Synthesis, X‐ray and Electronic Structure of Trinickel Tetradecker Sandwich Complexes {(η 5 ‐C 5 H 5 )Ni[μ,η 5 ‐(CR 1 ) 2 (BR 2 ) 2 CR 3 ]} 2 Ni

The reduction of the 18‐VE complexes (η 5 C 5 H 5 )Ni[η 5 ‐(CR 1 ) 2 (BR 2 ) 2 CR 3 ] ( 2a, b ) with potassium in THF leads to the paramagnetic anions 2 – , which react with NiBr 2 · DME to give the triple‐decker complexes (η 5 ‐C 5 H 5 )Ni[μ,η 5 ‐(CR 1 ) 2 (BR 2 ) 2 CR 3 ]Ni(η 5 ‐C 5 H 5 ) ( 3a, b ) and the tetradecker {(η 5 ‐C 5 H 5 )Ni[μ,η 5 ‐(CR 1 ) 2 (BR 2 ) 2 CR 3 ]} 2 Ni ( 4b ). Surprisingly, 4b is also obtained by heating the double‐decker complex 2b and H 2 AlOCMe 3 in THF. The reaction between 2c and AlH 3 in Et 2 O leads to the substitution of the ethoxy groups at the boron atoms and to the formation of the tetradecker 4d with four B–H groups. Heating of the 1,3‐benzodiborole ligand 1e and [(C 5 H 5 )Ni(CO)] 2 in toluene yields small amounts of 2e and 3e , and the tetradecker 4e . X‐ray structure analyses were performed for 4d and 4e . Complex 4d is centrosymmetric with relatively short nickel‐to‐bridging ligand distances (1.69, 1.70 Å). In the tetradecker 4e the two bridging ligands adopt a synclinal conformation (rotating about 36° resp. 41° against each other). The Ni–Ni distance (3.42, 3.43) is slightly longer than in 4d (3.39 Å). The electronic structures of the tripledecker cation 3 + (32 VE) and of the tetradecker 4 (46 VE) have been investigated by means of the Fenske‐Hall method. The findings are in agreement with the experimental results that 3 + is a triplet species and 4 possesses a closed‐shell electron configuration.