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Electronic Excited States of Tungsten(0) Arylisocyanides Hana Kvapilová, a,b Wesley Sattler, c Aaron Sattler, c Igor V. Sazanovich, d Ian P. Clark, d Michael Towrie, d Harry B. Gray, c, * Stanislav Záliš, a, * Antonín Vlček a,e, * a J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague, Czech Republic b Department of Inorganic Chemistry, University of Chemistry and Technology, Prague, Technická 5, CZ-166 28 Prague 6, Czech Republic c Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA d Central Laser Facility, Research Complex at Harwell, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0QX, United Kingdom, e Queen Mary University of London, School of Biological and Chemical Sciences, Mile End Road, London E1 4NS, United Kingdom e-mail: zalis@jh-inst.cas.cz, hbgray@caltech.edu, a.vlcek@qmul.ac.uk Supplementary Information S1

Supplementary Information Contents: Optimized W(CNAryl) 6 molecular structures in THF. p. S3-S6 Calculated intense electronic transitions: orbital composition, energies and electronic density differences. p. S7-S23 Coplanar (C i ) W(CNPh) 6 p. S7-S9 Orthogonal (T d ) W(CNPh) 6 p. S10-S13 Orthogonal W(CNXy) 6.. p. S13-S15 Coplanar W(CNdipp) 6 p. S16-S18 Coplanar W(CNdipp OMe 2 ) 6.. p. S18-S20 Simulated W(CNPh) 6 UV-vis spectra.. p. S20-21 MO composition.. p. S21-22 Nine lowest triplets of W(CNdipp) 6 p. S23 Spin-Orbit TDDFT calculation of coplanar (C i ) W(CNdipp) 6.. p. S24-S26 Experimental and simulated TRIR spectra. Visualization of ν(c N) vibrations. p. S27-S34 Spin density distribution in the T1 state of W(CNXy) 6 and W(CNdipp) 6. p. S35 Spin-orbit electronic transitions of coplanar (C i ) W(CNdipp) 6 at optimized T1 geometry p. S36-37 Characterization of the vertically excited T1 state of W(CNdipp) 6 and W(CNdippPh OMe 2 ) 6. p. S38-39 Electronic transitions of the W(CNPh) 6 model complex p. S40-42 S2

Optimized W(CNAryl) 6 molecular structures in THF W(CNPh) 6 coplanar, symmetry C i W(CNPh) 6 orthogonal, symmetry T d trans-ph 0.0-0.3 trans-ph 90 E = -2011.808979 au; G = -2011.301744 au E = -2011.8072154 au; G = -2011.298682 au Figure S1a. DFT-optimized structures of W(CNPh) 6 in THF. E = electronic energy, G = free energy, in Hartree atomic units. S3

W(CNXy) 6 - orthogonal W(CNXy) 6 - coplanar trans-ph 56-87 trans-ph 1-7 E = -2483. 0620422 au; G = -2482.236635 au E = -2483.0619587 au; G = -2482.238759 au Figure S1b. DFT-optimized structures of W(CNXy) 6 in THF. E = electronic energy, G = free energy, in Hartree atomic units. S4

W(CNdipp) 6 - coplanar W(CNdipp) 6 - orthogonal trans-ph 3-4 trans-ph 65-86 E = -3425.3953488 au; G = -3423.908804 au E = -3425.3881867 au; G = -3423.901449 au Figure S1c. DFT-optimized structures of W(CNdipp) 6 in THF. E = electronic energy, G = free energy, in Hartree atomic units. S5

W(CNdippPh OMe 2 ) 6 - coplanar W(CNdippPh OMe 2 ) 6 - orthogonal trans-ph 2.6 3.9 torsion angle b/w dipp and Ph OMe 2 within the ligand ~ 36 trans-ph 64, 79, 87 torsion angle b/w dipp and Ph OMe 2 within the ligand ~ 36 Figure S1d. DFT-optimized structure of W(CNdippPh OMe 2 ) 6 in THF. S6

Calculated intense electronic transitions: Orbital composition, energies and electronic density differences W(CNPh) 6 coplanar conformation in the C i symmetry group Table S1. Intense singlet transitions of W(CNPh) 6 in the coplanar structure, C i symmetry. Orbital contributions >10 % shown, TDDFT PBE0/PCM-pentane. Simulated spectrum in Figure S7a. State Orbital Contribution (%) Energy / cm -1 Wavelength / nm Oscillator Strength 7 HOMO (a g ) LUMO+1 (a u ) (32) 22533 444 0.7014 HOMO-1 (a g ) LUMO (a u ) (26) 8 HOMO-2 (a g ) LUMO+2 (a u ) (30) 22579 443 0.6960 HOMO (a g ) LUMO (a u ) (29) HOMO (a g ) LUMO+2 (a u ) (10) 9 HOMO-1 (a g ) LUMO+2 (a u ) (34) 22642 442 0.6991 HOMO-2 (a g ) LUMO+1 (a u ) (29) HOMO (a g ) LUMO+1 (a u ) (10) 43 HOMO (a g ) LUMO+13 (a u ) (23) 34584 289 1.3774 HOMO-2 (a g ) LUMO+13 (a u ) (22) HOMO-1 (a g ) LUMO+14 (a u ) (15) HOMO-1 (a g ) LUMO+12 (a u ) (11) 44 HOMO-2 (a g ) LUMO+14 (a u ) (22) 34599 289 1.3737 HOMO (a g ) LUMO+12 (a u ) (20) HOMO-2 (a g ) LUMO+12 (a u ) (19) HOMO (a g ) LUMO+14 (a u ) (14) 45 HOMO-1 (a g ) LUMO+12 (a u ) (22) HOMO-2 (a g ) LUMO+13 (a u ) (19) HOMO-1 (a g ) LUMO+14 (a u ) (19) 34618 289 1.3725 S7

LUMO+14 coplanar (0.57 ev, a u ) LUMO+13 coplanar (0.54 ev, a u ) LUMO+12 coplanar (0.52 ev, a u ) LUMO+2 coplanar (-0.96 ev, a u ) LUMO+1 coplanar (-0.97 ev, a u ) LUMO coplanar (-0.98 ev, a u ) HOMO coplanar (-4.41 ev, a g ) HOMO-1 coplanar (-4.41 ev, a g ) HOMO-2 coplanar (-4.42 ev, a g ) Figure S2a. Molecular orbitals participating in intense singlet electronic transitions of W(CNPh) 6 in the coplanar structure, C i symmetry. TDDFT PBE0/PCM-pentane. Symmetry representations and calculated orbital energies in parentheses. S8

state 7, 444 nm (2.79 ev) state 8, 443 nm (2.80 ev) state 9, 442 nm (2.81 ev) state 43, 289 nm (4.29 ev) state 44, 289 nm (4.29 ev) state 45, 289 nm (4.29 ev) Figure S2b. Electron density changes accompanying intense singlet transitions of the coplanar conformer of W(CNPh) 6, C i symmetry. Purple and blue colors indicate increasing and diminishing electron density, respectively. Transitions in nm (ev). S9

W(CNPh) 6 orthogonal conformation in the T d symmetry group Table S2. Intense singlet transitions of W(CNPh) 6 in the orthogonal structure, T d symmetry. Orbital contributions >10 % shown, TDDFT PBE0/PCM-pentane. Individual orbitals in degenerate sets are distinguished by apostrophes. Simulated spectrum in Figure S7b. State Orbital Contribution (%) Energy / cm -1 Wavelength / nm Oscillator Strength 16 HOMO (t 2 ) LUMO+1 (25) (t 2 ) 26620 376 1.4498 HOMO (t 2 ) LUMO+1 (25) (t 2 ) HOMO (t 2 ) LUMO (23) (t 1 ) HOMO (t 2 ) LUMO (23) (t 1 ) 17 HOMO (t 2 ) LUMO+1 (25) (t 2 ) 26620 376 1.4498 HOMO (t 2 ) LUMO+1 (25) (t 2 ) HOMO (t 2 ) LUMO (23) (t 1 ) HOMO (t 2 ) LUMO (23) (t 1 ) 18 HOMO (t 2 ) LUMO+1 (25) (t 2 ) 26620 376 1.4498 HOMO (t 2 ) LUMO+1 (25) (t 2 ) HOMO (t 2 ) LUMO (23) (t 1 ) HOMO (t 2 ) LUMO (23) (t 1 ) 52 HOMO (t 2 ) LUMO+7 (21) (t 2 ) 39978 250 0.2370 HOMO (t 2 ) LUMO+7 (21) (t 2 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) 53 HOMO (t 2 ) LUMO+7 (21) (t 2 ) 39978 250 0.2369 HOMO (t 2 ) LUMO+7 (21) (t 2 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) 54 HOMO (t 2 ) LUMO+7 (21) (t 2 ) HOMO (t 2 ) LUMO+7 (21) (t 2 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) HOMO (t 2 ) LUMO+6 (22) (t 1 ) 39978 250 0.2370 S10

LUMO+7 orthogonal (1.54 ev, t 2 ) LUMO+7 orthogonal (1.54 ev, t 2 ) LUMO+7 orthogonal (1.54 ev, t 2 ) LUMO+6 orthogonal (1.35 ev, t 1 ) LUMO+6 orthogonal (1.35 ev, t 1 ) LUMO+6 orthogonal (1.35 ev, t 1 ) LUMO+1 orthogonal (-0.68 ev, t 2 ) LUMO+1 orthogonal (-0.68 ev, t 2 ) LUMO+1 orthogonal (-0.68 ev, t 2 ) Figure S3a. Molecular orbitals participating in intense singlet electronic transitions of W(CNPh) 6 in the orthogonal structure, T d symmetry. TDDFT PBE0/PCM-pentane. Symmetry representations and calculated orbital energies in parentheses. S11

LUMO orthogonal (-0.69 ev, t 1 ) LUMO orthogonal (-0.69 ev, t 1 ) LUMO orthogonal (-0.69 ev, t 1 ) HOMO orthogonal (-4.42 ev, t 2 ) HOMO orthogonal (-4.42 ev, t 2 ) HOMO orthogonal (-4.42 ev, t 2 ) Figure S3a (continued). Molecular orbitals participating in intense singlet electronic transitions of W(CNPh) 6 in the orthogonal structure, T d symmetry. TDDFT PBE0/PCM-pentane. Symmetry representations and calculated orbital energies in parentheses. S12

state 16, 376 nm (3.30 ev) state 17, 376 nm (3.30 ev) state 18, 376 nm (3.30 ev) state 52, 250 nm (4.96 ev) state 53, 250 nm (4.96 ev) state 54, 250 nm (4.96 ev) Figure S3b. Electron density changes accompanying intense singlet electronic transitions of W(CNPh) 6 in the orthogonal structure, T d symmetry. Purple and blue colors indicate increasing and diminishing electron density, respectively. TDDFT PBE0/PCM-pentane. Transitions in nm (ev). W(CNXy) 6 orthogonal conformation Table S3. Intense singlet transitions of W(CNXy) 6 in the orthogonal conformation. Orbital contributions >10 % shown, TDDFT PBE0/PCM-THF. State Orbital Contribution (%) Energy / cm -1 Wavelength / nm Oscillator Strength 16 HOMO-2 LUMO+2 (31) 26987 371 1.1749 HOMO LUMO+3 (24) HOMO-2 LUMO+5 (14) HOMO LUMO+1 (12) 17 HOMO-1 LUMO+5 (46) 27079 369 1.0121 HOMO LUMO+4 (16) HOMO LUMO (12) 18 HOMO-2 LUMO+4 (38) HOMO-1 LUMO+3 (22) 27456 364 1.2402 S13

LUMO+5 (-0.55 ev) LUMO+4 (-0.62 ev) LUMO+3 (-0.65 ev) LUMO+2 (-0.69 ev) LUMO+1 (-0.98 ev) LUMO (-0.99 ev) HOMO (-4.50 ev) HOMO-1 (-4.56 ev) HOMO-2 (-4.57 ev) Figure S4a. Molecular orbitals participating in intense singlet electronic transitions of W(CNXy) 6 in the orthogonal conformation. Calculated orbital energies in parentheses. TDDFT PBE0/PCM-THF. S14

State 16, 371 nm (3.35 ev) State 17, 369 nm (3.36 ev) State 18, 364 nm (3.40 ev) Figure S4b. Electron density changes accompanying intense singlet electronic transitions of the orthogonal conformer of W(CNXy) 6. Purple and blue colors indicate increasing and diminishing electron density, respectively. TDDFT PBE0/PCM-THF. Transitions in nm (ev). S15

W(CNdipp) 6 coplanar conformation Table S4. Intense singlet transitions of W(CNdipp) 6 in the coplanar conformation. Orbital contributions >10 % shown, TDDFT PBE0/PCM-THF. State Orbital Contribution (%) Energy / cm -1 Wavelength / nm Oscillator Strength 1 HOMO LUMO+1 (44) 20458 489 0.0076 HOMO-2 LUMO (16) HOMO-1 LUMO (12) 2 HOMO LUMO+2 (44) 20504 488 0.0073 HOMO-2 LUMO+1 (12) HOMO-1 LUMO+2 (11) 3 HOMO LUMO (47) 20801 481 0.0083 HOMO-1 LUMO+2 (23) HOMO-2 LUMO+1 (19) 4 HOMO-1 LUMO (51) 21307 469 0.0475 HOMO LUMO+1 (30) 5 HOMO-2 LUMO (40) 21344 469 0.0516 HOMO LUMO+2 (30) HOMO-2 LUMO+1 (10) 6 HOMO-2 LUMO+2 (47) 21563 464 0.0275 HOMO-1 LUMO+1 (43) 7 HOMO LUMO (42) 22230 450 0.3558 HOMO-1 LUMO+2 (25) HOMO-2 LUMO+1 (24) 8 HOMO-2 LUMO (22) 23085 433 0.6190 HOMO-1 LUMO+2 (22) HOMO-2 LUMO+1 (18) HOMO LUMO+2 (11) 9 HOMO-1 LUMO+1 (29) 23110 433 0.6228 HOMO-2 LUMO+2 (25) HOMO-1 LUMO (14) HOMO LUMO+1 (10) 43 HOMO-2 LUMO+13 (23) 33442 299 0.8625 HOMO-1 LUMO+12 (20) HOMO LUMO+14 (20) HOMO-2 LUMO+12 (11) 44 HOMO-2 LUMO+14 (33) 33801 296 1.2752 HOMO-2 LUMO+12 (15) HOMO-1 LUMO+13 (14) HOMO LUMO+13 (13) 45 HOMO-1 LUMO+14 (43) HOMO-2 LUMO+13 (13) HOMO LUMO+12 (12) 33834 296 1.2521 S16

LUMO+14 (0.37 ev) LUMO+13 (0.30 ev) LUMO+12 (0.29 ev) LUMO+2 (-1.01 ev) LUMO+1 (-1.01 ev) LUMO (-1.03 ev) HOMO (-4.47 ev) HOMO-1 (-4.55 ev) HOMO-2 (-4.56 ev) Figure S5a. Molecular orbitals participating in intense singlet electronic transitions of W(CNdipp) 6 in the coplanar conformation. Calculated orbital energies in parentheses. TDDFT PBE0/PCM-THF. S17

State 7, 450 nm (2.76 ev) State 8, 433 nm (2.86 ev) State 9, 433 nm (2.87 ev) Figure S5b. Electron density changes accompanying intense singlet transitions of the coplanar conformer of W(CNdipp) 6. Purple and blue colors indicate increasing and diminishing electron density, respectively. TDDFT PBE0/PCM-THF. Transitions in nm (ev). W(CNdippPh OMe2 ) 6 coplanar conformation Table S5. Intense singlet transitions of W(CNdippPh OMe2 ) 6 in the coplanar conformation. Orbital contributions >10 % shown, TDDFT PBE0/PCM-THF. State Orbital Contribution (%) Energy / cm -1 Wavelength / nm Oscillator Strength 7 HOMO LUMO (36) 20750 482 0.6531 HOMO-1 LUMO+2 (25) HOMO-2 LUMO+1 (23) 8 HOMO-2 LUMO+2 (28) 21426 467 1.1438 HOMO-1 LUMO+1 (26) HOMO-2 LUMO (19) HOMO LUMO+2 (13) 9 HOMO-2 LUMO+1 (30) HOMO-1 LUMO+2 (28) HOMO-1 LUMO (18) HOMO LUMO+1 (11) 21436 467 1.1710 S18

LUMO+2 (-1.26 ev) LUMO+1 (-1.27 ev) LUMO (-1.28 ev) HOMO (-4.46 ev) HOMO-1 (-4.55 ev) HOMO-2 (-4.55 ev) Figure S6a. Molecular orbitals participating in intense singlet transitions of W(CNdippPh OMe2 ) 6 in the coplanar structure. TDDFT PBE0/PCM-THF. S19

State 7, 482 nm (2.57 ev) State 8, 467 nm (2.66 ev) State 9, 467 nm (2.66 ev) Figure S6b. Electron density changes accompanying intense singlet transitions of the coplanar conformer of W(CNdippPh OMe2 ) 6. Purple and blue colors indicate increasing and diminishing electron density, respectively. TDDFT PBE0/PCM-THF. Transitions in nm (ev). 2.0 300000 CNPh coplanar, C i 250000 1.5 Intensity 200000 150000 100000 50000 1.0 0.5 Oscillator Strength 0 0.0 200 250 300 350 400 450 500 550 600 Wavelength (nm) Figure S7a. TDDFT-calculated spectrum of W(CNPh) 6 in the coplanar structure, C i symmetry. PBE0/PCM-pentane. Experimental spectrum in 2-MeTHF glass is reported in Mann, K. R.; Gray, H. B.; Hammond, G. S. J. Am. Chem. Soc. 1977, 99, 306-307. S20

350000 2.0 300000 CNPh orthogonal, T d Intensity 250000 200000 150000 100000 1.5 1.0 0.5 Oscillator Strength 50000 0 0.0 200 250 300 350 400 450 500 550 600 Wavelength (nm) Figure S7b. TDDFT-calculated spectrum of W(CNPh) 6 in the orthogonal structure, T d symmetry. PBE0/PCM-pentane. Experimental spectrum in 2-MeTHF glass is reported in Mann, K. R.; Gray, H. B.; Hammond, G. S. J. Am. Chem. Soc. 1977, 99, 306-307. Table S6a. Contribution of atomic groups to the three highest occupied and three lowest unoccupied molecular orbitals in the coplanar conformer of W(CNPh) 6, calculated in the C i symmetry group. W(CNPh) 6 C i symmetry % contribution of atomic groups Molecular orbital Orbital symmetry Orbital energy (ev) all CN all Ph W LUMO+2 a u -1.14 38 58 4 LUMO+1 a u -1.15 38 59 4 LUMO a u -1.16 38 58 4 HOMO a g -4.64 37 16 47 HOMO-1 a g -4.65 36 15 48 HOMO-2 a g -4.66 37 14 48 S21

Table S6b. Contribution of atomic groups to the three highest occupied and three lowest unoccupied molecular orbitals of W(CNdipp) 6 (coplanar conformer). W(CNdipp) 6 % contribution of atomic groups Molecular orbital Orbital energy (ev) all CN all dipp W LUMO+2-1.01 38 58 3 LUMO+1-1.01 39 59 3 LUMO -1.03 37 59 4 HOMO -4.47 34 19 47 HOMO-1-4.55 36 16 48 HOMO-2-4.56 35 17 48 Table S6c. Contribution of atomic groups to the three highest occupied and three lowest unoccupied molecular orbitals of W(CNdippPh OMe 2 ) 6 (coplanar conformer). W(CNdippPh OMe 2 ) 6 % contribution of atomic groups Molecular orbital Orbital energy (ev) all CN all dipp all Ph OMe 2 W LUMO+2-1.26 28 51 19 2 LUMO+1-1.26 27 51 18 2 LUMO -1.28 27 52 18 3 HOMO -4.46 33 18 3 45 HOMO-1-4.54 35 16 2 46 HOMO-2-4.55 34 17 2 46 S22

Table S7. Lowest singlet-triplet transitions of W(CNdipp) 6 (coplanar structure). Orbital contributions >10 % shown, TDDFT PBE0/PCM-THF. Orbital Contribution (%) Energy / cm -1 Wavelength / nm HOMO LUMO+1 (41) 18085 553 HOMO-2 LUMO (19) HOMO-2 LUMO+1 (11) HOMO-1 LUMO (10) HOMO LUMO+2 (44) 18095 553 HOMO-1 LUMO+1 (18) HOMO-2 LUMO+2 (10) HOMO-1 LUMO (10) HOMO LUMO (49) 18132 552 HOMO-1 LUMO+2 (18) HOMO-2 LUMO+1 (11) HOMO-2 LUMO+1 (34) 19803 505 HOMO-1 LUMO+2 (30) HOMO LUMO (19) HOMO-2 LUMO (33) 20132 497 HOMO LUMO+1 (27) HOMO-1 LUMO+2 (10) HOMO LUMO+2 (26) 20168 496 HOMO-1 LUMO (21) HOMO-1 LUMO+2 (13) HOMO-2 LUMO+1 (12) HOMO-1 LUMO+1 (10) HOMO-1 LUMO+1 (43) 21124 473 HOMO-1 LUMO (25) HOMO LUMO (15) HOMO-1 LUMO (17) 21150 473 HOMO LUMO+1 (16) HOMO-2 LUMO+1 (15) HOMO-1 LUMO+2 (14) HOMO-2 LUMO+2 (12) HOMO-2 LUMO+2 (45) HOMO LUMO+2 (16) HOMO-2 LUMO+1 (15) 21200 472 S23

Spin-orbit TDDFT calculation of coplanar (C i ) W(CNdipp) 6 Table 8a. Selected lowest spin-free electronic transitions of W(CNdipp) 6 (coplanar, C i ) calculated at the ground state geometry. TDDFT/PBE0/COSMO-THF. HOMO denotes the almost degenerated set of HOMO, HOMO-1 and HOMO-2; LUMO the almost degenerated set of LUMO, LUMO+1 and LUMO+2; LUMO the almost degenerated set of LUMO+12, LUMO+13 and LUMO+14. State Main components (%) Energy/ Wavelength/ Oscillator ev nm strength a 3 A u mixed HOMO LUMO (a g a u ) 2.30 539 - b 3 A u mixed HOMO LUMO (a g a u ) 2.30 539 - c 3 A u mixed HOMO LUMO (a g a u ) 2.30 539 d 3 A u mixed HOMO LUMO (a g a u ) 2.48 500 - e 3 A u mixed HOMO LUMO (a g a u ) 2.50 496 f 3 A u mixed HOMO LUMO (a g a u ) 2.51 494 a 1 A u mixed HOMO LUMO (a g a u ) 2.53 490 0.005 b 1 A u mixed HOMO LUMO (a g a u ) 2.54 488 0.005 c 1 A u mixed HOMO LUMO (a g a u ) 2.57 482 0.005 g 3 A u mixed HOMO LUMO (a g a u ) 2.63 471 - h 3 A u mixed HOMO LUMO (a g a u ) 2.63 471 - i 3 A u mixed HOMO LUMO (a g a u ) 2.64 470 - d 1 A u mixed HOMO LUMO (a g a u ) 2.64 470 0.022 e 1 A u mixed HOMO LUMO (a g a u ) 2.65 468 0.023 f 1 A u mixed HOMO LUMO (a g a u ) 2.67 464 0.005 g 1 A u mixed HOMO LUMO (a g a u ) 2.78 446 0.308 h 1 A u mixed HOMO LUMO (a g a u ) 2.89 429 0.510 i 1 A u mixed HOMO LUMO (a g a u ) 2.90 427 0.511 j 1 A u mixed HOMO LUMO (a g a u ) 4.27 290 0.704 k 1 A u mixed HOMO LUMO (a g a u ) 4.34 286 1.037 l 1 A u mixed HOMO LUMO (a g a u ) 4.34 286 1.018 S24

Table 8b. The selected lowest spin-orbit electronic transitions of W(CNdipp) 6 (coplanar, C i ) calculated at the ground state geometry. E is the energy difference from the lowest SO excited state. SO states are expressed as linear combinations of spin-free states listed in Table 8a. SO- TDDFT/PBE0/COSMO-THF. SO state Contributions of spin free states Energy (cm -1 ) E (cm -1 ) Oscillator Strength aa u a 3 A u (71%) + c 3 A u (14%) + b 3 A u (7%) + d 3 A u (3%) 18372 0 0.000 ba u a 3 A u (62%) + b 3 A u (25%) + c 3 A u (6%) + d 1 A u (1.2%) 18379 7 0.001 ca u a 3 A u (47%) + b 3 A u (24%) + c 3 A u (22%) + a 1 A u (0.8%) 18388 16 0.000 da u b 3 A u (54%) + a 3 A u (37%) + c 1 A u (2%) + a 1 A u (1.6%) 18402 30 0.000 ea u b 3 A u (59%) + a 3 A u (31%) + d 3 A u (3%) + f 1 A u (1.0%) + a 1 A u (0.5%) + c 1 A u (0.4%) 18404 32 0.000 fa u b 3 A u (79%) + a 3 A u (8%) + c 3 A u (6%) + b 1 A u (1.9%) + e 1 A u (0.4%) + d 1 A u (0.3%) 18414 42 0.000 ga u c 3 A u (81%) + a 3 A u (8%) + b 3 A u (4%) + a 1 A u (0.8%) + e 1 A u (0.5%) 18448 76 0.000 ha u c 3 A u (82%) + b 3 A u (9%) + a 3 A u (5%) + b 1 A u (1.5%) + d 1 A u (0.4%) 18453 81 0.000 ia u c 3 A u (63%) + b 3 A u (19%) + a 3 A u (11%) + c 1 A u (2.0%) + f 1 A u (0.4%) 18468 96 0.000 ja u d 3 A u (69%) + e 3 A u (15%) + f 3 A u (8%) + a 1 A u (3.2%) + g 1 A u (1.0%) + d 1 A u (0.7%) + h 1 A u (0.5%) 19863 1491 0.006 ka u d 3 A u (56%) + f 3 A u (18%) + e 3 A u (10%) + b 1 A u (1.5%) + g 1 A u (1.2%) + d 1 A u (1.1%) 19917 1545 0.006 la u d 3 A u (60%) + e 3 A u (11%) + f 3 A u (11%) + b 1 A u (3.9%) + e 1 A u (1.3%) + i 1 A u (0.7%) 19923 1551 0.005 ma u g 1 A u (93.9%) 22547 4175 0.290 na u h 1 A u (97.5%) 23411 5039 0.498 oa u i 1 A u (97.5%) 23440 5068 0.499 pa u j 1 A u (98.9%) 34468 16096 0.696 qa u k 1 A u (99.2%) 35018 16646 1.029 ra u l 1 A u (99.2%) 35050 16678 1.010 S25

intensity / a.u. 300 350 400 450 500 550 600 Wavelength / nm intensity / a.u. 300 350 400 450 500 550 600 Wavelength / nm Figure S8. Comparison of simulated spin-free (black) and spin-orbit (red) simulated UV-vis absorption spectra of W(CNdipp) 6 in THF (coplanar, C i ). Top: fwhm 1000 cm 1, Bottom fwhm 3000 cm 1. TDDFT and SO-TDDFT/COSMO-THF. S26

Experimental and simulated TRIR spectra. Visualization of ν(c N) vibrations. 0.002 0.000 Absorbance -0.002-0.004-0.006-0.008-0.010 1850 1900 1950 2000 2050 2100 1800 1900 2000 2100 Wavenumber / cm -1 Figure S9. Nanosecond TRIR spectra of W(CNdipp) 6 in pentane measured at selected time delays in the 2-1000 ns range after 0.8 ns, 355 nm laser-pulse excitation. Spectra evolve in the direction of the arrows; Inset: detail of the transient bands, blue and red bars show calculated ground- and excited-state wavenumbers (Table S9). The main excited-state band is upshifted by 59 cm 1 relative to the ground-state band. 0.01 Absorbance 0.00-0.01-0.02-0.03-0.04 1850 1900 1950 2000 2050 Wavenumber / cm-1 Figure S10. Femto-picosecond TRIR spectra of W(CNdippPh OMe2 ) 6 measured in THF after 400 nm, 50 fs excitation, evolving in the direction of the arrows. Spectra were measured at 0 fs and then at every 50 fs until 750 fs. S27

Table S9a. DFT-calculated ν(c N) energies and intensities of the optimized ground state and lowest triplet state of W(CNXy) 6 (orthogonal). Wavenumbers scaled by a factor of 0.940. Ground State UKS Triplet Energy (cm -1 ) Intensity (km.mol -1 ) Energy (cm -1 ) Intensity (km.mol -1 ) 1950 13291 1904 11333 1957 13038 1919 520 1958 13025 1980 9 1985 50 1992 9200 1987 0.6 2004 8156 2104 0.3 2088 1 Table S9b. DFT-calculated ν(c N) energies and intensities of the optimized ground state and lowest triplet state of W(CNdipp) 6 (coplanar). Wavenumbers scaled by a factor of 0.940. Ground State UKS Triplet Energy (cm -1 ) Intensity (km.mol -1 ) Energy (cm -1 ) Intensity (km.mol -1 ) 1951 13697 1909 10885 1954 13515 1920 0.004 1956 11238 1985 9663 1982 0.0001 1988 0.007 1984 0.0001 2017 7603 2102 0.0000 2089 0.0000 Table S9c. DFT-calculated ν(c N) energies and intensities of the optimized ground state and lowest triplet state of W(CNdippPh OMe2 ) 6 (coplanar). Wavenumbers scaled by a factor of 0.940. Ground State UKS Triplet Energy (cm -1 ) Intensity (km.mol -1 ) Energy (cm -1 ) Intensity (km.mol -1 ) 1950 17917 1919 1.00 1951 17603 1935 7692 1953 13523 1986 11386 1981 0.021 1993 0.13 1982 0.32 2016 9306 2099 0.0072 2088 0.01 S28

Ground state 1950 cm -1 1957 cm -1 1958 cm -1 UKS-calculated lowest triplet state 1904 cm -1 (downshifted) 1992 cm -1 (upshifted) 2004 cm -1 (upshifted) Figure S11a. Ground state (top) and lowest triplet excited state (bottom) IR-active vibrations of W(CNXy) 6. Methyl substituents are represented by spheres. The orange color denotes the XyNC-W-CNXy moiety with the highest excited-state spin-density localization. S29

Ground state 1951 cm -1 1954 cm -1 1956 cm -1 UKS-calculated lowest triplet state 1909 cm -1 (downshifted) 1985 cm -1 (upshifted) 2017 cm -1 (upshifted) Figure S11b. Ground state (top) and lowest triplet excited state (bottom) IR-active vibrations of W(CNdipp) 6. i Pr substituents are represented by spheres. The orange color denotes the dippnc- W-CNdipp moiety with the highest excited-state spin-density localization. S30

Ground state 1950 cm -1 1951 cm -1 1953 cm -1 UKS-calculated lowest triplet state 1935 cm -1 (downshifted) 1986 cm -1 (upshifted) 2016 cm -1 (upshifted) Figure S11c. Ground state (top) and lowest triplet excited state (bottom) IR-active vibrations of W(CNdippPh OMe2 ) 6. i Pr substituents are represented by spheres. The orange color denotes the dippph OMe 2 NC-W-CNdippPh OMe 2 moiety with the highest excited-state spin-density localization. S31

Intensity, Absorbance CNXy calculated GS CNXy calculated UKS CNXy measured FTIR 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure S12a. Comparison of DFT-calculated ground state (blue) and lowest triplet excited state (red) IR spectra and experimental ground-state FTIR (black) spectrum of W(CNXy) 6. PBE0/PCM-THF, FTIR measured in THF. Intensity, Absorbance CNXy calculated CNXy measured 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure S12b. Comparison of DFT-calculated differential spectrum (lowest triplet state ground state, red) and experimental 1.5 ns TRIR (black) spectrum of W(CNXy) 6. PBE0/PCM-THF. TRIR measured in THF. S32

CNdipp calculated GS CNdipp calculated UKS CNdipp measured FTIR Intensity, Absorbance 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure 13a. Comparison of DFT-calculated ground state (blue) and lowest triplet excited state (red) IR spectra and experimental ground-state FTIR (black) spectrum of W(CNdipp) 6. PBE0/PCM-THF. FTIR measured in THF. Intensity, Absorbance CNdipp calculated CNdipp measured 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure S13b. Comparison of DFT-calculated differential spectrum (lowest triplet state ground state, red) and experimental TRIR (black) spectrum of W(CNdipp) 6. PBE0/PCM-THF, TRIR measured in THF. S33

Intensity, Absorbance CNdippPh OMe2 calc. GS CNdippPh OMe2 calc. UKS CNdippPh OMe2 measured 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure S14a. Comparison of DFT-calculated ground state (blue) and lowest triplet excited state (red) IR spectra and experimental ground-state FTIR (black) spectrum of W(CNdippPh OMe 2 ) 6. PBE0/PCM-THF. FTIR measured in THF. CNdippPh OMe2 calculated CNdippPh OMe2 measured Intensity, Absorbance 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 Wavenumber (cm -1 ) Figure S14b. Comparison of the DFT-calculated differential spectrum (lowest triplet state ground state, red) and experimental TRIR (black) spectrum of W(CNdippPh OMe2 ) 6. PBE0/PCM- THF, TRIR measured in THF. S34

Spin density distribution in the T1 state of W(CNXy) 6 and W(CNdipp) 6 W(CNXy) 6 conformation coplanar in one axis W(CNXy) 6 - conformation coplanar in three axes E = -2482.982359 a.u.; G = -2482.163156 a.u. E = -2482.9825555 a.u.; G = -2482.163645 a.u. Figure S15a. Spin density distribution in the optimized structures of the lowest triplet excited state of W(CNXy) 6. DFT-UKS, PBE0/PCM-THF. W(CNdipp) 6 - coplanar conformation Figure S15b. Spin density distribution in the optimized structure of the lowest triplet excited state of W(CNdipp) 6. DFT-UKS, PBE0/PCM-THF. S35

Spin-orbit electronic transitions of coplanar (C i ) W(CNdipp) 6 at optimized T1 geometry Table 10a. Spin-free electronic transitions of W(CNdipp) 6 (coplanar, C i ) calculated at the a 3 A u UKS-optimized geometry. Transition energies in ev. TDDFT/PBE0/COSMO-THF. State Main components (%) Calculated Oscillator transitions strength a 3 A u mixed HOMO LUMO (a g a u ) 2.05 - b 3 A u, c 3 A u mixed HOMO LUMO (a g a u ) 2.26, 2.27 - d 3 A u mixed HOMO LUMO (a g a u ) 2.34 - a 1 A u mixed HOMO LUMO (a g a u ) 2.35 0.026 e 3 A u, f 3 A u mixed HOMO LUMO (a g a u ) 2.43, 2.46 - b 1 A u mixed HOMO LUMO (a g a u ) 2.46 0.009 g 3 A u mixed HOMO LUMO (a g a u ) 2.47 - c 1 A u mixed HOMO LUMO (a g a u ) 2.48 0.007 d 1 A u mixed HOMO LUMO (a g a u ) 2.50 0.019 e 1 A u mixed HOMO LUMO (a g a u ) 2.59 0.084 h 3 A u, i 3 A u mixed HOMO LUMO (a g a u ) 2.59, 2.62 - f 1 A u mixed HOMO LUMO (a g a u ) 2.62 0.159 g 1 A u mixed HOMO LUMO (a g a u ) 2.68 0.164 h 1 A u mixed HOMO LUMO (a g a u ) 2.82 0.460 i 1 A u mixed HOMO LUMO (a g a u ) 2.89 0.508 S36

Table 10b. Spin-orbit electronic transitions of W(CNdipp) 6 (coplanar, C i ) calculated at the 3 A u UKS-optimized geometry. E is the energy difference from the lowest SO excited state. SO states are expressed as linear combinations of spin-free states listed in Table 10a. SO- TDDFT/PBE0/COSMO-THF. Contributions of spin free SO state states aa u a 3 A u (95%) + d 3 A u (2%) + e 3 A u (1.5%) ba u a 3 A u (95%) + d 3 A u (2%) + b 1 A u (1.1%) + a 1 A u (0.5%) ca u a 3 A u (96%) + e 3 A u (2%) + a 1 A u (1.8%) + b 1 A u (0.2%) da u c 3 A u (50%) + b 3 A u (42%) + e 1 A u (0.5%) + c 1 A u (0.2%) ea u b 3 A u (62%) + c 3 A u (35%) + e 1 A u (0.9%) + c 1 A u (0.7%) + d 1 A u (0.2%) + f 1 A u (0.2%) fa u b 3 A u (70%) + c 3 A u (22%) + f 3 A u (5%) + f 1 A u (0.5%) + e 1 A u (0.5%) + g 1 A u (0.4%) + a 1 A u (0.3%) ga u c 3 A u (56%) + b 3 A u (36%) + d 1 A u (1.1%) + c 1 A u (0.7%) + b 1 A u (0.2%) + e 1 A u (0.2%) + f 1 A u (0.2%) ha u c 3 A u (77%) + b 3 A u (15%) + c 1 A u (1.5%) + f 1 A u (0.2%) + g 1 A u (0.2%) + d 1 A u (0.2%) ia u c 3 A u (42%) + b 3 A u (52%) + d 1 A u (1.7%) + c 1 A u (0.7%) + b 1 A u (0.3%) + e 1 A u (0.2%) + f 1 A u (0.2%) + g 1 A u (0.2%) ja u d 3 A u (56%) + e 3 A u (13%) + a 1 A u (21.1%) + f 1 A u (0.9%) + g 1 A u (0.4%) + e 1 A u (0.2%) + h 1 A u (0.2%) ka u d 3 A u (85%) + e 3 A u (9%) + a 3 A u (3%) + b 1 A u (0.2%) la u d 3 A u (88%) + a 3 A u (4%) + b 1 A u (4.6%) + a 1 A u (2.0%) + c 1 A u (0.6%) + f 1 A u (0.3%) ma u a 1 A u (58.1%) + b 1 A u (1.1%) + f 1 A u (0.2%) + g 1 A u (0.2%) + d 3 A u (24%) + e 3 A u (12%) Energy (cm -1 ) E (cm -1 ) 16409 0 16414 5 16422 13 18085 1676 18093 1684 18105 1696 18122 1713 18133 1724 18145 1736 18556 2147 18843 2434 18859 2450 19048 2639 S37

Characterization of vertically excited T1 state of W(CNdipp) 6 and W(CNdippPh OMe 2 ) 6 Table S11. Differences of Mulliken charges between the vertically excited lowest triplet state and the ground state (TDDFT, left column) and between the optimized relaxed lowest triplet state T1 and the ground state (right column). PBE0/PCM-THF. W(CNdipp) 6 TDDFT triplet 1 Optimized lowest triplet W 0.393 0.207 1 CN 0.033-0.126 2 CN 0.008 0.021 3 CN -0.171 0.045 1 i PrPh 0.002-0.319 2 i PrPh -0.075 0.074 3 i PrPh -0.190 0.098 W(CNdippPh OMe 2 ) 6 TDDFT triplet 1 W 0.378 0.214 1 CN -0.017-0.095 2 CN 0.034 0.022 3 CN -0.090 0.043 1 i PrPh -0.055-0.263 2 i PrPh -0.046 0.068 3 i PrPh -0.152 0.089 1 PhOMe -0.009-0.126 2 PhOMe -0.008 0.021 3 PhOMe -0.034 0.026 Optimized lowest triplet S38

Figure S16. Electron density redistribution accompanying the lowest vertical triplet excitation of W(CNdipp) 6 (left) and W(CNdippPh OMe 2 ) 6 (right). Axes are oriented in the same was as in figures showing the relaxed lowest triplet T1 (Figures 5, S15b). DFT-UKS, PBE0/PCM-THF. S39

Electronic transitions of the W(CNPh) 6 model complex. Coplanar and orthogonal conformations of the W(CNPh) 6 model (Figures 1-bottom, S1a) can be calculated using C i and T d symmetries, respectively. The corresponding MO schemes are shown in Figures S17 and S18, respectively. C i coplanar conformation: The three a g HOMO orbitals (Figure S2a, S17) are predominantly π(w C)/π*(C N), while the set of three a u LUMOs is made up of π*(c N, Aryl) and π(n Aryl) ligand orbitals, localized along one or two molecular axes. HOMO LUMO transitions are symmetry-allowed and the three transitions that also are overlap-allowed give rise to the lowest intense band. Calculated electron-density differences (Figure S2b-top) show that the corresponding transitions are localized along one or two axes and combine ππ* excitation of the C N C(Ph) moiety with W Ph MLCT, and a small LLCT contribution. The LUMO set in coplanar (C i ) W(CNPh) 6 is followed in energy by several groups of ligand-localized orbitals, transitions to which are either symmetry- (a g ) or overlap- forbidden (Figure S17). The strong UV band around 300 nm is due to transitions from HOMO to highlying π*(c N) a u orbitals (containing ca. 12% W 6p) localized along one or two axes (Figures S2a-top, Table S1). These UV transitions are W CN MLCT in character, localized along one or two axes, with an LLCT contribution from the other pair(s) of CNPh ligands (Figure S2bbottom). Molecular orbitals and electronic transitions of the coplanar (i.e. predominant) conformation of W(CNdipp) 6 and W(CNdippPh OMe2 ) 6 are analogous, only some of the forbidden transitions gain more intensity because of the lack of symmetry, see the main text, Tables S4, S5 and Figures S5, S6. S40

Figure S17. Molecular orbitals and most intense transitions of W(CNPh) 6 in the coplanar conformation of the C i symmetry. Simulated absorption spectra are shown in Figure S7a. T d orthogonal conformation: The MO compositions (Figures S4a, S18) are virtually identical to those in the coplanar structure. The HOMO set is triply degenerate (t 2 ) of a combined π(w C)/π*(C N) character. Upon going from the coplanar to the orthogonal conformation, the a u and a g π*(c N, Aryl)/π(N C(Aryl)) sets of empty orbitals shift up and down in energy, becoming LUMO+1(t 2 ) and LUMO(t 1 ), respectively (Figure S3a, S18). HOMO LUMO(t 1 ) transitions are only weakly allowed. They do not result in a distinct feature in the simulated W(CNPh) 6 spectrum but are manifested by a shoulder observed and calculated around 450 nm for W(CNXy) 6, whose orthogonal structure is distorted and the T d symmetry broken (Figures 2, 3). The strong W(CNPh) 6 band calculated at 375 nm (and observed in the experimental spectrum of W(CNXy) 6 at 380 nm and as the "middle band"-shoulder in the W(CNdipp) 6 and W(CNdippPh OMe2 ) 6 spectra) arises from transitions that combine excitations from HOMO(t 2 ) to LUMO(t 1 ) and LUMO+1(t 2 ) (Table S2). The latter orbitals contain ~9% metal contribution. They are the S41

counterparts of the a u LUMO of the coplanar conformation. (In W(CNXy) 6, only ligands along two axes have coplanar orientation, the third being "intermediate", see Figure 1; the T d symmetry is broken and the three most intense transitions at 370 nm (calc) / 380 nm (exp) are localized along individual axes, Figures 5, S4b.) The corresponding transitions combine the ππ*(c N C(Ph)) and MLCT Ph excitations, as in the coplanar conformation (Figure S3b-top). The LUMO+1 (t 2 ) of W(CNPh) 6 are followed in energy by several sets of ligandlocalized orbitals, transitions into which are very weak. The π*(c N) orbitals localized along two molecular axes occur as t 1 and t 2 sets (Figure S18). Transitions from the HOMO set to the higher π*(c N) t 2 set of orbitals occur in the UV region, around 250 nm, giving rise to the second absorption band (Figure S7b). Calculated electron-density differences confirm the W CN MLCT character, combined with small LLCT contributions (Figure S3b-bottom). Figure S18. Molecular orbitals and most intense transitions of W(CNPh) 6 in the orthogonal conformation of the T d symmetry. Simulated absorption spectra are shown in Figure S7b. Molecular orbitals and electronic transitions of the orthogonal (i.e. predominant) conformation of W(CNXy) 6 (Figures 5, S4, Table S3) and of the orthogonal (minor) conformations of W(CNdipp) 6 and W(CNdippPh OMe2 ) 6 are analogous, only some of the forbidden transitions gain more intensity because of the lack of symmetry. S42

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