A d¹⁰ Ag(i) amine-borane σ-complex and comparison with a d⁸ Rh(i) analogue: Structures on the η¹ to η²:η² continuum

H₃B·NMe₃ σ-complexes of d⁸ [(L1)Rh][BAr^F₄] and d¹⁰ [(L1)Ag][BAr^F₄] (where L1 = 2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine) have been prepared and structurally characterised. Analysis of the molecular and electronic structures reveal important but subtle differences in the nature of the bonding in these σ-complexes, which differ only by the identity of the metal centre and the d-electron count. With Rh the amine-borane binds in an η²:η² fashion, whereas at Ag the unsymmetrical {Ag⋯H₃B·NMe₃} unit suggests a structure lying between the η²:η² and η¹ extremes.