Contributions to the Chemistry of Boron, 210. η 2 ‐Transition Metal Complexes of the Ligand 9‐Fluorenylidene(2,2,6,6‐tetramethylpiperidino)borane

The boraolefin 9‐fluorenylidene(tetramethylpiperidino)borane L forms complexes 1 — 4 with the isolobal fragments Fe‐(CO) 4 , CpCo(CO), Cp′Mn(CO) 2 , and C 6 H 6 Cr(CO) 2 , respectively. Compounds Fe L (CO) 3 PR 3 (PR 3 =PMe 3 , PhPMe 2 , Ph 2 ‐PMe, PCl 3 , PhPCl 2 , 1 a — e ) are obtained photochemically from Fe(CO) 5 , L , and PR 3 in toluene solution. An X‐ray structure analysis 1 and 1 a reveals a distorted trigonal‐bipyramidal geometry, with the boron atom of 1 in an equatorial position. 2 contains two independent molecules in the asymmetric unit. The two molecules are enantiomers with Co atoms in a pseudotetrahedral environment. MO calculations demonstrate that the distortion found in 1 is due to electronic rather than steric effects as exemplified by the model compound Fe(CO) 4 (H 2 N—B=CH 2 ) ( 6 ). π‐Back bonding from the metal‐centred 2a′ orbital to the boron‐centred π BC * orbital contributes significantly to the stabilization of the energy‐minimized structure 8 . Experimental evidence for this back bonding is provided by the shorter M–B bond in 1 and 2 as compared with the M–C bond to the boraoelfin unit. However, the reverse is true for the manganese complex 3 .