Synthesis of the 17-electron cations [FeL(L')(NO)(2)](+) (L,L'=PPh(3), OPPh(3)): Structure and bonding in four-co-ordinate metal dinitrosyls, and implications for the identity of paramagnetic iron dinitrosyl complex catalysts

The complex [FeL(2)(NO)(2)] (L = PEt(3) 1a, L = PPh(3) 1b or L(2) = dppe 1c) prepared from [{Fe(mu-I)(NO)(2)}(2)] and PPh(3) or Ph(2)PCH(2)CH(2)PPh(2) (dppe) {in the presence and absence of [Co(cp)(2)] (cp = eta(5)-C5H5) respectively} undergo one-electron oxidation at a platinum electrode in CH2Cl2. The complex [{Fe(mu-dppm)(NO)(2)}(2)] 2, prepared from [{Fe(mu-I)(NO)(2)}(2)] and Ph(2)PCH(2)PPh(2) (dppm) in the presence of [Co(cp)(2)], undergoes two sequential one-electron oxidations. Complex 1b with [Fe(cp)(2)](+) gave 1b(+), X-ray studies of which show a distorted tetrahedral geometry with near C-2 upsilon symmetry. Oxidation of 1b leads to substantial lengthening of the Fe-P bonds and changes in the P-Fe-P and N-Fe-N angles. These changes are consistent with significant Fe-P pi-bonding character in the singly occupied molecular orbital of 1b(+). Cation 1b(+) reacts with halide ions, giving [FeX(PPh(3))(NO)(2)] (X = Cl or I) and then [FeX(2)(NO)(2)](-), and with OPPh(3) to give [Fe(OPPh(3))(PPh(3))(NO)(2)](+) 3(+). X-Ray studies on the last, as its [PF6](-) salt, show a distorted tetrahedral geometry; the co-ordination angles at iron approach trigonal bipyramidal with the PPh(3) ligand in one apical site and the other apical site vacant. The complex [Fe(OPPh(3))(2)(NO)(2)](+) 4(+) resulted from the reaction between [{Fe(mu-I)(NO)(2)}(2)] and OPPh(3) in the presence of TlPF6. An analysis of the ESR spectra of the paramagnetic cations 1b(+), 3(+) and 4(+), together with extended-Huckel MO calculations on models of 1b(+) and 3(+), suggest that the complex catalysts formed from [{Fe(mu-Cl)(NO)(2)}(2)] and Ag+ or Tl+ are also four-co-ordinate 17-electron radicals. A crystallographic database study of four-co-ordinate dinitrosyl complexes of iron and other metals confirms that the N-Fe-N and O ... Fe ... O angles are linearly related. Consideration of these geometric effects, and those resulting from oxidation of 1b, in the light of a model proposed by Summerville and Hoffmann provides insight into the bonding in these and related species.