Synthesis and structural characterization of [κ³-B,S,S- B(mim^R)₃]Ir(CO)(PPh₃)H (R = Bu^t, Ph) and [κ⁴-B(mim^But)₃]M(PPh₃)Cl (M = Rh, Ir): Analysis of the bonding in metal borane compounds

A series of iridium and rhodium complexes that feature M→B dative bonds, namely [κ³-B,S,S-B(mim^R)₃]Ir(CO)- (PPh₃)H (R = Bu^t, Ph) and [κ⁴-B(mim ^But)₃]M(PPh₃)CI (M = Rh, Ir), has been synthesized via (i) the reactions of Ir(PPh₃)₂(CO)Cl with [Tm^But]TI and [Tm^Ph]Li and (ii) the reactions of (COD)M(PPh₃)Cl with [Tm^But]K. The complexes have been structurally characterized by X-ray diffraction, thereby demonstrating the presence of a M→B dative bond in each complex. The nature of the M→B interaction in these complexes has been addressed by computational methods which indicate that the metal centers possess a d⁶ configuration. The d⁶ configuration is in accord with the value predicted by using a method that employs the valence to determine dⁿ, but is not in accord with the d⁸ configuration that is predicted using the oxidation number. Thus, even though B(mim^R)₃ may be regarded as a neutral closed-shell ligand, coordination to a dⁿ transition metal via the boron results in the formation of a complex in which the metal center possesses a d^n-2 configuration.