Ten-vertex polyhedral azametallaborane chemistry: a unique nido-6,9 to nido-6,8-cluster isomerization

Reaction of the arachno-4-NB(8)H(12)](-) anion with RhCl(2)(eta(5)-C(5)Me(5))](2) in CH(2)Cl(2) at room temperature affords a mixture of red '6,9' isomer 9-(eta(5)-C(5)Me(5))-nido-6,9-NRhB(8)H(11)] () and its yellow '6,8' isomer, 8-(eta(5)-C(5)Me(5))-nido-6,8-NRhB(8)H(11)] (). Under the same conditions, reactions of with IrCl(2)(eta(5)-C(5)Me(5))](2) and RuCl(2)(eta(6)-MeC(6)H(4)-4-(iso)Pr)](2) give the '6,8' isomers, yellow 8-(eta(5)-C(5)Me(5))-nido-6,8-NIrB(8)H(11)] () and red 8-(eta(6)-MeC(6)H(4)-4-(iso)Pr)-nido-6,8-NRuB(8)H(11)] (), respectively. In contrast, IrCl(PPh(3))(3)] yields orange 9,9-(PPh(3))(2)-9-H-nido-6,9-NIrB(8)H(11)] (), which exhibits the '6,9' configuration. Compound isomerizes quantitatively in solution to give . At high temperatures, compound gives the yellow '6,8' species, 8,8-(PPh(3))(2)-8-H-nido-6,8-NIrB(8)H(11)] (), in low yields. Possible mechanisms for the unprecedented 6,9 --> 6,8 isomerization are discussed.