Chloride abstraction (using NaBAr＇(4), Ar＇ = 3,5-(CF3)(2)C6H3) from Ir(H)(2)Cl((PBu2Ph)-Bu-t)(2) gives cis,trans-Ir(H)(2)((PBu2Ph)-Bu-t)(2)(+), which has two agostic interactions with methyl C-H groups on different Bu-t groups. The molecule exists as diastereomers, due to stereochemistry at P. Chloride can be similarly abstracted from ortho-metalated IrH(eta(2)-(C6H4PBu2)-Bu-t)Cl((PBu2Ph)-Bu-t) to give square-pyramidal IrH(eta(2)-(C6H4PBu2)-Bu-t)((PBu2Ph)-Bu-t)(+), which has only one agostic interaction, involving a (BuC)-Bu-t-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 PCy2Ph yields only the tris-phosphine complex Ir(H)(2)(PCy2Ph)(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 degrees). The analogous Ir(H)(2)((PPr2Ph)-Pr-i)(3)(+) has similar inter-phosphorus angles, but no agostic interaction. Geometrical optimization of IrH2L3+ (PCy2Ph, (PPr2Ph)-Pr-i) 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.