Experimental and theoretical investigations on the synthesis, structure, reactivity, and bonding of the stannylene-iron complex bis{{bis(2-tert-butyl-4,5,6-trimethyl-phenyl)}Sn}Fe(eta(6)-toluene) (Sn-Fe-Sn)

The pi-(arene)bis(stannylene) complex bis{(bis(2-tert-butyl-4,5,6-trimethyl phenyl)}Sn)Fe(eta(6)-toluene) (Sn-Fe-Sn, 15) is accessible in high yields b y a metal-atom-mediated synthesis between iron atoms, toluene, and the stan nylene [{bis(2-tert-butyl-4,5,6-trimethylphenyl)}Sn] (3). Complex 15 has a half-sandwich structure with short Fe Sn bonds (2.432(1) Angstrom) and a tr igonal-planar coordination at both the Fe and Sn atoms. The distance betwee n the two Sn centers is 3.56 Angstrom. Complex 15 is stable under ambient c onditions and displays a pi-arene lability, so far rarely observed for (are ne)iron complexes; this leads to an irreversible substitution of the arena and formation of fivefold-coordinated zerovalent iron complexes. The pi-are ne lability of the title compound is a result of the Fe - Sn bonding situat ion, which can be interpreted, on the basis of an extended Huckel molecular orbital calculation, as being solely a donation of the 5 sigma lone-pair o f Sn into empty or half-filled acceptor d orbitals on Fe. As the calculatio ns reveal, there is little backbonding from the iron to the tin, and the st rong sigma donation leads to an increased occupation of the pi-antibonding orbitals of the eta(6)-arene, which are mainly responsible for the experime ntally observed arene lability. Fe and Sn Mossbauer spectra support the pol ar character of Sndelta+-->Fedelta- with strong a donation from tin to iron , but significantly low iron-to-tin pi backdonation.