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In search of complexes exhibiting functional and structural characteristics of the active centers of nitrogenases, the Fe" complexes [Fe(L)('bi~Sl)] with the pentadentate amine-thioether-thiolate ligand ' N H S~)~-(L = N2H4, CH3-OH, THF, pyridine, NH3, NHCH3NH2, P(OMe)3, P(OPh)3; ' N~s 4 '~-= dianion of 2,2'-bis[(2-mercaptophenyl)-thioldiethylamine), [Fe('N&)],, and the free ligand ['N&'-H2].HCl were synthesized and characterized. The molecular structures of the complexes with L = N2H4, NH3, and CH3OH have been elucidated by X-ray structure analyses. The crystal data for [ F~( N~H~) ( " H S~' ) ] (la) are orthorhombic space group Pbca, a = 1102.1 (7), b = 1567.2 (8), c = 2206.6 (14) pm, 2 = 8, and R / R , = 0.046/0.046. The crystal data for [Fe(CH30H)('NHSl)] (2) are orthorhombic space group Pbca, a = 1152.8 (3), b = 1471.5 (9, c = 2237.0 (5) pm, 2 = 8, and R / R , = 0.069/0.054. The crystal data for [Fe(NH3)('NHS,')] (5) are orthorhombic space group Pbca, a = 1107.0 (9, b = 1547.1 (8), c = 2171.9 (9) pm, 2 = 8, and R / R , = 0.040/0.035. u donor ligands (N2H4 or NHj) cause high-spin states; u donor, T acceptor ligands (CO, NO, or N z H~) , however, cause low-spin states of the iron centers. The high-spin 18e-complexes (L = N&, NH3, CH3OH) exhibit considerably larger F e S and Fe-N distances than the low-spin 18e-compounds (L = CO, '/2(N2H2)). The 19e-complex [F~(NO)("HS~')] with low-spin FelI takes an intermediate position.The correlation between coligands L, spin states, and bond lengths is interpreted in terms of molecular orbital schemes, which also explain why the 18e-low-spin complexes are highly substitution inert and the 18e-high-spin complexes are very labile. With regard to the active centers of "Fe-only" nitrogenases, [Fe-('NHS~)] complexes provide a model. Interrelation of iron spin states, coordination of either u or u -~ ligands, and substitution lability or inertness may not only support the reduction steps leading from N2 via N2Hz over N2H4 to NH3, but also enable the final step in a catalytic cycle, the substitution of NH3 by N2.

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two-carbon backbone (TcOEn(A0)J the bite angle of the nitrogens is 82.5 (l)', the nitrogens are deprotonated and planar, and the N-Tc distances are short. Adding a third carbon to the backbone to make a six-membered ring (TcOPn(AO),) introduces some change, as shown by the increased bite angle (92.7O). However, the nitrogens are still deprotonated and planar and the Tc-N distances remain nearly the same. The five-and sixmembered ring systems are able to accommodate the planar nitrogens without much strain. Increasing the backbone size to five carbons gives an eight-membered ring (TcO,Pent(AO),). This increased ring size makes it difficult to maintain a small bite angle and short Tc-N distances without considerable puckering of the ring. This same argument must apply to the seven-membered ring system with a four-carbon backbone (TcO,Bn(AO),). In the larger ring systems ring strain inhibits deprotonation of the nitrogens by favoring tetrahedral sp3 nitrogens and long Tc-N distances. ; ?COE~(AO)~, 95180-93-1; 99TcOzBn(AO),, 109959-89-9; 99Tc02Pent(A0)2, 109959-90-2; 99Tc0zPent(AO)2.2CHC13, 109959-91-3; Pent(AO),, 109929-73-9; chloroxime, 3238-16-2; NH499T~04, 34035-97-7; SnC4H4O6, 8 15-85-0; 1 ,5-diaminopentane, 462-94-2. Supplementary Material Available: Tables of positional parameters for hydrogen atoms, thermal parameters, bond distances and angles, and least-squares planes (13 pages); listings of observed and calculated structure factors (37 pages). Ordering information is given on any current masthead page. The complicated high-spin (HS, S = 2) + low-spin (LS, S = 0) transition in the solid iron(I1) complex [FeL(CN)2].H20 of a N302 macrocyclic Schiff-base ligand L has been studied in detail. When the sample is cooled slowly, a transition from the HS form to a form with approximately equal proportions of HS and LS is observed (T,( = 207 K). The transition is reversible (T: = 222 K) and, due to the pronounced hysteresis of ATc = 15 K, thermodynamically first order. On reasonably fast cooling (10-20 K min-I) from 300 K to temperatures between 150 and 160 K, the HS form is retained but transforms after several minutes into the LS form. The lower the temperature, the greater the LS fraction. The transformation is irreversible and fast, and below -1 SO K it is practically complete. On warming of the LS form, a transformation to the HS:LS = 1:l form is observed at T, = 157 K. If the sample is rapidly quenched to 77 K, the HS form is frozen in, an irreversible transformation to LS setting in if the sample is warmed to above 110 K. The kinetics of the HS -LS relaxation has been investigated between 115.7 and 130.0 K. The rate constant for the process varies between k = 6.77 X and 2.31 X lo-' s-I, the activation energy being AE = 7.3 kcal mol-'. In X-ray diffraction, the transformations are characterized by the replacement of individual diffraction patterns, thus establishing that the unit cells of the forms involved are all different. The quadrupole splitting for the LS form A E Q~ is characterized by a significant depe...

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