Small Heteroborane Cluster Systems. 5. Factors Affecting the 2D ¹¹B-¹¹B (Boron-11) Cosy Nmr Spectra of Terminal and Bridge-Substituted Pentaborane Cluster Systems

The ¹¹B-¹¹B correlation NMR spectra (2D ¹¹B-¹¹B COSY NMR) and T₁ relaxation times for pentaborane(9), B₅H₉, and 10 terminal and bridge-substituted pentaborane clusters, including phosphinopentaboranes, silylpentaboranes, and metal pentaborane complexes, have been measured. Both JrB and “long-range” ²J_PBB couplings, observed the first time in boranes, were measured for the (μ-phosphino)pentaborane compounds. The long-range ²J_PBB couplings were not directly observable in the normal ID spectra but were clearly resolved in the cross peaks of the 2D spectra. The magnitudes of these J_PB and ²J_PBB coupling constants were related to the calculated MO cage bonding scheme for the clusters. A direct relationship between the MNDO-calculated boron-boron bond orders and the intensity of the developed cross peaks in the 2D ¹¹B-¹¹B COSY NMR spectra for adjacent cage boron atoms was observed which relates the calculated electron density localized in bonding interactions to observed NMR parameters in the clusters. The relationship among the MNDO-calculated bond order, T, relaxation time, and the appearance of cross peaks in COSY spectra has been evaluated. The first double-quantum ¹¹B-¹¹B correlation NMR experiment for the framework cage elucidation of a borane cluster is reported.