Complexes with interactions between metals and aliphatic groups of boron compounds. 1. Synthesis and reactivity of (.eta.5-cyclopentadienyl)cobalt .eta.5-1,3-diborolene complexes, a novel type of sandwich compound with a pentacoordinated carbon atom. Molecular structure of (.eta.5-cyclopentadienyl)cobalt 1,3,4,5-tetraethyl-2-methyl-1,3-diborolene. Extended Hueckel and INDO MO studies