(a) and Ni(Pyipa) - Nature

a

b

3

1.2 T (cm mol K)

-1

2 1.5 1

0

1

0.9

0.5 0

1.1

3.

3.

-1

T (cm mol K)

2.5

1.3

50

100

150 T (K)

200

250

300

0.8 0

50

100

150

200

250

T (K)

Supplementary Figure 1. χMT = f(T) curve for Co(Pyipa)2 (a) and Ni(Pyipa)2 (b). Solid line corresponds to the best fit.

1

a

b 2

M (μ0B)

M (μ0B)

2

1

1 6K 4K 2K 0

0 0

15000 H /T(Oe/K)

30000

0

15000 H/T (Oe/K)

30000

Supplementary Figure 2. Reduced magnetization plots (M = f(µ0H/T) for Co(Pyipa)2 (a) and Ni(Pyipa)2 (b).

2

9000 8000 7000 6000 5000

Envelope

4000

counts/s

experimental N1s

3000 2000 1000

404

402

400

398

396

394

Binding Energy (eV)

P2p3/2 P2p1/2 Envelope

3000 2500 2000 1500 1000

conts/s

experimental P2p

500 0

136

134

132

130

128

Binding Energy (eV) Supplementary Figure 3. XPS of Co((Pyipa))2; at the Co2p edge (top), the N1s edge (middle) and the P2p edge (bottom).

3

9000

Envelope

8000 7000 6000 5000 4000

counts/s

experimental N1s

3000 2000 1000

404

402 400 398 396 Binding Energy (eV)

394

experimental P2p

2500

P2p3/2

2000

P2p1/2

1500

Envelope

1000

counts/s

3000

500 0

136

134 132 130 Binding Energy (eV)

128

Supplementary Figure 4. XPS of Ni((Pyipa))2; at the Ni2p edge (top), the N1s edge (middle) and the P2p edge (bottom)

4

Supplementary Figure 5. XPS of Co((Pyipa))2; at the Co2p edge (top), the N1s edge (middle) and the P2p edge (bottom) of fresh (red) and 2-day incubated (blue) samples.

5

Supplementary Figure 6. XPS of Ni((Pyipa))2; at the Ni2p edge (top), the N1s edge (middle) and the P2p edge (bottom) of fresh (red) and 2-day incubated (blue) samples.

6

Supplementary Figure 7. AFM images. a, Fe3O4; b, surface profile of Fe3O4 as indicated by the blue line; c monolayer of Ni(Pyipa)2; d, surface profile of Ni(Pyipa)2 as indicated by the blue line.

7

1600

800

Fe2O2PO3H

-

800

400

0 223

400

200

223.5

224 m/z

224.5

225

0 209

Co(C5H4NCHN)2+

300

700

210

210.2 210.4 210.6

CoC5H4NCHNH2+

600

Intensity (counts)

250 200 150

500 400 300

100

200

50

100

0 268.4

209.2 209.4 209.6 209.8 m/z

800

350

Intensity (counts)

CoFeOPO3-

600

Intensity (counts)

Intensity (counts)

1200

268.6

268.8

269 m/z

269.2

269.4

0 165.5

165.7

165.9

166.1

166.3

166.5

m/z

Supplementary Figure 8. Representative Tof-SIMS spectra of Co(Pyipa)2 on epitaxial F3O4(111) for the negative (top) and positive (bottom) modes.

8

4000

900

NiNC2H4PO3-

3500

NiFeO2PO3H-

Intensity (counts)

Intensity (counts)

3000 2500 2000 1500

600

300

1000 500 0 178.2

178.4

178.6

178.8 179 m/z

179.2

179.4

0 225

1200

226.5

227

Ni(C5H4NCHN)2+

120

900 600 300 0 163.5

226 m/z

150

NiC5H4NCHN+ Intensity (counts)

Intensity (counts)

1500

225.5



90 60 30

163.7

163.9 m/z

164.1

164.3

0 267.5

267.7

267.9

268.1

268.3

268.5

m/z

Supplementary Figure 9. Representative Tof-SIMS spectra of Ni(Pyipa)2 on epitaxial F3O4(111) for the negative (top) and positive (bottom) modes.

9

Supplementary Figure 10. Tof-SIMS spectra of Ni(Pyipa)2 (a and c) and Co(Pyipa)2 (b and d) on epitaxial F3O4(111) for the negative (top) and positive (bottom) modes.

10

3.5

XAS (a.u.)

3.0

L3

2.5 2.0

σ+ σ-

1.5

θ = 45o L2

XMCD (a.u.)

1.0 0.5 0.0 -0.5 700

720 710 Energy (e.V.)

730

740

Supplementary Figure 11. XAS/XMCD of bare epitaxial Fe3O4(111). Iron L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 45° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field.

11

a 3.0 L3

XAS (a.u.)

2.5

σ+ σ-

2.0 1.5

θ = 45o

1.0

L2

XMCD (a.u.)

0.5 0.0 -0.5 700

710 720 Energy (e.V.)

b

730

740

XAS (a.u.)

3.0 L3

2.5 2.0

σ+ σ-

1.5

θ = 45o

XMCD (a.u.)

1.0

L2

0.5 0.0 -0.5 700

720 710 Energy (e.V.)

730

740

Supplementary Figure 12. XAS/XMCD of epitaxial F3O4(111). Iron L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 45° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field. a Co(Pyipa)2 monolayer was deposited onto the Fe3O4; b Ni(Pyipa)2 monolayer was deposited onto the Fe3O4.

12

a

4.0 L2

XMCD (a.u.)

XAS (a.u.)

L3

σ+ σ-

3.8

θ = 0o

0.0

CoPyipa2

-0.1 770

780 790 Energy (e.V.)

800

b L2

4.0

XMCD (a.u.)

XAS (a.u.)

L3 σ+ σθ = 0o

3.8

0.0

-0.1 NiPyipa2 840

850

860 Energy (e.V.)

870

Supplementary Figure 13. XAS/XMCD of Co(Pyipa)2 and Ni(Pyipa)2. a, Cobalt L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 0° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field. b, Nickel L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 0° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field. Note that the non-flat background for the XMCD for Co comes from the XMCD signal of the Fe L2,3 edges and can be removed by recording the XMCD on a pure Fe3O4 surface in the energy range of the Co L2,3 edges. This step is what has been done to obtain Figures S11c and S13.

13

a 0.002

XMCD (a.u.)

0.0 -0.002 -0.004 -0.006 Fe Co

-0.008 θ=0

CoPyipa2

-0.010 770 b

o

780 790 Energy (e.V.)

800

0.005

XMCD (a.u.)

0.0

-0.005

-0.010 Fe Co θ = 45o CoPyipa2

-0.015 770

780 790 Energy (e.V.)

800

c 0.002

XMCD (a.u.)

0.0 -0.002 -0.004 -0.006 -0.008 -0.010 Co-Fe background θ = 45o

-0.012 -0.014 770

780

790

CoPyipa2 800

Energy (e.V.)

Supplementary Figure 14. XMCD of Co(Pyipa)2 and Fe3O4 at the Cobalt L2,3 edge spectra recorded at T = 2 K, θ = 0° (a), and θ = 45° (b, and c) using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field. The black line in a and b indicated the background signal arising from the Fe3O4 substrate. The background was subtracted to produce Figure S11c.

14

a

XAS (a.u.)

5 L3

4 3

L2

2

XMCD (a.u.)

1

b

0 -1 -2 780 Energy (e.V.) 5.4 4.5

XAS (a.u.)

800

L3

3.6 L2

2.7 1.8

XMCD (a.u.)

0.9 0.0 -0.9 -1.8 860 Energy (e.V.)

880

Supplementary Figure 15. XAS/XMCD of a bulk, dropped casted, layer of Co(Pyipa)2 and Ni(Pyipa)2. a, Cobalt L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 45° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field. b, Nickel L2,3 edge XAS and XMCD spectra recorded at T = 2 K, and θ = 45° using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field.

15

a

Fe - Surface

0.2

b

CoPyipa2

Fe - Surface

0.2

o

NiPyipa2

o

θ=0

θ=0 0.008

0.004 0.1

0.004 0.0

0.0

-0.004 -0.1

XMCD (a.u.)

XMCD (a.u.)

0.1

0.002 0.0

0.0

-0.002 -0.1 -0.004

-0.008 -0.2

-0.2 -8

-6

-4

-2

0 μ0H(T)

2

4

6

8

-8

-6

-4

-2

0 μ0H(T)

2

4

6

8

Supplementary Figure 16. Element-specific field dependence of the magnetization of the Co atoms of Co(Pyipa)2, the Ni atoms of Ni(Pyipa)2 and of the ferrimagnetic surface (Fe). Hysteresis curves in the ± 6 T field range (multiplied by -1) of the Co atoms (blue), Ni atoms (green) and Fe atoms (grey) obtained at the L2,3 edges XMCD maxima at T = 2 K, and θ = 0°. (Monochromatized X-rays are set at the energy of the maximum absolute value of the XMCD signal (i.e. hν = 777.5 eV for Co, hν = 851 eV for Ni, and hν = 707 eV for Fe) then the external magnetic field is switched step by step from +6.5 T down to -6.5 T and back to +6.5 T. At each step the magnetic field is switched from left to right circular polarization to yield the element specific magnetization curves.

16

Supplementary Figure 17. XAS/XMCD of Co(Pyipa)2 deposited onto an Al2O3 terminated Fe3O4. Cobalt L2,3 edge XAS and XMCD spectra recorded at T = 2 K, θ = 0° (red line), and θ = 45° (black line) using left (σ+) and right hand (σ-) circularly polarized light in 6.5 T field.

17

45

XMCD (a.u.)

0.01

0.0

-0.01

-8

-6

-4

-2

0 μ0H(T)

2

4

6

8

Supplementary Figure 18. Element-specific field dependence of the magnetization of the Co atoms of Co(Pyipa)2 on the ferrimagnetic surface (Fe) separated by an Al2O3 insulating layer. Hysteresis curves (multiplied by -1) of the Co atoms obtained at the L2,3 edges XMCD maxima at T = 2 K, θ = 45°. (Monochromatized X-rays are set at the energy of the maximum absolute value of the XMCD signal (i.e. hν = 777.5 eV for Co) then the external magnetic field is switched step by step from +6.5 T down to -6.5 T and back to +6.5 T. At each step the magnetic field is switched from left to right circular polarization to yield the element specific magnetization curves.

18

Solution 1

Solution 2

MII

MII

FeA

FeB

FeC

MII = NiII;

E1 = 5J1 + 5 J2 + 25 J3 2 2

MII = CoII; E1 = 15 J1 + 15 J2 + 25 J3 2 4 2

FeA MII = NiII;

FeB

E2 = - 5J1 - 5 J2 + 25 J3 2 2

MII = CoII; E2 = - 15 J1 - 15 J2 + 25 J3 2 4 2

Solution 3

Solution 4

MII

MII

FeA

FeB

MII = NiII;

E3 = 5J1 - 5 J2 - 25 J3 2 2

FeC

MII = CoII; E3 = 15 J1 - 15 J2 - 25 J3 2 4 2

FeC

FeA

FeB

MII = NiII;

E4 = - 5J1 +5 J2 - 25 J3 2 2

FeC

MII = CoII; E4 = - 15 J1 + 15 J2 - 25 J3 2 4 2

Supplementary Figure 19. DFT broken symmetry solutions and their analytical energies as functions of the magnetic exchange coupling parameters J1, J2 and J3. Solutions 1 and 2 correspond to the situation when the surface FeIII ions are ferromagnetically coupled and the MII ions are ferro- and antiferromagnetically coupled to them, respectively; solutions 3 and 4 correspond to the situation when the surface FeIII ions are antiferromagnetically coupled and the MII ions are ferro- and antiferromagnetically coupled to them, respectively. Although solutions 1 and 2 (ferromagnetic coupling between Fe ions of the surface) are lower in energy than solutions 3 and 4 (antiferromagnetic coupling between Fe ions of the surface), the coupling between the complexes and the substrate is independent from the nature of the coupling within the substrate because equations E1 – E2 and E3 – E4 are a function of J1 and J2 only

19

Supplementary Table 1. Crystallographic data for complex Co(Pyipa)2 and Ni(Pyipa)2. Compound Formula fw Crystal size / mm3 Crystal system Space group a, Å b, Å c, Å α, ° β, ° γ, ° Cell volume, Å3 Z T, K F000 µ / mm–1

θ range / ° Refl. collected Refl. unique Rint GOF Refl. obs. (I>2σ(I)) Parameters wR2 (all data) R value (I>2σ(I)) Largest diff. peak and hole (e-.Å-3)

Co(Pyipa)2 C16CoH20N4O6P2, 485.23 0.11 x 0.07 x 0.04 monoclinic C 2/c 26.3780(13) 9.0590(4) 16.5517(7) 90 102.2860(10) 90 3864.6(3) 8 100(1) 1992 1.097 1.58 – 30.51 29 403 5 844 0.0616 1.031 3 926 676 0.1299 0.0524 -0.967; 1.996

Ni(Pyipa)2 C16H20N4NiO6P2, 484.99 0.24 x 0.06 x 0.01 monoclinic C 2/c 25.9071(8) 9.1614(3) 16.4713(4) 90 100.9560(10) 90 3838.1(2) 8 100(1) 2000 1.221 1.60 – 30.51 29 473 5 540 0.0340 1.036 4 321 266 0.1401 0.0540 -1.268 ; 1.642

20

Supplementary Table 2. XPS peak ratio of the fresh and 2-day incubated samples. Ni samples fresh 2-days incubation

%Ni 14 14

%N 57 54

%P 29 32

Co samples fresh 2-days incubation

%Co 14 14

%N 54 52

%P 32 34

theoretical

1

4

2

21

Supplementary Table 3. Ions detected by Tof-SIMs for Co(Pyipa)2. Important peaks are highlighted in yellow. Co system

detected mass (m/z)

theoretical mass (m/z)

Co

58.93

58.933

CoPO3-

137.90

137.892

150.89

150.888

138.90

138.900

CoPO3FeO

209.83

209.822

Co(PO3)2FeO-

288.78

288.780

304.76

304.775

180,93 (sh)

180.934

N(CH2)2CoPO3

179,90 (sh)

179.926

Co+

58,93 (sh)

58.933

59.94

59.941

163,98 (sh)

163.978

164,99 (weak)

164.986

165.99

165.994

CoC5H4NCHNCH2

178 (weak)

177.994

CoC5H4NCHNCH3+

179 (weak)

179.002

CoC5H4NCHNC2H5+

193,02 (weak)

193.017

268.99

269.024

-

FeOPO3- -

CoPO3H -

Co(PO3)2FeO2- -

N(CH2)2CoPO3H -

+

CoH +

CoC5H4NCHN CoC5H4NCHNH+ +

CoC5H4NCHNH2 +

+

Co(C5H4NCHN)2

22

Supplementary Table 4. Ions detected by Tof-SIMs for Ni(Pyipa)2. Important peaks are highlighted in yellow. Ni system

detected mass (m/z)

theoretical mass (m/z)

57.94

57.935

58.94

58.943

NiFeOPO3H

209.83

209.831

-

NiFeO2PO3H

225.83

225.826

NiNC2H4PO3-

178.92

178.928

Ni+

57.94

57.935

58.94

58.943

113.87

113.870

129.87

129.865

162.99

162.981

163.99

163.988

NiC5H4NCHNCH2

176.99

176.996

NiC5H4NCHNCH3+

178.00

178.004

NiC5H4NCHNC2H5+

192.02

192.020

268.02

268.026

-

Ni NiH- -

+

NiH NiFe+ +

NiFeO NiC5H4NCHN+ +

NiC5H4NCHNH +

+

Ni(C5H4NCHN)2

23

Supplementary Table 5. Energies in a.u. of the four broken symmetry solutions calculated at the B3LYP level and depicted in Figure S16 for the Ni and Co complexes. M/E(a.u.) E1 E2 E3 E4 Ni -8187.1889526 -8187.1897272 -8187.1843507 -8187.1852164 Co -8061.646066 -8061.640701 -8061.639038 -8061.637022

24

Supplementary Discussion Ab initio calculations of the complexes. Ab initio calculations using the spin-orbit state interaction (SOSI) method, which treats the spin-orbit coupling (SOC)1,2 in the wave function theory (WFT)- based framework (implemented in the MOLCAS code3) permits the calculation of the ZFS parameters from first principles and the determination of the orientation of the main axes of the anisotropy tensor according to a well-established method, which is known to provide accurate results.4-10 The calculations, in very good agreement with experimental data, lead to DNi = -3.4 cm-1 and ENi = 0.7 cm-1, which corresponds to a weak separation between the ground MS = ±1 (not degenerate because E ≠ 0 for a non Kramers sub-levels) and the excited MS = 0 of 3.4 cm-1. The easy axis of magnetization was found to be perpendicular to the C2 symmetry axis of the molecule and makes an angle of 10° with the Namine-Ni-Namine direction. For Co(Pyipa)2, the calculation gives DCo = + 32.3 cm-1 and ECo = 4.1 cm-1, which corresponds to a large energy separation between the ground MS = ±1/2 and the excited MS = ± 3/2 sub-levels of 64.6 cm-1 (= 2|D|). XPS analysis. For the 2-day incubated samples, XPS fitting was performed using CasaXPS and IgorPro Wavemetrics. Photoemitted electron data spectra were decomposed using mixed Lorenzian and Gaussian (1/3;2/3) peaks for carbon, phosphorus and nitrogen. In the case of metal, e.g. cobalt and nickel, the peaks are composed of Gaussian and Lorenzian peaks. The energetic splitting for 2P 1/2 and 2P 3/2 for the phosphorus element was fixed at 0.7 eV, and the intensity set at 1/3 to 2/3 respectively. The atomic ratio was determined using the surface area of each peak for each element taking in consideration the Scofield factors. For the asdissolved samples, the areas of the peaks were measured after background correction of Shirley type,11 and then corrected using Scofield parameters (Ni-2p3/2 : 14.610 ; Co-2p3/2 : 12.620 ; N-1s : 1.80 et P-2p : 1.192). These procedures allow quantification of the relative percentages of the three elements M (Ni or Co), N and P for the four samples shown in supplementary Table 2. Density Functional Theory – Geometrical optimization of the complexes on the surface. In our calculations, we have considered an 8 ML slab of 4 x 4 unit cells of Fe3O4 oriented along the (111) direction and two k-points. We have set the molecule in three different initial configurations, namely a physisorbed configuration and two chemisorbed configurations, considering the anchoring of the molecule with two

25

equivalent or non-equivalent oxygen atoms of the surface. Then, we have proceeded to a DFT molecular dynamic simulation at room temperature to reproduce the experimental conditions of molecular deposition on the surface, followed by a structural optimization at 0K to determine the final configuration on the surface. This well-established procedure12 gives us a stable structure in the three configurations, as well as the molecular adsorption energies: Physisorption: -2.51 eV/molecule Chemisorption (non equivalent O): -8.14 eV/molecule Chemisorption (equivalent O): -10.11 eV/molecule The latter configuration seems to be the most stable, defining the molecular orientation observed in the experiments and more importantly the site where the molecules are linked to the Fe surface atoms. Sum rule calculations. For CoII (in a monolayer of Co(Pyipa)2) the number of hole was taken to be nh = 3, which results, at θ = 0°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 0.39 ± 0.03 µB and a spin magnetic moment MS = -g0µB⟨SZ⟩ = 1.19 ± 0.08 µB, corresponding to a total magnetic moment M = ML + MS = 1.58 ± 0.11 µB. For CoII (in a monolayer of Co(Pyipa)2) the number of hole was taken, again, to be nh = 3, which results, at θ = 45°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 0.44 ± 0.03 µB and a spin magnetic moment MS = -g0µB⟨SZ⟩ = 1.39 ± 0.08 µB, corresponding to a total magnetic moment M = ML + MS = 1.83 ± 0.11 µB. For NiII (in a monolayer of Ni(Pyipa)2) the number of hole was taken, again, to be nh = 2, which results, at θ = 45°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 0.35 ± 0.02 µB and a spin magnetic moment MS = g0µB⟨SZ⟩ = 1.39 ± 0.12 µB, corresponding to a total magnetic moment M = ML + MS = 1.74 ± 0.14 µB (the error bars are estimated to 10 % and stem mainly from the normalization procedure and the background subtraction). For NiII (in a monolayer of Ni(Pyipa)2) the number of hole was taken to be nh = 2, which results, at θ = 45°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 0.34 ± 0.02 µB and a spin magnetic moment MS = -g0µB⟨SZ⟩ = 1.50 ± 0.12 µB, corresponding to a total magnetic moment M = ML + MS = 1.84 ± 0.14 µB (the error bars are estimated to 10 % and stem mainly from the normalization procedure and the background subtraction). The values for both Co and Ni in a thick monolayer are comparable to those found in the monolayer at θ = 45°. For CoII (in a monolayer of Co(Pyipa)2 deposited onto Al2O3) the number of hole was taken to be nh = 3, which 26

results, at θ = 0°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 0.60 ± 0.03 µB and a spin magnetic moment MS = -g0µB⟨SZ⟩ = 1.52 ± 0.08 µB, corresponding to a total magnetic moment M = ML + MS = 2.12 ± 0.14 µB (the error bars are estimated to 10 % and stem mainly from the normalization procedure and the background subtraction). For CoII (in a monolayer of Co(Pyipa)2 deposited onto Al2O3) the number of hole was taken, again, to be nh = 3, which results, at θ = 45°, in an orbit magnetic moment ML = -µB⟨LZ⟩ = 1.45 ± 0.03 µB and a spin magnetic moment MS = -g0µB⟨SZ⟩ = 1.39 ± 0.08 µB, corresponding to a total magnetic moment M = ML + MS = 2.84 ± 0.14 µB (the error bars are estimated to 10 % and stem mainly from the normalization procedure and the background subtraction). Density Functional Theory – calculation of the magnetic coupling between the molecules the surface. These calculations have been performed using the B3LYP functional of the GAUSSIAN 09 package.13,14 First, the structure of each complex anchored to the surface was optimized. The effect of the surface onto the complex was modeled as follows: a piece containing three FeIII centers and the coordinated O atoms (see Figure 5) was cut into the structure that was predetermined by the molecular dynamics technique. The external oxo anions were protonated. It is important to note that even if the surface model is approximated, it is exactly the same for both complexes. The structure of the model surface was frozen and the structure of the whole complex was optimized for the Ms = 18/2 (Co(Pyipa)2) or Ms = 17/2 (Ni(Pyipa)2) solution corresponding to a ferromagnetic coupling between the three FeIII and the CoII or NiII metal centers. Very small spin contamination is observed: for Co(Pyipa)2, S = 9.03, compared to an expected value of S = 9.00 without spin contamination; for Ni(Pyipa)2, S = 8.52 vs an expected value of S = 8.50. For both complexes, the spin density of the central the FeIII centers are almost the same and independent of the Ms values. A substantial spin density delocalization toward the O atoms bound to the Fe centers is observed in both cases (as usual in metal oxides).15-19 The CoII and NiII ions have spin densities of 1.33 and 0.86, respectively. Unexpectedly, the coordination sphere of Ni(Pyipa)2 is slightly smaller than that of Co(Pyipa)2 as can be seen from the differences in bond distances: NiN distances are between 2.03Å and 2.15Å (average 2.10Å); Ni-O distances are 2.03Å and 2.08Å; Co-N distances are between 2.05Å and 2.19Å (average 2.14Å); Co-O distances are 2.09Å and 2.24Å. The distance

27

from the metal center to the closest FeIII ion is shorter by 0.5 Å for Ni (Pyipa)2 than that of Co (Pyipa)2 (4.27Å vs. 4.77Å). Second, the solution corresponding to a ferromagnetic coupling between the FeIII centers and an antiferromagnetic coupling with the CoII and NiII ions was computed (Ms = 12/2 and Ms = 13/2 for Co- and Ni(Pyipa)2, respectively). It should be noted that the extraction of the magnetic couplings J1 and J2 is independent of the nature of the surface couplings, since the equations E1-E2 and E3-E4 which have been used are functions of J1 and J2 only. As expected, the spin contamination of these solutions is larger but it remains small: S = 6.28 (exp. 6.00) for Co(Pyipa)2; S = 6.67 (exp. 6.50) for Ni(Pyipa)2. The spin densities on the iron centers are almost the same as in the high-spin solution (variations of less than 0.02); that of CoII and NiII are 1.40 and -0.87, respectively.

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