Molecular orbital study of the structure and stability of transition metal polyhedral borane complexes. Position of bridging hydrogens

Abstract

The dependence of bridging hydrogen atoms on terminal hydrogen positions in nido-metalloboranes has been studied using the model compounds B₄H₈X (6, X = BH) and B₄H₈Fe(CO)₃ (1d). Calculations on B₅H₉ at both MNDO and extended Huckel levels have shown that the bridging hydrogen positions are controlled by the terminal hydrogen positions. Bridging hydrogen atoms might be observed above the Bn plane (towards the cap), if the caps have sufficiently diffuse orbitals. Model studies at both MNDO and extended Huckel levels of B₄H₈X [X = BeH⁻, Li⁻ and Fe(CO)₃] supported these results. The effects of increasing ring size from B4H8 to B5H10 are also studied. Suitable metal fragments for polyhedral borane ligands (rings) having five, six, seven, 11 and 13 vertex boron atoms can be selected using the ring-cap overlap match. Calculations have shown that B₆H₆⁴⁻ (13) has more diffuse orbitals compared to B₁₁H₁₁⁴⁻ (14) and B₁₃H₁₃⁴⁻ (15). Compounds 14 and 15 preferred caps with less diffuse orbitals because of the rigidity of the ligands. The metal-ring boron distances in metalloboranes are controlled by this ring-cap compatibility. The concept of topological charge stabilization is also used in predicting the stabilities of various isomers of metalloboranes.