Computational and Experimental Test of Steric Influence on Agostic Interactions:  A Homologous Series for Ir(III)

Chloride abstraction (using NaBAr‘ 4, Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3 ) from Ir(H) 2 Cl(P t Bu 2 Ph) 2 gives cis,trans -Ir(H) 2 (P t Bu 2 Ph) 2 +, which has two agostic interactions with methyl C−H groups on different t Bu groups. The molecule exists as diastereomers, due to stereochemistry at P. Chloride can be similarly abstracted from ortho-metalated IrH(η 2 -C 6 H 4 P t Bu 2 )Cl(P t Bu 2 Ph) to give square-pyramidal IrH(η 2 -C 6 H 4 P t Bu 2 )(P t Bu 2 Ph) +, which has only one agostic interaction, involving a t BuC−H bond; steric constraints on each phosphine leave no more C−H bonds available to donate to the remaining empty Ir(III) orbital. The smaller ligand PCy 2 Ph yields only the tris-phosphine complex Ir(H) 2 (PCy 2 Ph) 3 +, and this is shown to have a square-pyramidal structure with one agostic cyclohexyl group and large P ax −Ir−P angles (104−106°). The analogous Ir(H) 2 (P i Pr 2 Ph) 3 + has similar inter-phosphorus angles, but no agostic interaction. Geometrical optimization of IrH 2 L 3 + (PCy 2 Ph, P i Pr 2 Ph) with the hybrid quantum mechanics/molecular mechanics (QM/MM) method (IMOMM) at the IMOMM (B3LYP:MM3) and IMOMM (MP2:MM3) levels permits a more detailed understanding of the influence of steric factors on the occurrence of an agostic bond. The MP2/MM3 method gives the results in closer agreement with experiment. Steric factors place the agostic bond in the vicinity of the metal center but at a distance that is too long to be considered as bonding. The electron-donating ability of the C−H bond and the electron accepting capacity of the metal center, which are introduced only at the QM level, bring the two partners in a bonding situation.