Structural insights into transition-metal–aminodiphosphine (PNP) complexes bearing [ <i>M</i> Cl _<i>n</i> (PNP) ₂ ] ( <i>M</i>  = Co, Ru, Cr or Mo; <i>n</i>  = 1 or 2) cores in the solid state

The molecular structures of two novel cobalt aminodiphosphine (PNP) complexes (1 and 2) are reported, namely, bis[bis(diphenylphosphanyl)(pentyl)amine-κ²P,P']chloridocobalt(III) di-μ-chlorido-bis[dichloridocobalt(II)], [CoCl(C₂₉H₃₁NP₂)₂][Co₂Cl₆], and bis[bis(diphenylphosphanyl)(propan-2-yl)amine-κ²P,P']chloridocobalt(III) di-μ-chlorido-bis[dichloridocobalt(II)], [CoCl(C₂₇H₂₇NP₂)₂][Co₂Cl₆], featuring variation in the N-atom substituent, i.e. n-pentyl in complex 1 and isopropyl in complex 2. The asymmetric unit of complex 1 contains a five-coordinated cationic [CoCl{κ²-P,P-(N)-C₅H₁₁}₂]²⁺ species and a [Co₂(μ₂-Cl)₂Cl₄]^2- anion, while complex 2 includes a five-coordinated cation [CoCl{κ²-P,P-(N)-C₃H₇}₂], half a [Co₂(μ₂-Cl)₂Cl₄]^2- anion, and disordered diethyl ether and dichloromethane solvent molecules. The impact of ligand-induced strain, particularly due to the small bite angles of the PNP aminodiphosphine ligands, was examined in the context of geometric constraints and their influence on stability and reactivity. A Cambridge Structural Database (CSD) survey, along with a noncovalent interaction (NCI) analysis of the analogous [TMCl_n(PNP)₂] (where TM = transition metal and n = 1 or 2) core, revealed an inverse correlation between P-TM-P bite angles and N...TM contact distances. This correlation is attributed to the influence of the van der Waals radius of the metal: smaller metals allow wider bite angles and stronger N...TM contacts, whereas larger metals favour narrower bite angles and longer N...TM distances. NCI analysis indicated significant steric repulsion at the TM...N contacts, reflecting strain imposed by ligand geometry. A comparison of sign(λ₂)ρ eigenvalues suggests that Mo-P bonds exhibit weaker attractive interactions relative to Co-P and Ru-P bonds, with Cr-P bonds being the weakest. These findings provide pointers to structural and electronic factors governing coordination in PNP-ligated transition-metal complexes, offering rational design and catalytic and material applications.