Highly Stable Neutral and Positively Charged Dicarbollide Sandwich Complexes

Novel sandwich metallacarboranes commo ‐[3,3′‐Ni(8‐SMe 2 ‐1,2‐C 2 B 9 H 10 ) 2 ] ( 1 ), commo ‐[3,3′‐Co(8‐SMe 2 ‐1,2‐C 2 B 9 H 10 ) 2 ] + ( 2 + ), commo ‐[3,3′‐Ru(8‐SMe 2 ‐1,2‐C 2 B 9 H 10 ) 2 ] ( 4 ) and commo ‐[3,3′‐Fe(8‐SMe 2 ‐1,2‐C 2 B 9 H 10 ) 2 ] ( 5 ) have been prepared by reaction of [10‐SMe 2 ‐7,8‐ nido ‐C 2 B 9 H 10 ] − with NiCl 2 ⋅ 6 H 2 O, CoCl 2 , [RuCl 2 (dmso) 4 ] and [FeCl 2 (dppe)], respectively. Reduction of 2 + with metallic Zn leads to the neutral and isolable complex commo ‐[3,3′‐Co(8‐SMe 2 ‐1,2‐C 2 B 9 H 10 ) 2 ] ( 3 ). Theoretical calculations using the ZINDO/1 semiempirical method show three energy minima for complexes 1 – 3 and 5 that agree with the presence of three different rotamers in solution at low temperature, while four relative energy minima have been found for 4 . The calculated rotational energy barriers for complexes 1 – 5 have been found in the range 5.2±0.2 and 11.5±0.2 kcal mol −1 . These values are in agreement with the experimental data calculated for complexes 2 + and 5 . Only one rotamer is found in the X‐ray crystal structure of complexes 1 – 3 , while two are observed for 4 . Neutral complexes 1 , 3 and 4 exhibit a gauche conformation, whereas a cisoid conformation is found for the 2 + ion. Rotamers evident from X‐ray diffraction studies are in agreement with the global energy minimum calculated by the ZINDO/1 method. The electrochemical studies conducted on 1 , 3 , 4 and 5 support the proposal that the charge‐compensated ligand [10‐SMe 2 ‐7,8‐ nido ‐C 2 B 9 H 10 ] − stabilises lower oxidation states in metals than the dianionic [7,8‐ nido ‐C 2 B 9 H 11 ] 2− and even the [C 5 H 5 ] − ligands.