Experimental and DFT studies on Hexacoordinated acyl(alkyl)and Pentacooordinated Hydroxyalkyl(phosphinite)erhodium(III). Catalytic Hydrolysis of Ammonia Borane

[Rh(nbd)Cl]2 (nbd=norbornadiene) reacts with quinoline‐8‐carbaldehyde (C9H6NCHO) and pyrazole (Hpz) affording, depending on Rh/Hpz ratio, acyl‐alkyl [Rh(μ‐Cl)(C9H6NCO)(Hpz)(σ‐C7H9)]2 (1) or [RhCl(C9H6NCO)(Hpz)2(σ‐norbornenyl)] (2), stereoselectively as confirmed by DFT calculations. Hydrogen bond between NH‐pyrazole and O‐acyl occurs. 1 and 2 react with diphenylphosphine oxide (SPO) giving 16e [RhCl(C9H6NC(nbyl))(Ph2PO)(Hpz)] (3) through SPO κ1‐P‐coordination, C−C bond coupling between acyl and norbornenyl and outer‐sphere O(P)‐to‐O(C) hydrogen transfer. 3 shows distorted trigonal bipyramidal structure with weak intramolecular (C)−OH⋅⋅⋅O(=P) and NH⋅⋅⋅Cl hydrogen bonds. DFT calculations indicate this 16e structure being favoured by σ‐ and π‐type orbital multi‐overlapping between pyrazole and both the OH⋅⋅⋅O and chlorine and by intermolecular forces in the crystal. Solutions of 3 contain an equilibrium between trigonal bipyramid and a square‐planar pyramid with OH interacting with phosphoryl and NH, confirmed by DFT 13C NMR calculations. 1 is very efficient homogeneous catalyst for H3N−BH3 hydrolysis to release hydrogen under air. Acyl‐pyrazole complex efficient homogeneous catalyst for hydrolytic H2 release from ammonia‐borane. PPh2(O)promotes tandem C−C coupling/outer sphere hydrogen transfer and stable pyrazole 16e species. DFT studies allow understanding experimental X‐ray structures and observed species in solution.