Synthesis and structural characterization of [kappa3-B,S,S-B(mimR)3]Ir(CO)(PPh3)H (R = Bu(t), Ph) and [kappa4-B(mim(Bu(t))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 [kappa(3)-B,S,S-B(mim(R))3]Ir(CO)(PPh3)H (R = But, Ph) and [kappa4-B(mim(Bu)t)3]M(PPh3)Cl (M = Rh, Ir), has been synthesized via (i) the reactions of Ir(PPh3)2(CO)Cl with [Tm(Bu)t]Tl and [Tm(Ph)]Li and (ii) the reactions of (COD)M(PPh3)Cl with [Tm(Bu)t]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 d(6) configuration. The d(6) configuration is in accord with the value predicted by using a method that employs the valence to determine d(n)(), 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 d(n) transition metal via the boron results in the formation of a complex in which the metal center possesses a d(n-2) configuration.