Planar d* complexes

Photooxidation of coordinated oxalate has been known since the earliest studies of transition metal photochemistry (42). In these reactions oxalate ligand is photooxidized to CO2, and up to two metal centers are reduced by one electron (e.g. ferrioxalate). We wondered whether the oxalate ligand could be a two-electron photoreductant, by simultaneous or rapid sequential electron transfer, with metals prone to 2e redox processes. Application of this concept to l6e square planar d complexes, Equation 15, was attractive because it should produce solvated I4e metal complexes that are inorganic analogues of... 

In synthesis (b), the initial product is a 5-coordinate (sp) iridium(III) hydride complex, which is rapidly oxidized in solution to the planar iridium(II) complex. Both of the compounds are paramagnetic with one unpaired electron, as expected for square planar d complexes. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(ii). in such palladium(ll) olefin complexes (1), which are square planar d complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled tt-orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin tt -orbital (back donation), leads to an electrophilic activation of the alkene. ... 

Available evidence supports the following schemes for the oxidative additions of H2 (as well as other non-polar molecules) and organic halides (RX), respectively, to square planar d -complexes such as those of Rh(I) and Ir(I), [e.g., Ir(I)Cl(CO)(PPh3)2] ... 

The most widely studied adducts of square-planar d complexes are those with the oxygen molecule Oo see References 30 and 31 for review). In these derivatives, the two oxygen atoms are bound equivalently to the metal atom, and the bonding scheme is often compared to that proposed for ethylene adducts. Presumably, the O2 donates a pair of electrons to the bond, but the metal also donates strongly to the bond. When the metal atom can back donate very strongly, irreversible O2 binding is expected. 

This mechanism is related to the ligand replacement reaction in square-planar d complexes (IS) where the first step is addition on a square-planar complex giving two trigonal-bipyramidal isomers. Isomer... 

Figure 1. Possible mechanism for formation of acyl complex. Application of substitution mechanism on square planar d complexes.

The typically faster rate for associative substitution does not preclude dissociation of ligand occurring as the first step of reactions other than ligand substitutions (Equation 5.10). For example, dissociation of ligands from square-planar, d complexes occurs as the first step of reductive eliminations (Chapter 8). Thus, dissociation of ligands from this class of compound can occur, but association to form a five-coordinate intermediate is typically faster. 

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