Supporting Information

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1 Supporting Information A Trinitrosyl Iron Complex (a {Fe(NO) 3 } 10 TNIC) Stabilized by an N- Heterocyclic Carbene and the Cationic and Neutral {Fe(NO) 2 } 9/10 Products of Its NO Release Chung-Hung Hsieh and Marcetta Y. Darensbourg* Department of Chemistry, Texas A & M University, College Station, TX *marcetta@mail.chem.tamu.edu SI Table of Contents Page # 1. Full X-ray Structure Data and Tables for Complex 1 S2 2. Full X-ray Structure Data and Tables for Complex 2 S7 3. Full X-ray Structure Data and Tables for Complex 3 S12 4. Full X-ray Structure Data and Tables for Complex 4 S17 5. The IR Spectrum of Complex 4 in THF Solution. S22 6. Table of IR Stretching Frequencies of Complexes 1-4. S22 7. Cyclic Voltammogram of Complex 3. S23 8. Cyclic Voltammogram of Complex 1. S23 S1

2 Table S1. Crystal data and structure refinement for [(IMes)Fe(NO) 3 ][BF 4 ] (1). Identification code m Empirical formula C21 H25 B F4 Fe N5 O3.50 Formula weight Temperature 110(2) K Wavelength Å Crystal system Triclinic Space group P-1 Unit cell dimensions a = 8.252(4) Å α= (6). b = (5) Å β= (6). c = (8) Å γ = (6). Volume (11) Å 3 Z 2 Density (calculated) Mg/m 3 Absorption coefficient mm -1 F(000) 562 Crystal size 0.20 x 0.14 x 0.14 mm 3 Theta range for data collection 2.54 to Index ranges -9<=h<=9, -12<=k<=12, -18<=l<=18 Reflections collected Independent reflections 3996 [R(int) = ] Completeness to theta = % Absorption correction Semi-empirical from equivalents Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 3996 / 78 / 353 Goodness-of-fit on F Final R indices [I>2sigma(I)] R1 = , wr2 = R indices (all data) R1 = , wr2 = Largest diff. peak and hole and e.å -3 S2

3 Figure S1. ORTEP drawing and labeling scheme of complex 1 with thermal ellipsoids drawn at 50% probability. The counter ion was found to be disordered over multiple positions. The disordered counter ion required refinement of similarity, bond distance and/or bond angle restraints. These restraints were needed for refinement of chemically reasonable counter ion. A partially occupied water was found in the final stages of refinement. Figure S2. The alternate view of complex 1 in capped stick rendition. S3

4 Table S2. Bond lengths [Å] and angles [ ] for [(IMes)Fe(NO) 3 ][BF 4 ] (1). Fe(1)-N(1) 1.681(5) C(19)-H(14B) Fe(1)-N(3) 1.691(5) C(19)-H(14C) Fe(1)-N(2) 1.696(5) C(17)-C(16) 1.386(8) Fe(1)-C(1) 1.999(5) C(17)-H(15) N(4)-C(1) 1.360(6) C(2)-H(16) N(4)-C(3) 1.398(7) C(12)-H(17A) N(4)-C(4) 1.442(6) C(12)-H(17B) C(4)-C(9) 1.379(7) C(12)-H(17C) C(4)-C(5) 1.401(7) N(2)-O(2) 1.140(6) C(1)-N(5) 1.347(6) C(11)-H(18A) N(5)-C(2) 1.384(7) C(11)-H(18B) N(5)-C(13) 1.432(6) C(11)-H(18C) C(8)-C(7) 1.383(7) C(16)-C(20) 1.512(8) C(8)-C(9) 1.392(7) C(10)-H(20A) C(8)-H(3) C(10)-H(20B) C(13)-C(18) 1.381(7) C(10)-H(20C) C(13)-C(14) 1.405(7) C(20)-H(21A) N(1)-O(1) 1.147(6) C(20)-H(21B) C(9)-C(12) 1.498(7) C(20)-H(21C) C(18)-C(17) 1.379(7) C(21)-H(26A) C(18)-C(21) 1.504(7) C(21)-H(26B) N(3)-O(3) 1.144(6) C(21)-H(26C) C(14)-C(15) 1.394(7) O(1W)-H(1W) C(14)-C(19) 1.490(7) O(1W)-H(2W) C(6)-C(7) 1.382(8) B11-F (10) C(6)-C(5) 1.386(7) B11-F (11) C(6)-H(8) B11-F (11) C(5)-C(10) 1.494(8) B11-F (12) C(7)-C(11) 1.482(7) B12-F (11) C(3)-C(2) 1.328(7) B12-F (12) C(3)-H(12) B12-F (12) C(15)-C(16) 1.380(8) B12-F (13) C(15)-H(13) C(19)-H(14A) S4

5 N(1)-Fe(1)-N(3) 111.7(2) N(1)-Fe(1)-N(2) 114.9(2) N(3)-Fe(1)-N(2) 111.5(2) N(1)-Fe(1)-C(1) (19) N(3)-Fe(1)-C(1) 105.8(2) N(2)-Fe(1)-C(1) 107.4(2) C(1)-N(4)-C(3) 110.3(4) C(1)-N(4)-C(4) 125.5(4) C(3)-N(4)-C(4) 123.8(4) C(9)-C(4)-C(5) 122.9(4) C(9)-C(4)-N(4) 120.0(4) C(5)-C(4)-N(4) 117.1(4) N(5)-C(1)-N(4) 104.8(4) N(5)-C(1)-Fe(1) 128.0(3) N(4)-C(1)-Fe(1) 127.2(3) C(1)-N(5)-C(2) 110.8(4) C(1)-N(5)-C(13) 124.6(4) C(2)-N(5)-C(13) 124.5(4) C(7)-C(8)-C(9) 122.0(5) C(7)-C(8)-H(3) C(9)-C(8)-H(3) C(18)-C(13)-C(14) 122.5(4) C(18)-C(13)-N(5) 119.7(4) C(14)-C(13)-N(5) 117.8(4) O(1)-N(1)-Fe(1) 176.0(4) C(4)-C(9)-C(8) 117.5(4) C(4)-C(9)-C(12) 122.1(4) C(8)-C(9)-C(12) 120.4(5) C(17)-C(18)-C(13) 117.4(4) C(17)-C(18)-C(21) 121.2(4) C(13)-C(18)-C(21) 121.4(5) O(3)-N(3)-Fe(1) 171.9(5) C(15)-C(14)-C(13) 117.4(4) C(15)-C(14)-C(19) 121.2(5) C(13)-C(14)-C(19) 121.4(4) C(7)-C(6)-C(5) 122.5(5) C(7)-C(6)-H(8) C(5)-C(6)-H(8) C(6)-C(5)-C(4) 116.8(5) C(6)-C(5)-C(10) 121.9(5) C(4)-C(5)-C(10) 121.2(5) C(6)-C(7)-C(8) 118.2(5) C(6)-C(7)-C(11) 119.7(5) C(8)-C(7)-C(11) 122.1(5) C(2)-C(3)-N(4) 106.5(4) C(2)-C(3)-H(12) N(4)-C(3)-H(12) C(16)-C(15)-C(14) 121.4(5) C(16)-C(15)-H(13) C(14)-C(15)-H(13) C(14)-C(19)-H(14A) C(14)-C(19)-H(14B) H(14A)-C(19)-H(14B) C(14)-C(19)-H(14C) H(14A)-C(19)-H(14C) H(14B)-C(19)-H(14C) C(18)-C(17)-C(16) 122.5(5) C(18)-C(17)-H(15) C(16)-C(17)-H(15) C(3)-C(2)-N(5) 107.6(5) C(3)-C(2)-H(16) N(5)-C(2)-H(16) C(9)-C(12)-H(17A) C(9)-C(12)-H(17B) H(17A)-C(12)-H(17B) C(9)-C(12)-H(17C) H(17A)-C(12)-H(17C) H(17B)-C(12)-H(17C) O(2)-N(2)-Fe(1) 172.6(5) C(7)-C(11)-H(18A) C(7)-C(11)-H(18B) H(18A)-C(11)-H(18B) S5

6 C(7)-C(11)-H(18C) C(18)-C(21)-H(26B) H(18A)-C(11)-H(18C) H(26A)-C(21)-H(26B) H(18B)-C(11)-H(18C) C(18)-C(21)-H(26C) C(15)-C(16)-C(17) 118.7(5) H(26A)-C(21)-H(26C) C(15)-C(16)-C(20) 119.4(5) H(26B)-C(21)-H(26C) C(17)-C(16)-C(20) 121.9(5) H(1W)-O(1W)-H(2W) C(5)-C(10)-H(20A) F41-B11-F (8) C(5)-C(10)-H(20B) F41-B11-F (8) H(20A)-C(10)-H(20B) F11-B11-F (8) C(5)-C(10)-H(20C) F41-B11-F (7) H(20A)-C(10)-H(20C) F11-B11-F (8) H(20B)-C(10)-H(20C) F31-B11-F (8) C(16)-C(20)-H(21A) F42-B12-F (11) C(16)-C(20)-H(21B) F42-B12-F (11) H(21A)-C(20)-H(21B) F12-B12-F (11) C(16)-C(20)-H(21C) F42-B12-F (10) H(21A)-C(20)-H(21C) F12-B12-F (11) H(21B)-C(20)-H(21C) F32-B12-F (11) C(18)-C(21)-H(26A) Symmetry transformations used to generate equivalent atoms: S6

7 Table S3. Crystal data and structure refinement for [(IMes)Fe(CO)(NO) 2 ] (2). Identification code fem Empirical formula C22 H24 Fe N4 O3 Formula weight Temperature 110(2) K Wavelength Å Crystal system Triclinic Space group P-1 Unit cell dimensions a = 7.610(5) Å α= (11). b = 8.319(5) Å β= (10). c = (12) Å γ = (10). Volume (12) Å 3 Z 2 Density (calculated) Mg/m 3 Absorption coefficient mm -1 F(000) 468 Crystal size 0.24 x 0.22 x 0.18 mm 3 Theta range for data collection 3.47 to Index ranges -9<=h<=9, -9<=k<=9, -22<=l<=22 Reflections collected Independent reflections 3879 [R(int) = ] Completeness to theta = % Absorption correction Semi-empirical from equivalents Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 3879 / 0 / 272 Goodness-of-fit on F Final R indices [I>2sigma(I)] R1 = , wr2 = R indices (all data) R1 = , wr2 = Largest diff. peak and hole and e.å -3 S7

8 Figure S3. ORTEP drawing and labeling scheme of complex 2 with thermal ellipsoids drawn at 50% probability. Figure S4. The alternate view of complex 2 in capped stick rendition. S8

9 Table S4. Bond lengths [Å] and angles [ ] for [(IMes)Fe(CO)(NO) 2 ] (2). Fe(1)-N(1) 1.657(2) C(13)-H(13B) Fe(1)-N(2) 1.694(3) C(13)-H(13C) Fe(1)-C(1) 1.777(4) C(14)-C(19) 1.378(4) Fe(1)-C(2) 1.989(3) C(14)-C(15) 1.397(4) O(1)-N(1) 1.184(3) C(15)-C(16) 1.394(4) C(1)-O(3) 1.133(4) C(15)-C(21) 1.489(4) N(2)-O(2) 1.164(3) C(16)-C(17) 1.369(5) C(2)-N(4) 1.359(3) C(16)-H(16) C(2)-N(3) 1.361(3) C(17)-C(18) 1.384(4) N(3)-C(3) 1.384(3) C(17)-C(22) 1.515(4) N(3)-C(5) 1.435(3) C(18)-C(19) 1.390(4) C(3)-C(4) 1.338(4) C(18)-H(18) C(3)-H(3) C(19)-C(20) 1.501(4) N(4)-C(4) 1.380(3) C(20)-H(20A) N(4)-C(14) 1.445(3) C(20)-H(20B) C(4)-H(4) C(20)-H(20C) C(5)-C(6) 1.382(4) C(21)-H(21A) C(5)-C(10) 1.392(4) C(21)-H(21B) C(6)-C(7) 1.390(4) C(21)-H(21C) C(6)-C(12) 1.506(5) C(22)-H(22A) C(7)-C(8) 1.376(5) C(22)-H(22B) C(7)-H(7) C(22)-H(22C) C(8)-C(9) 1.373(5) C(8)-C(13) 1.512(4) N(1)-Fe(1)-N(2) (12) C(9)-C(10) 1.388(4) N(1)-Fe(1)-C(1) (12) C(9)-H(9) N(2)-Fe(1)-C(1) (13) C(10)-C(11) 1.482(5) N(1)-Fe(1)-C(2) (10) C(11)-H(11A) N(2)-Fe(1)-C(2) (10) C(11)-H(11B) C(1)-Fe(1)-C(2) (12) C(11)-H(11C) O(1)-N(1)-Fe(1) (19) C(12)-H(12A) O(3)-C(1)-Fe(1) 175.7(2) C(12)-H(12B) O(2)-N(2)-Fe(1) 174.4(2) C(12)-H(12C) N(4)-C(2)-N(3) (19) C(13)-H(13A) N(4)-C(2)-Fe(1) (17) S9

10 N(3)-C(2)-Fe(1) (17) C(2)-N(3)-C(3) 111.5(2) C(2)-N(3)-C(5) (19) C(3)-N(3)-C(5) (19) C(4)-C(3)-N(3) 106.7(2) C(4)-C(3)-H(3) N(3)-C(3)-H(3) C(2)-N(4)-C(4) (19) C(2)-N(4)-C(14) (19) C(4)-N(4)-C(14) (19) C(3)-C(4)-N(4) 106.8(2) C(3)-C(4)-H(4) N(4)-C(4)-H(4) C(6)-C(5)-C(10) 122.5(3) C(6)-C(5)-N(3) 118.0(2) C(10)-C(5)-N(3) 119.5(3) C(5)-C(6)-C(7) 117.4(3) C(5)-C(6)-C(12) 121.0(3) C(7)-C(6)-C(12) 121.7(3) C(8)-C(7)-C(6) 122.1(4) C(8)-C(7)-H(7) C(6)-C(7)-H(7) C(9)-C(8)-C(7) 118.6(3) C(9)-C(8)-C(13) 120.6(4) C(7)-C(8)-C(13) 120.9(4) C(8)-C(9)-C(10) 122.1(3) C(8)-C(9)-H(9) C(10)-C(9)-H(9) C(9)-C(10)-C(5) 117.4(3) C(9)-C(10)-C(11) 121.3(3) C(5)-C(10)-C(11) 121.4(3) C(10)-C(11)-H(11A) C(10)-C(11)-H(11B) H(11A)-C(11)-H(11B) C(10)-C(11)-H(11C) H(11A)-C(11)-H(11C) H(11B)-C(11)-H(11C) C(6)-C(12)-H(12A) C(6)-C(12)-H(12B) H(12A)-C(12)-H(12B) C(6)-C(12)-H(12C) H(12A)-C(12)-H(12C) H(12B)-C(12)-H(12C) C(8)-C(13)-H(13A) C(8)-C(13)-H(13B) H(13A)-C(13)-H(13B) C(8)-C(13)-H(13C) H(13A)-C(13)-H(13C) H(13B)-C(13)-H(13C) C(19)-C(14)-C(15) 122.8(2) C(19)-C(14)-N(4) 118.6(2) C(15)-C(14)-N(4) 118.5(3) C(16)-C(15)-C(14) 117.1(3) C(16)-C(15)-C(21) 120.3(3) C(14)-C(15)-C(21) 122.6(2) C(17)-C(16)-C(15) 122.1(3) C(17)-C(16)-H(16) C(15)-C(16)-H(16) C(16)-C(17)-C(18) 118.5(3) C(16)-C(17)-C(22) 120.6(3) C(18)-C(17)-C(22) 121.0(3) C(17)-C(18)-C(19) 122.4(3) C(17)-C(18)-H(18) C(19)-C(18)-H(18) C(14)-C(19)-C(18) 117.1(3) C(14)-C(19)-C(20) 121.3(2) C(18)-C(19)-C(20) 121.6(3) C(19)-C(20)-H(20A) C(19)-C(20)-H(20B) H(20A)-C(20)-H(20B) C(19)-C(20)-H(20C) H(20A)-C(20)-H(20C) S10

11 H(20B)-C(20)-H(20C) C(17)-C(22)-H(22A) C(15)-C(21)-H(21A) C(17)-C(22)-H(22B) C(15)-C(21)-H(21B) H(22A)-C(22)-H(22B) H(21A)-C(21)-H(21B) C(17)-C(22)-H(22C) C(15)-C(21)-H(21C) H(22A)-C(22)-H(22C) H(21A)-C(21)-H(21C) H(22B)-C(22)-H(22C) H(21B)-C(21)-H(21C) Symmetry transformations used to generate equivalent atoms: S11

12 Table S5. Crystal data and structure refinement for [(IMes)Fe(SPh)(NO) 2 ] (3). Identification code fe Empirical formula C27 H32 Fe N4 O3.50 S Formula weight Temperature 110(2) K Wavelength Å Crystal system Monoclinic Space group P2(1)/c Unit cell dimensions a = (12) Å α= 90. b = 8.202(6) Å β= (9). c = (17) Å γ = 90. Volume 2975(4) Å 3 Z 4 Density (calculated) Mg/m 3 Absorption coefficient mm -1 F(000) 1168 Crystal size 0.16 x 0.12 x 0.10 mm 3 Theta range for data collection 1.83 to Index ranges -21<=h<=21, -10<=k<=10, -28<=l<=28 Reflections collected Independent reflections 6786 [R(int) = ] Completeness to theta = % Absorption correction Semi-empirical from equivalents Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 6786 / 0 / 331 Goodness-of-fit on F Final R indices [I>2sigma(I)] R1 = , wr2 = R indices (all data) R1 = , wr2 = Largest diff. peak and hole and e.å -3 S12

13 Figure S5. ORTEP drawing and labeling scheme of complex 3 with thermal ellipsoids drawn at 50% probability. A partially occupied included water was found in the final stages of refinement. Figure S6. The alternate view of complex 3 in capped stick rendition. S13

14 Table S6. Bond lengths [Å] and angles [ ] for [(IMes)Fe(SPh)(NO) 2 ] (3). Fe(1)-N(2) 1.667(3) C(13)-C(18) 1.504(4) Fe(1)-N(1) 1.677(3) C(14)-C(15) 1.391(4) Fe(1)-C(1) 2.045(3) C(14)-H(14A) Fe(1)-S(1) (17) C(15)-C(16) 1.509(5) S(1)-C(2) 1.760(4) C(16)-H(16A) C(1)-N(3) 1.353(4) C(16)-H(16B) C(1)-N(4) 1.355(3) C(16)-H(16C) N(1)-O(1) 1.182(3) C(17)-H(17A) O(2)-N(2) 1.186(3) C(17)-H(17B) C(2)-C(7) 1.398(6) C(17)-H(17C) C(2)-C(3) 1.402(5) C(18)-H(18A) N(3)-C(9) 1.388(4) C(18)-H(18B) N(3)-C(10) 1.453(4) C(18)-H(18C) C(3)-C(4) 1.419(7) C(19)-C(24) 1.381(5) C(3)-H(3A) C(19)-C(20) 1.392(4) N(4)-C(8) 1.374(4) C(20)-C(21) 1.395(4) N(4)-C(19) 1.458(4) C(20)-C(26) 1.509(5) C(4)-C(5) 1.324(8) C(21)-C(22) 1.376(5) C(4)-H(4A) C(21)-H(21A) C(5)-C(6) 1.403(8) C(22)-C(23) 1.391(5) C(5)-H(5A) C(22)-C(27) 1.507(5) C(6)-C(7) 1.385(6) C(23)-C(24) 1.403(4) C(6)-H(6A) C(23)-H(23A) C(7)-H(7A) C(24)-C(25) 1.513(5) C(8)-C(9) 1.349(5) C(25)-H(25A) C(8)-H(8A) C(25)-H(25B) C(9)-H(9A) C(25)-H(25C) C(10)-C(15) 1.377(5) C(26)-H(26A) C(10)-C(11) 1.394(5) C(26)-H(26B) C(11)-C(12) 1.398(5) C(26)-H(26C) C(11)-C(17) 1.507(5) C(27)-H(27A) C(12)-C(13) 1.384(5) C(27)-H(27B) C(12)-H(12A) C(27)-H(27C) C(13)-C(14) 1.388(5) O(1W)-H(1W) S14

15 O(2W)-H(1W2) O(2W)-H(2W2) N(2)-Fe(1)-N(1) (13) N(2)-Fe(1)-C(1) (11) N(1)-Fe(1)-C(1) (11) N(2)-Fe(1)-S(1) (9) N(1)-Fe(1)-S(1) (10) C(1)-Fe(1)-S(1) (9) C(2)-S(1)-Fe(1) (14) N(3)-C(1)-N(4) 103.7(2) N(3)-C(1)-Fe(1) 126.6(2) N(4)-C(1)-Fe(1) 129.6(2) O(1)-N(1)-Fe(1) 167.8(2) O(2)-N(2)-Fe(1) 169.6(2) C(7)-C(2)-C(3) 117.9(4) C(7)-C(2)-S(1) 124.5(3) C(3)-C(2)-S(1) 117.6(4) C(1)-N(3)-C(9) 111.8(3) C(1)-N(3)-C(10) 124.2(2) C(9)-N(3)-C(10) 124.0(2) C(4)-C(3)-C(2) 119.1(5) C(4)-C(3)-H(3A) C(2)-C(3)-H(3A) C(1)-N(4)-C(8) 111.7(3) C(1)-N(4)-C(19) 125.8(2) C(8)-N(4)-C(19) 122.5(2) C(5)-C(4)-C(3) 122.3(5) C(5)-C(4)-H(4A) C(3)-C(4)-H(4A) C(4)-C(5)-C(6) 119.5(5) C(4)-C(5)-H(5A) C(6)-C(5)-H(5A) C(7)-C(6)-C(5) 119.9(5) C(7)-C(6)-H(6A) C(5)-C(6)-H(6A) C(6)-C(7)-C(2) 121.3(4) C(6)-C(7)-H(7A) C(2)-C(7)-H(7A) C(9)-C(8)-N(4) 107.0(3) C(9)-C(8)-H(8A) N(4)-C(8)-H(8A) C(8)-C(9)-N(3) 105.8(3) C(8)-C(9)-H(9A) N(3)-C(9)-H(9A) C(15)-C(10)-C(11) 123.7(3) C(15)-C(10)-N(3) 118.4(3) C(11)-C(10)-N(3) 118.0(3) C(10)-C(11)-C(12) 116.9(3) C(10)-C(11)-C(17) 122.1(3) C(12)-C(11)-C(17) 121.0(3) C(13)-C(12)-C(11) 121.5(3) C(13)-C(12)-H(12A) C(11)-C(12)-H(12A) C(12)-C(13)-C(14) 118.8(3) C(12)-C(13)-C(18) 120.0(3) C(14)-C(13)-C(18) 121.2(3) C(13)-C(14)-C(15) 122.0(3) C(13)-C(14)-H(14A) C(15)-C(14)-H(14A) C(10)-C(15)-C(14) 117.0(3) C(10)-C(15)-C(16) 122.1(3) C(14)-C(15)-C(16) 121.0(3) C(15)-C(16)-H(16A) C(15)-C(16)-H(16B) H(16A)-C(16)-H(16B) C(15)-C(16)-H(16C) H(16A)-C(16)-H(16C) H(16B)-C(16)-H(16C) C(11)-C(17)-H(17A) C(11)-C(17)-H(17B) H(17A)-C(17)-H(17B) S15

16 C(11)-C(17)-H(17C) C(24)-C(23)-H(23A) H(17A)-C(17)-H(17C) C(19)-C(24)-C(23) 117.4(3) H(17B)-C(17)-H(17C) C(19)-C(24)-C(25) 122.1(3) C(13)-C(18)-H(18A) C(23)-C(24)-C(25) 120.5(3) C(13)-C(18)-H(18B) C(24)-C(25)-H(25A) H(18A)-C(18)-H(18B) C(24)-C(25)-H(25B) C(13)-C(18)-H(18C) H(25A)-C(25)-H(25B) H(18A)-C(18)-H(18C) C(24)-C(25)-H(25C) H(18B)-C(18)-H(18C) H(25A)-C(25)-H(25C) C(24)-C(19)-C(20) 123.3(3) H(25B)-C(25)-H(25C) C(24)-C(19)-N(4) 118.3(3) C(20)-C(26)-H(26A) C(20)-C(19)-N(4) 118.4(3) C(20)-C(26)-H(26B) C(19)-C(20)-C(21) 116.6(3) H(26A)-C(26)-H(26B) C(19)-C(20)-C(26) 121.1(3) C(20)-C(26)-H(26C) C(21)-C(20)-C(26) 122.3(3) H(26A)-C(26)-H(26C) C(22)-C(21)-C(20) 122.9(3) H(26B)-C(26)-H(26C) C(22)-C(21)-H(21A) C(22)-C(27)-H(27A) C(20)-C(21)-H(21A) C(22)-C(27)-H(27B) C(21)-C(22)-C(23) 118.2(3) H(27A)-C(27)-H(27B) C(21)-C(22)-C(27) 121.8(3) C(22)-C(27)-H(27C) C(23)-C(22)-C(27) 120.1(3) H(27A)-C(27)-H(27C) C(22)-C(23)-C(24) 121.6(3) H(27B)-C(27)-H(27C) C(22)-C(23)-H(23A) H(1W2)-O(2W)-H(2W2) Symmetry transformations used to generate equivalent atoms: S16

17 Table S7. Crystal data and structure refinement for [(NO) 2 Fe(bmedach)Fe(NO)][BF 4 ] (4). Identification code 2feins Empirical formula C4.50 H9 B0.50 F2 Fe N2.50 O1.50 S Formula weight Temperature 110(2) K Wavelength Å Crystal system Orthorhombic Space group Pca21 Unit cell dimensions a = (5) Å α= 90. b = 7.999(3) Å β= 90. c = (5) Å γ = 90. Volume (10) Å 3 Z 8 Density (calculated) Mg/m 3 Absorption coefficient mm -1 F(000) 1024 Crystal size 0.36 x 0.22 x 0.13 mm 3 Theta range for data collection 2.65 to Index ranges -18<=h<=18, -10<=k<=10, -20<=l<=20 Reflections collected Independent reflections 4306 [R(int) = ] Completeness to theta = % Absorption correction Semi-empirical from equivalents Max. and min. transmission and Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 4306 / 79 / 281 Goodness-of-fit on F Final R indices [I>2sigma(I)] R1 = , wr2 = R indices (all data) R1 = , wr2 = Absolute structure parameter (11) Largest diff. peak and hole and e.å -3 S17

18 Figure S7. ORTEP drawing and labeling scheme of complex 4 with thermal ellipsoids drawn at 50% probability. The counter ion was found to be disordered over multiple positions. Figure S8. The alternate view of complex 4 in capped stick rendition. S18

19 Table S8. Bond lengths [Å] and angles [ ] for [(NO) 2 Fe(bme-dach)Fe(NO)][BF 4 ] (4). Fe(1)-N(1) 1.669(2) C(6)-C(7) 1.547(3) Fe(1)-N(2) 1.673(2) C(6)-H(6A) Fe(1)-S(2) (9) C(6)-H(6B) Fe(1)-S(1) (9) C(7)-H(7A) Fe(1)-Fe(2) (8) C(7)-H(7B) Fe(2)-N(3) 1.668(2) C(8)-C(9) 1.501(3) Fe(2)-N(5) 2.026(2) C(8)-H(8A) Fe(2)-N(4) (19) C(8)-H(8B) Fe(2)-S(1) (8) C(9)-H(9A) Fe(2)-S(2) (8) C(9)-H(9B) S(1)-C(1) 1.831(3) B11-F (5) S(2)-C(9) 1.830(2) B11-F (5) O(1)-N(1) 1.174(3) B11-F (5) O(2)-N(2) 1.169(3) B11-F (5) O(3)-N(3) 1.147(3) B12-F (5) N(4)-C(8) 1.493(3) B12-F (5) N(4)-C(7) 1.498(3) B12-F (4) N(4)-C(5) 1.503(3) B12-F (4) N(5)-C(6) 1.481(3) N(5)-C(2) 1.492(3) N(1)-Fe(1)-N(2) (10) N(5)-C(3) 1.509(3) N(1)-Fe(1)-S(2) (7) C(1)-C(2) 1.505(4) N(2)-Fe(1)-S(2) (7) C(1)-H(1A) N(1)-Fe(1)-S(1) (7) C(1)-H(1B) N(2)-Fe(1)-S(1) (7) C(2)-H(2A) S(2)-Fe(1)-S(1) 92.63(3) C(2)-H(2B) N(1)-Fe(1)-Fe(2) (7) C(3)-C(4) 1.512(3) N(2)-Fe(1)-Fe(2) (7) C(3)-H(3A) S(2)-Fe(1)-Fe(2) (18) C(3)-H(3B) S(1)-Fe(1)-Fe(2) 51.57(2) C(4)-C(5) 1.509(3) N(3)-Fe(2)-N(5) (9) C(4)-H(4A) N(3)-Fe(2)-N(4) (9) C(4)-H(4B) N(5)-Fe(2)-N(4) 78.50(8) C(5)-H(5A) N(3)-Fe(2)-S(1) 98.51(7) C(5)-H(5B) N(5)-Fe(2)-S(1) 88.01(6) S19

20 N(4)-Fe(2)-S(1) (6) N(3)-Fe(2)-S(2) (7) N(5)-Fe(2)-S(2) (6) N(4)-Fe(2)-S(2) 87.19(6) S(1)-Fe(2)-S(2) 92.52(3) N(3)-Fe(2)-Fe(1) (8) N(5)-Fe(2)-Fe(1) (6) N(4)-Fe(2)-Fe(1) (6) S(1)-Fe(2)-Fe(1) 51.83(2) S(2)-Fe(2)-Fe(1) 51.62(2) C(1)-S(1)-Fe(2) 98.86(9) C(1)-S(1)-Fe(1) (8) Fe(2)-S(1)-Fe(1) 76.59(3) C(9)-S(2)-Fe(1) (8) C(9)-S(2)-Fe(2) 98.93(8) Fe(1)-S(2)-Fe(2) 76.37(3) O(1)-N(1)-Fe(1) (19) O(2)-N(2)-Fe(1) 166.6(2) O(3)-N(3)-Fe(2) 165.8(2) C(8)-N(4)-C(7) (17) C(8)-N(4)-C(5) (17) C(7)-N(4)-C(5) (18) C(8)-N(4)-Fe(2) (15) C(7)-N(4)-Fe(2) (13) C(5)-N(4)-Fe(2) (15) C(6)-N(5)-C(2) (18) C(6)-N(5)-C(3) 109.8(2) C(2)-N(5)-C(3) (17) C(6)-N(5)-Fe(2) (14) C(2)-N(5)-Fe(2) (15) C(3)-N(5)-Fe(2) (14) C(2)-C(1)-S(1) (16) C(2)-C(1)-H(1A) S(1)-C(1)-H(1A) C(2)-C(1)-H(1B) S(1)-C(1)-H(1B) H(1A)-C(1)-H(1B) N(5)-C(2)-C(1) (19) N(5)-C(2)-H(2A) C(1)-C(2)-H(2A) N(5)-C(2)-H(2B) C(1)-C(2)-H(2B) H(2A)-C(2)-H(2B) N(5)-C(3)-C(4) (19) N(5)-C(3)-H(3A) C(4)-C(3)-H(3A) N(5)-C(3)-H(3B) C(4)-C(3)-H(3B) H(3A)-C(3)-H(3B) C(5)-C(4)-C(3) 114.7(2) C(5)-C(4)-H(4A) C(3)-C(4)-H(4A) C(5)-C(4)-H(4B) C(3)-C(4)-H(4B) H(4A)-C(4)-H(4B) N(4)-C(5)-C(4) 112.9(2) N(4)-C(5)-H(5A) C(4)-C(5)-H(5A) N(4)-C(5)-H(5B) C(4)-C(5)-H(5B) H(5A)-C(5)-H(5B) N(5)-C(6)-C(7) (18) N(5)-C(6)-H(6A) C(7)-C(6)-H(6A) N(5)-C(6)-H(6B) C(7)-C(6)-H(6B) H(6A)-C(6)-H(6B) N(4)-C(7)-C(6) (18) N(4)-C(7)-H(7A) C(6)-C(7)-H(7A) N(4)-C(7)-H(7B) C(6)-C(7)-H(7B) S20

21 H(7A)-C(7)-H(7B) F41-B11-F (5) N(4)-C(8)-C(9) (19) F41-B11-F (5) N(4)-C(8)-H(8A) F11-B11-F (5) C(9)-C(8)-H(8A) F41-B11-F (5) N(4)-C(8)-H(8B) F11-B11-F (5) C(9)-C(8)-H(8B) F31-B11-F (5) H(8A)-C(8)-H(8B) F42-B12-F (4) C(8)-C(9)-S(2) (17) F42-B12-F (4) C(8)-C(9)-H(9A) F12-B12-F (4) S(2)-C(9)-H(9A) F42-B12-F (4) C(8)-C(9)-H(9B) F12-B12-F (4) S(2)-C(9)-H(9B) F32-B12-F (4) H(9A)-C(9)-H(9B) Symmetry transformations used to generate equivalent atoms: S21

22 0.12 Absorbance Wavenumber (cm -1 ) Figure S9. The IR spectrum of complex 4 in THF solution. Table S9. The IR stretching frequencies of complexes 1-4 in THF solution and, for comparison, Fe(CO) 2 (NO) 2. Complexes ν(co) cm -1 ν(no) cm -1 [(IMes)Fe(NO) 3 ][BF 4 ] (1) 1932(s), 1831(s), 1804(vs) [(IMes)Fe(CO)(NO) 2 ] (2) 1986(s) 1744(s), 1702(vs) [(IMes)Fe(SPh)(NO) 2 ] (3) 1763(s), 1715(vs) [(NO) 2 Fe(bme-dach)Fe(NO)][BF 4 ] (4) 1795(s), 1763(s), 1740(vs) Fe(CO) 2 (NO) (s), 2038(vs) 1807(s), 1762(vs) S22

23 * 10 µa * mv Figure S10. Cyclic voltammogram of complex 3 at scan rate 100 mv/s in 2 mm THF solution with 100 mm [n-bu 4 N][BF 4 ] as supporting electrolyte; using a glassy carbon electrode, the potential was measure relative to Ag/AgNO 3 electrode. * is the redox process of Cp 2 Fe/Cp 2 Fe +, (E a -E c ) = 370 mv. 10 µa mv Figure S11. Cyclic voltammogram of complex 1 at scan rate 200 mv/s in 2 mm THF solution with 100 mm [n-bu 4 N][BF 4 ] as supporting electrolyte; using a glassy carbon electrode, the potential was measured relative to Ag/AgNO 3 electrode and referenced to Cp 2 Fe/Cp 2 Fe + E 1/2 = 0.00 mv. S23