis leads to the dissociation of a carbonyl ligand rather than to homolysis of the intermetallic bond. The quantum yields of these particular reactions are dependent on both the metal and the ligands in the complex. The photodissociation of a carbonyl ligand has also been proposed as the initial step in the photochemical disproportionation reactions of these complexes/ In this case two electron donor ligands such as THF or a tertiary phosphine induce the photodisproportionation of the complex (CO)sM-M(C0)3 (diimine) (M = Mn, Re) into a cationic complex M(CO)3(diimine)L and an anionic complex M(CO)s (Refs. 99 and 100) ... 

---