Ir-Donor ligands

Saito, T. In Early Transition Metal Clusters with ir-Donor Ligands Chisholm, M. H Ed. VCH New York, 1995. 

Formation of polycyclic oligomers, not necessarily the cuboidal tetramer, is characteristic of the phosphaalkynes.58 The P=C and As=C bonds can serve as ir-donor ligands in much the same way as nitriles and acetylenes. The fact that the carbon atom is more nucleophilic than the P atom has been explained on the basis of electron density mapping for Bu C=P.59... 

The reactions outlined above and in Table 1 can be generalized to other lone-pair (n-donor) ligands. ir-Donor ligands such as olefins, also behave in an analogous fashion, but Mo(CO) CjHj-h , produced by normal photosubstitution, yields an intermediate hydride upon irradiation ... 

A systematic DFT (density functional theory) computational study examining the correlation between stereochemistry and spin state in four-coordinate transition-metal complexes has been reported. " One outcome was the development of a magic cube for the prediction of the preferred spin state of tetrahedral complexes with electron configurations between d and cfi (those for which high and low spin are possible within a tetrahedral ligand field). The factors predicted to predispose tetrahedral complexes to a low spin-state include (1) no ir-donor ligands, (2) a metal oxidation state +4, and (3) a metal from the second or third transition series. This DFT study provides an excellent discussion of the factors that dictate the geometry of four-coordinate complexes. 

Coordinated polyenes and arenes are a simple, but important, class of ir-donor ligand. Although we considered an rj -arene ligand to be a six-electron donor in Section 1.1, because it contains resonance structures with three double bonds, each of which are two-electron donors, the bonding of such ligands to the metal is more complex. These ligands are more accurately described by a combination of a- and ir-donation from the molecular orbitals of the entire ring. This combination of interactions is illustrated by the molecular orbitals of a sandwich compound, such as bis(benzene)chromium. 

FIGURE The effect of turning on the it interaction between a ir-donor ligand and the metal. The occupied, and relatively stable, lone pair (tt) orbitals of the ligand are shown on the right. Their effect is to destabilize the filled d, orbitals of the complex and so decrease A. This is effectively a repulsion between two lone pairs, one on the metal and the other on the ligand. 

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