Tetracyanomanganate(II) and its salts of divalent first-row transition metal ions

Article abstract of DOI:10.1021/ic0012726

The first known paramagnetic, tetrahedral cyanide complex, [Mn^II(CN)₄]^2-, is formed by the photoinduced decomposition of [Mn^IV(CN)₆]^2- in nonaqueous solutions or by thermal decomposition in the solid state. In acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (λ_max = 25 700 cm^-1, ε = 3700 cm^-1 M^-1) causes the homolytic cleavage of cyanide radicals and reduction of Mn^IV. Free cyanide in dichloromethane leads to the isolation of polycyanide oligomers such as [C₁₂N₁₂]^2- and [C₄N₄]-, which was crystallographically characterized as the PPN⁺ salt C₄₀H₃₀N₅P₂: monoclinic space group = 12/a, a = 18.6314(2) A, b = 9.1926(1) A, c = 20.8006(1), β = 106.176(2)°, Z = 4]. In the solid state Mn^IV-CN bond homolysis is thermally activated above 122 °C, according to differential scanning calorimetry measurements, leading to the reductive elimination of cyanogen. The [Mn^II(CN)₄]^2- ion has a dynamic solution behavior, as evidenced by its concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and lability of [Mn^II(CN)₄]^2- in solution, its reaction with divalent first-row transition metal cations leads to the formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the metal ions and multiple structural and cyano-linkage isomers. α-Mn^II[Mn^II(CN)₄] has an interpenetrating sphalerite- or diamond-like network structure with a unit cell parameter of a = 6.123 A (P43m space group) while a β-phase of this material has a noninterpenetrating disordered lattice containing tetrahedral [Mn^II(CN)₄]^2-. Linkage isomerization or cyanide abstraction during formation results in α-Mn^II[Co^II(CN)₄] and Mn^II[Ni^II(CN)₄] lattice compounds, both containing square planar tetracyanometalate centers. α-Mn^II[Co^II(CN)₄] is irreversibly transformed to its β-phase in the solid state by heating to 135 °C, which causes a geometric isomerization of [Co^II(CN)₄]^2- from square planar (v_CN = 2114 cm^-1, S = 1/2) to tetrahedral (v_CN = 2158 cm^-1, S = 3/2) as evidenced by infrared and magnetic susceptibility measurements. Mn^II[Ni^II(CN)₄] is the only phase formed with Ni^II due to the high thermodynamic stability of square planar [Ni^II(CN)₄]^2-.

Full text of DOI:10.1021/ic0012726

1926
Inorg. Chem. 2001, 40, 1926-1935
Tetracyanomanganate(II) and Its Salts of Divalent First-Row Transition Metal Ions
Jamie L. Manson,† Wayne E. Buschmann,‡ and Joel S. Miller*
Department of Chemistry, University of Utah, 315 South 1400 E. Rm. 2124,
Salt Lake City, Utah 84112-0850
ReceiVed September 27, 2000
The first known paramagnetic, tetrahedral cyanide complex, [MnII(CN)4]2-, is formed by the photoinduced
decomposition of [MnIV(CN)6]2- in nonaqueous solutions or by thermal decomposition in the solid state. In
acetonitrile or dichloromethane, photoexcitation into the ligand-to-metal charge transfer band (λmax ) 25 700
cm-1, ꢀ ) 3700 cm-1 M-1) causes the homolytic cleavage of cyanide radicals and reduction of MnIV. Free cyanide
in dichloromethane leads to the isolation of polycyanide oligomers such as [C12N12]2- and [C4N4]-, which was
crystallographically characterized as the PPN+ salt C40H30N5P2: monoclinic space group ) I2/a, a ) 18.6314(2)
Å, b ) 9.1926(1) Å, c ) 20.8006(1), â )106.176(2)°, Z ) 4]. In the solid state MnIV-CN bond homolysis is
thermally activated above 122 °C, according to differential scanning calorimetry measurements, leading to the
reductive elimination of cyanogen. The [MnII(CN)4]2- ion has a dynamic solution behavior, as evidenced by its
concentration-dependent electronic and electron paramagnetic spectra, that can be attributed to aggregation of the
coordinatively and electronically unsaturated (four-coordinate, 13-electron) metal center. Due to dynamics and
lability of [MnII(CN)4]2- in solution, its reaction with divalent first-row transition metal cations leads to the
formation of lattice compounds with both tetrahedral and square planar local coordination geometries of the
metal ions and multiple structural and cyano-linkage isomers. R-MnII[MnII(CN)4] has an interpenetrating sphalerite-
or diamond-like network structure with a unit cell parameter of a ) 6.123 Å (P4h3m space group) while a â-phase
of this material has a noninterpenetrating disordered lattice containing tetrahedral [MnII(CN)4]2-. Linkage
isomerization or cyanide abstraction during formation results in R-MnII[CoII(CN)4] and MnII[NiII(CN)4] lattice
compounds, both containing square planar tetracyanometalate centers. R-MnII[CoII(CN)4] is irreversibly transformed
to its â-phase in the solid state by heating to 135 °C, which causes a geometric isomerization of [CoII(CN)4]2-
1
3
from square planar (νCN ) 2114 cm-1, S ) /2) to tetrahedral (νCN ) 2158 cm-1, S ) /2) as evidenced by
infrared and magnetic susceptibility measurements. MnII[NiII(CN)4] is the only phase formed with NiII due to the
high thermodynamic stability of square planar [NiII(CN)4]2-
.
Introduction
member of a new class of paramagnetic, tetrahedral, and coor-
dinatively unsaturated cyanometalates. This result is unusual
since percyano complexes of the first-row transition metals are
well-known to favor coordinatively saturated octahedral geom-
etries.2,7 Occasionally, however, there are deviations from this
trend. Some well-characterized nonoctahedral examples include
seven-coordinate [VIII(CN)7]4- 7d and the diamagnetic four-
coordinate square planar-d8 [MII(CN)4]2- (e.g., M ) Ni, Pd,
Pt) and tetrahedral-d10 (e.g., M ) ZnII, CdII, HgII) complexes.2
The rare, d7 low-spin, S ) 1/2, square planar [CoII(CN)4]2- attests
to the strong ligand-field strength imposed by the cyanide
ligand.7b,c
Photochemical and thermal decomposition of homoleptic
metal cyanide complexes is documented with several examples.1-3
The photochemical processes fall into two general categories,
photooxidation-reduction and photosubstitution reactions.1
Photooxidation-reduction reactions are typically initiated by a
ligand-to-metal charge transfer (LMCT) transition into an
excited state that favors ligand-metal bond homolysis. Photo-
substitution (e.g., aquation), in contrast, is typically initiated
by ligand-field excited states. Thermolysis of cyanometalates
in the solid state can result in loss of cyanide radicals in the
form of cyanogen3 accompanied by reduction of the metal
center. This type of process has also been reported for Li2MnIVF6
in the solid state as a preparation of Li2MnIIF4.4
[MnII(CN)4]2- elicits an interesting question regarding its
ligand-field properties; since it is tetrahedral and consists of a
strong ligand field, CN-, is it high- or low-spin? All known
homoleptic cyano complexes are low-spin;2,7 however, virtually
all known tetrahedral metal complexes are high-spin resulting
from the decreased crystal field stabilization with respect to
Both photochemical5 and thermal decomposition of [MnIV-
(CN)6]2- lead to the formation of [MnII(CN)4]2- (1),6 the first
† Present address: Chemistry and Materials Science Divisions, Argonne
National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439.
‡ Present address: Bioscience Division, Los Alamos National Laboratory,
Los Alamos, NM 87544.
(1) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds;
Academic Press: New York, 1970.
(2) (a) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition
Metals; Academic Press: New York, 1976. (b) Shriver, D. F. Struct.
Bonding 1966, 1, 33.
(3) (a) Buschmann, W. E.; Ensling, J.; Gu¨tlich, P.; Miller, J. S. Chem.s
Eur. J. 1999, 5, 3019. (b) Mohai, V. B. Z. Anorg. Allg. Chem. 1972,
392, 287. (c) Seifer, G. B.; Belova, V. I.; Makarova, Z. A. Russ. J.
Inorg. Chem. (Engl.) 1964, 9, 844.
(4) (a) Wandner, K.-H.; Hoppe, R. Z. Anorg. Allg. Chem. 1988, 557, 153.
(b) Wandner, K.-H.; Hoppe, R. Z. Anorg. Allg. Chem. 1987, 546, 113.
(5) Buschmann, W. E.; Vazquez, C.; McLean, R. S.; Ward, M. D.; Jones,
N. C.; Miller, J. S. Chem. Commun. (Cambridge) 1997, 409.
(6) Buschmann, W. E.; Arif, A. M.; Miller, J. S. Angew. Chem., Int. Ed.
1998, 37, 781; Angew. Chem. 1998, 110, 813.
(7) (a) Dunbar, K. Prog. Inorg. Chem. 1996, 45, 823. (b) Meier, I. K.;
Pearlstein, R. M.; Ramprasad, D.; Pez, G. Inorg. Chem. 1997, 36,
1707. (c) Carter, S. J.; Foxman, B. M.; Stuhl, L. S. J. Am. Chem. Soc.
1984, 106, 4265. (d) Towns, R. L. R.; Levenson, R. A. J. Am. Chem.
Soc. 1972, 94, 4345.
10.1021/ic0012726 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/09/2001

Tetracyanomanganate(II) and Its Salts
Inorganic Chemistry, Vol. 40, No. 8, 2001 1927
octahedral complexes, although a few are low spin.8 From mag-
netic susceptibility and electron paramagnetic resonance (EPR)
studies, it was unambiguously determined that [Mn(CN)4]2- is
needles (80% yield based on K{B[C6H3(CF3)2]4}). IR (Nujol): 3118,
1608, 1355, 1279, 1164, 1141, 1122, 1098, 950, 929, 892, 853, 838,
744, 719, 712, 683, 670 cm-1. Mp: 120 °C dec. Anal. Calcd for C42H22-
BF24Fe: C, 48.09; H, 2.11. Found: C, 48.01; H, 2.20.
5
indeed high-spin (S ) /2) with an essentially temperature-
[PPN]2[MnIV(CN)6]. The previous method5 was improved by adding
a 1:6 MeCN/Et2O solution (70 mL) containing [Fe(C5H5)2]{B[C6H3-
(CF3)2]4} (0.7763 mmol, 0.8143 g) to a MeCN solution (35 mL)
containing [PPN]3[MnIII(CN)6] (0.7763 mmol, 1.418 g) while excluding
light (solutions are stable under red light). The blue color of the [Fe-
(C5H5)2]+ disappeared immediately upon mixing. Addition of another
20 mL of Et2O with stirring initiates crystallization from solution after
∼1 min. The product was allowed to settle for 30 min, isolated by
vacuum filtration, and recrystallized from MeCN/Et2O. Small, yellow
platelet crystals were isolated in 90% yield. IR νCN (Nujol, MeCN):
2132 cm-1. Raman νCN (solid): 2135 cm-1. Mp: 142 °C dec. Anal.
Calcd for C78H60N8P4Mn: C, 72.73; H, 4.70; N, 8.40. Found: C, 72.49;
H, 4.51; N, 8.58.
independent moment of 5.99 µB (5.92 µB predicted) and a Curie
θ value of 0 K.6
With the discovery of [MnII(CN)4]2-, a synthon to develop
novel magnetic 3-D networks akin to the Prussian Blue family
of magnetic materials emerged. Prussian Blue, FeIII4[FeII(CN)6]3‚
yH2O (14 < y < 16), is the prototype of a class of 3-D network
solids composed of FeII-C-N-FeIII linkages extended in three
dimensions.9 Several members of this class of materials mag-
netically order below an ordering temperature, Tc, which, in
several cases, exceeds room temperature.10
Herein, we report on the formation of [Mn(CN)4]2- in solution
and the solid state, its solution behavior, and a variety of network
solids formed by reacting [Mn(CN)4]2- with first-row transition
metal dications.
[Et4N]2[MnIV(CN)6]. A CH2Cl2 solution (5 mL) containing [Et4N]-
PF6 (0.220 mmol, 0.0606 g) was added to a CH2Cl2 solution (8 mL)
containing [PPN]2[MnIV(CN)6] (0.1060 mmol, 0.1366 g) while light
was excluded. Et2O (1 mL) was added, and the reaction mixture became
cloudy with a precipitate. The solid was isolated by vacuum filtration
and recrystallized from CH2Cl2/MeCN/Et2O. Yield: 50 mg of yellow
microcrystalline solid (50% yield). IR νCN (Nujol): 2132 cm-1. Mp:
123 °C dec. Anal. Calcd for C23.25Cl0.50H40.50MnN8 (includes 0.25 equiv
of CH2Cl2): C, 54.23; H, 8.28; N, 22.47. Found: C, 54.20; H, 8.74;
N, 22.71.
Photochemical Preparation of [PPN]2[MnII(CN)4]. A solution of
[PPN]2[Mn(CN)6] (1.546 g, 0.8470 mmol) in 10 mL of MeCN/
CH2Cl2, 1:1, was exposed to ambient fluorescent light until the color
had changed from yellow to red. The solution was then layered with
Et2O to crystallize out 0.314 g of burgundy-red prisms (30% yield).
IR νCN: 2205 cm-1 (Nujol), 2202 cm-1 (MeCN). Raman νCN (solid):
2109 cm-1. Dec: 206 °C (DSC, TGA). Anal. Calcd for C76H60-
MnN6P4: C, 73.84; H, 4.82; N, 6.80. Found: C, 73.62; H, 4.97; N,
6.75.
Thermochemical Preparation of [PPN]2[MnII(CN)4]. A 50.0 mg
sample of yellow [PPN]2[Mn(CN)6] was heated in vacuo at 140 °C for
14 h. The remaining solid was pale red and 4.0% (2.0 mg) less in mass
than prior to heating. Recrystallization from MeCN/CH2Cl2/Et2O gave
burgundy-red crystals in 90% yield. IR νCN: 2205 cm-1 (Nujol), 2202
cm-1 (MeCN). Dec: 206 °C (DSC, TGA).
[Et4N]2[MnII(CN)4]. A solution of [Et4N][PF6] (0.0606 g, 0.220
mmol) in 5 mL of 10:1 CH2Cl2/MeCN was added to a red solution of
[PPN]2[Mn(CN)4] (0.1236 g, 0.1000 mmol) in 5 mL of CH2Cl2, and a
fine precipitate began to form. The volume was approximately doubled
with Et2O to precipitate the product. The solid was isolated by vacuum
filtration and recrystallized from MeCN/CH2Cl2/Et2O. Yield: 270 mg
of dark burgundy-red twinned needles (64% yield). IR νCN: 2209 cm-1
(Nujol), 2201 cm-1 (MeCN). Mp: 137 °C (DSC). Dec: 248 °C (DSC,
TGA). Anal. Calcd for C20H40MnN6: C, 57.26; H, 9.61; N, 20.03.
Found: C, 57.29; H, 9.53; N, 19.99.
Experimental Section
All manipulations were performed under N2 or argon using standard
Schlenk techniques or a Vacuum Atmospheres inert atmosphere DriLab.
Dichloromethane was dried and distilled under N2 from CaH2. Aceto-
nitrile was dried and twice distilled under N2 from CaH2. Diethyl ether
(Et2O) and tetrahydrofuran (THF) were dried and distilled under N2
from sodium benzophenone ketyl radical. [MII(NCMe)6][TFPB]2 (M
) V, Cr, Mn, Fe, Co, Ni; TFPB ) B[C6H3(CF3)2]4) salts,11 K3[MnIII-
(CN)6],12a and [PPN]3[MnIII(CN)6]12b [PPN+ ) (Ph3P)2N+] were
prepared as previously described. [Et4N][PF6] was obtained by mixing
a 50 mL aqueous solution of [Et4N]Cl (Aldrich) (3.01 g, 18.2 mmol)
with a 50 mL aqueous solution of K[PF6] (Alfa) (3.34 g, 18.2 mmol).
The white precipitate was filtered off, washed with water several times,
air-dried, recrystallized from a minimum amount of hot MeOH, and
dried in vacuo.
[Fe(C5H5)2]{B[C6H3(CF3)2]4}. A 50 mL Et2O solution containing
p-benzoquinone (1.29 mmol, 0.140 g) and HCl (2.6 mmol, 2.6 mL as
a 1.0 M Et2O solution) was added via cannula to an Et2O solution (40
mL) containing K{B[C6H3(CF3)2]4} (2.355 mmol, 2.124 g) and Fe-
(C5H5)2 (2.59 mmol, 0.482 g). The reaction mixture immediately turned
blue in color. The product precipitated from solution by addition of
hexanes, was isolated by vacuum filtration, and then was recrystallized
from Et2O/hexanes and dried in vacuo. Yield: 1.985 g of deep blue
(8) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th
ed.; Harper Collins, 1993; p 403. Byrne, E. K.; Theopold, K. H. J.
Am. Chem. Soc. 1989, 111, 3887. Arnold, J.; Wilkinson, G.; Hussaun,
B.; Hursthouse, M. B. J. Chem. Soc., Chem. Commun. 1988, 1349.
Stavropoulos, P.; Savage, P. D.; Tooze, R. P.; Wilkinson, G. J. Chem.
Soc., Dalton Trans. 1987, 557.
(9) Ludi, A.; Gu¨del, H. U. Struct. Bonding 1973, 14, 1.
(10) (a) Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.; Verdaguer, M. Inorg.
Chem. 1999, 38, 229. Holmes, S. D.; Girolami, G. J. Am. Chem. Soc.
1999, 121, 5593. Hatlevik, Ø.; Buschmann, W. E.; Zhang, J.; Manson,
J. L.; Miller, J. S. AdV. Mater. 1999, 11, 914. Dujardin, E.; Ferlay,
S.; Phan, X.; Desplanches, C.; Moulin, C. C. D.; Sainctavit, P.;
Baudelet, F.; Dartyge, E.; Veillet, P.; Verdaguer, M. J. Am. Chem.
Soc. 1998, 120, 11347. Ferlay, S.; Mallah, T.; Ouahe`s, R.; Veillet, P.;
Verdaguer, M. Inorg. Chem. 1999, 38, 229. Verdaguer, M.; Bleuzen,
A.; Train, C. Garde, R.; Debiani, F. F.; Desplanches, C. Philos. Trans.
R. Soc. London (A) 1999, 357, 3159. (b) E.g.: Buschmann, W. E.;
Paulson, S. C.; Wynn, C. M.; Girtu, M.; Epstein, A. J.; White, H. S.;
Miller, J. S. AdV. Mater. 1997, 9, 645; Chem. Mater. 1998, 10, 1386.
Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature
1995, 378, 701. Entley, W. R.; Treadway, C. R.; Girolami, G. S. Mol.
Cryst. Liq. Cryst. 1995, 237, 153. Mallah, T.; Thie´baut, S.; Verdaguer,
M.; Veillet, P. Science 1993, 262, 1554. Greibler, W. D.; Babel, D.
Z. Naturforsch. 1982 87b, 832.
[PPN]2[C12N12]. A solution of [PPN]2[Mn(CN)6] (100 mg, 0.077
mmol) in 3 mL of CH2Cl2 was exposed to ambient fluorescent light
until the color had changed from yellow to red. This was slowly diluted
with Et2O by vapor diffusion while sitting on an optical bench for 1
month. Small pale-yellow crystals were isolated by vacuum filtration
in <5% yield. IR (Nujol): νCtN, 2299 (m), 2176 (m) cm-1; νCdN, 1559
(s) cm-1. UV-vis λmax (CH3CN): 288 nm (ꢀ ) 24 900 M-1 cm-1).
Anal. Calcd for C84H60N14P4: C, 72.62; H, 4.35; N, 14.11. Found: C,
72.20; H, 4.50; N, 13.94.
[PPN][C4N4]. A solution of [PPN]4[Mn(CN)6] (800 mg) in 22.5 mL
of 1.25:1 CH2Cl2/Et2O was allowed to stand for 10 months in ambient
fluorescent light. The red solution was removed from a small amount
of red precipitate and the solution reduced in volume to approximately
10 mL under reduced pressure. Fresh diethyl ether (10 mL) was layered
onto the solution, and deep-red prisms grew within 4 weeks. The
crystals were isolated by vacuum filtration yielding 1.1 mg of [PPN]-
[C4N4]. IR (Nujol): νCtN, 2191 (m), 2160 (m) cm-1; νCdN, 1524 (m)
cm-1. Composition was determined by single-crystal X-ray structural
analysis.
(11) Buschmann, W. E.; Miller, J. S. Chem.sEur. J. 1998, 4, 1731.
(12) (a) Brauer, G. Handbook of PreparatiVe Inorganic Chemistry, 2nd
ed.; Academic Press: New York, 1965; Vol. 2, p 1473. (b) Buschmann,
W. E.; Liable-Sands, L.; Rheingold, A. L.; Miller, J. S. Inorg. Chim.
Acta 1999, 284, 175. (c) Alexander, J. J.; Gray, H. B. J. Am. Chem.
Soc. 1968, 90, 4260.

1928 Inorganic Chemistry, Vol. 40, No. 8, 2001
Manson et al.
r-MnII[MnII(CN)4]. Solid [PPN]2[Mn(CN)4] (0.0925 mmol, 0.1144
g) and [Mn(NCMe)6][TFPB]2 (0.0925 mmol, 180 mg) were added
together in a 25 mL flask with magnetic stirring. Et2O (10 mL) was
added with stirring, which was followed by addition of 5 mL of
CH2Cl2. A red-brown solid was observed after stirring for 4 h, and the
mixture was stirred for an additional 0.5 h. The product was isolated
by vacuum filtration and washed several times with Et2O. The solid
was dried in vacuo for approximately 0.5 h, producing a fine red-brown
powder in quantitative yield. IR (Nujol) νCN: 2170 cm-1. This is the
first product formed without thermal treatment. TGA revealed a
continual weight loss from ∼30 to 500 °C indicating decomposition.
On one occasion the â-MnII[MnII(CN)4] was isolated using this
identical procedure, as evidenced by a different IR pattern and magnetic
Zero-field-cooled measurements were made in a residual field of -0.002
( 0.001 to 0.002 ( 0.001 Oe based on the fluxgate response. Ac-
susceptibility measurements were carried out at low temperature
(2-40 K) using Hdc ) 0 and 1 Oe oscillating fields of 10, 100, and
1000 Hz. The samples were loaded in an inert atmosphere glovebox in
a Delrin airtight holder. All magnetic data were corrected for the sample
holder contribution and for core diamagnetism as calculated from
Pascal’s constants. A value of -65 × 10-6 emu/mol was used for
[Mn(CN)4]2-
.
Thermal Analysis. Thermal properties of materials were studied
on a TA Instruments model 2910 differential scanning calorimeter
(DSC) and a TA Instruments model 2050 thermal gravimetric analyzer
(TGA). Temperature control is maintained between -150 and 500 °C,
and ambient and 1000 °C for the DSC and TGA instruments,
respectively. The TGA is located in a Vacuum Atmospheres inert
atmosphere glovebox in order to study oxygen and moisture sensitive
samples. TGA samples were handled in an Ar atmosphere and heated
under a N2 purge. DSC samples were weighed and hermetically sealed
under Ar in aluminum pans. Heating rates were 15 °C/min for TGA
and 5 °C/min for DSC experiments unless otherwise noted in the text.
Elemental analyses were performed at Atlantic Microlabs, Inc.
properties. IR (Nujol) νCN: 2159 cm-1
.
r-MnII[CoII(CN)4]. Solid samples of [PPN]2[Mn(CN)4] (0.0467
mmol, 0.0578 g) and [CoII(NCMe)6][SbF6]2 (0.0467 mmol, 0.0363 g)
were added together in a 20 mL flask with a magnetic stir bar. THF (3
mL) was added with stirring followed by the addition of 3 mL of
CH2Cl2. A blue-green solid was observed precipitating from solution
soon after and the reaction mixture stirred for 1 h. The product was
isolated by vacuum filtration followed by subsequent washing with THF
and CH2Cl2 and dried in vacuo for 2 h. IR (Nujol) νCN: 2210 (w),
2158 (sh), 2114 (s) cm-1. TGA revealed a continual weight loss from
∼30 to 500 °C indicating decomposition.
Results and Discussion
Monitoring the photolysis of [MnIV(CN)6]2- in CH2Cl2 by
IR shows the intensity of [MnIV(CN)6]2-, νCN ) 2135 cm-1
(Figure 1a, peak I), decreasing and the detectable production
of [MnII(CN)4]2-, νCN ) 2202 cm-1 (peak II), occurring by
exposure to ambient light at room temperature. An additional
absorption grows in at 1562 cm-1 (νCdN) accompanied by a
shoulder around 2170 cm-1 (νCtN), (peaks III and IV) ascribed
to the formation of {(1,1,2,2-tetracyano-1,2-ethanediyl)bis-
[imino(cyanomethylene)]}bis[cyanamide] ion(2-), [C12N12]2-
(2), reported previously.13 The integrated areas of peaks I, II,
and III are plotted as a function of time, t, in Figure 1b. The
observed rate for the disappearance of [MnIV(CN)6]2-, I, is fit
best to an exponential first-order rate constant during the first
125 min [ln(A0/A) vs t has a linear slope of 0.0106] whereas
the production of II fits best to a zero-order rate constant with
a linear slope of 0.00107 fit to the first 165 min. The rate of
photolysis is reduced as the concentration of the light-absorbing
[MnII(CN)4]2- increases, which reduces the number of photons
transmitted through solution, adding a significant nonlinear
component to the observed rate after ∼125 min. The formation
of [MnII(CN)4]2-, II, is not detected during the first 10 min of
photolysis, and [C12N12]2-, III, is not detected during the first
65 min.
â-Co[Mn(CN)4] was produced by thermolysis of a bulk sample of
the R-phase at 135 °C for 2 h. IR (Nujol) νCN: 2158 (s) cm-1
.
MnII[NiII(CN)4]. Solid samples of [PPN]2[Mn(CN)4] (0.128 mmol,
0.1582 g) and [Ni(NCMe)4][O3SCF3]2 (0.128 mmol, 77.2 mg) were
added together in a 20 mL flask with a stir bar. Approximately 6 mL
of THF was added, resulting in a pale yellow-green solution. After
stirring for about 0.5 h, 2 mL of CH2Cl2 was quickly added and
precipitation of a pale yellow solid occurred. The product was isolated
by vacuum filtration, washed numerous times with CH2Cl2, and dried
in vacuo for 1 h. IR (Nujol) νCN: 2159 cm-1. TGA revealed a continual
weight loss from ∼30 to 500 °C indicating decomposition.
Spectroscopic Measurements. Infrared spectra were obtained on a
Bio-Rad FTS-40 Fourier transform spectrometer in the range 600-
4000 ( 1 cm-1. Solids were analyzed as Nujol mulls sandwiched
between NaCl plates and solutions measured in a NaCl cell with a
0.100 mm path length. Variable-temperature infrared studies were
carried out using a custom-made heating attachment with an Ondyn
temperature controller in the standard transmittance configuration.
Spectra were recorded between 25 and 175 °C after allowing the
temperature to equilibrate for 10 min or greater prior to acquisition.
Raman spectra were obtained using a SPEX 1877, 0.6 m triple
spectrometer equipped with a SPEX Spectrum-1 CCD detector. The
radiation source was a Ti-sapphire Coherent 890 tunable laser energized
by a Coherent Innova 300 Ar ion laser. An excitation line of 764.925
nm was used.
UV-vis-near-IR spectra were made using a Cary-17D spectropho-
tometer with an On-Line Instrument Systems Inc. interface between
225 and 800 nm. Solution spectra were measured in 1 or 5 cm path
length airtight quartz UV cells using freshly prepared MeCN solutions.
Electron paramagnetic resonance (EPR) spectra were obtained with
a Bruker, model ESP 300E, spectrometer equipped with a X-band
microwave bridge with a model ER 041 XK frequency counter, and
an Oxford He cryostat controller. Measurements were made between
ambient temperature and 78 K. Measurements were made on solutions
in CH2Cl2 or CH2Cl2/MeCN mixtures in standard 3 mm fused quartz
EPR tubes.
X-ray Diffraction. The single-crystal X-ray structure of [PPN][C4N4]
was determined on a Bruker CCD diffractometer. Table 1 and the
Supporting Information summarize the data collection and refinement.
X-ray powder diffraction spectra were taken on a Rigaku Miniflex
diffractometer model 1GC2 (Cu KR). The reflection 2θ values were
corrected with an internal crystalline silicon standard.
The photoinduced decomposition of [MnIV(CN)6]2- in MeCN
is more complex with respect to CH2Cl2 as three new cyano
species are resolved in the IR, Figure 2a. The production of
[C12N12]2- does not occur under these conditions. The initial
2132 cm-1 absorption of [MnIV(CN)6]2- (peak V) rapidly
decreases in intensity while broader absorptions at 2100 cm-1
(peak VI) and at 2141 cm-1 (peak VII) increase in intensity.
The broad absorption for [MnII(CN)4]2- starts to grow in at 2200
cm-1 (peak VIII) and is soon accompanied by another absorp-
tion growing in at 2086 cm-1 (peak IX). The integrated areas
of peaks V-IX vs t are plotted in Figure 2b. The observed rate
Magnetic Measurements. Magnetic susceptibility measurements
were made between 2 and 300 K with a Quantum Design MPMS-5XL
SQUID ac/dc magnetometer equipped with the ultralow-field (∼0.005
Oe) accessory, reciprocating sample measurements system, and con-
tinuous low-temperature control with enhanced thermometry features.
(13) Buschmann, W. E.; Arif, A. M.; Miller, J. S. J. Chem. Soc., Chem.
Commun. 1995, 2343.

Tetracyanomanganate(II) and Its Salts
Inorganic Chemistry, Vol. 40, No. 8, 2001 1929
Table 1. Crystallographic Details for [PPN][C4N4]
chem formula C40H30N5P2
formula mass, Da 642.63
a
b
c
R
â
γ
V
Z
18.6314(2) Å
9.1926(1) Å
20.8006(1) Å
90o
space group
T
I2/a
293 K
λ
0.71073 Å
1.248 g cm-3
0.163 mm-1
0.0510
Fcalcd
µ
106.176(2)°
90°
R1(Fo)a
3421.50(5) Å3
4
Rw(Fo )b )
0.1427
2
a ∑||Fo| - |Fc||/∑|Fo|. b {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2
.
2
2
Figure 2. (a) IR spectra of a 10.0 mM MeCN solution of [PPN]2-
[MnIV(CN)6] exposed to ambient light for 330 min. Peak V,
[MnIV(CN)6]2-, loses intensity while peaks II and III gain intensity,
then peaks VIII, [MnII(CN)4]2-, and IX gain intensity and VI loses
intensity. (b) Integrated areas for peaks V-IX vs t. Curve fits (solid
lines) for peaks V (exponential) and VIII (linear) are extrapolated
beyond the fitted range.
having greater dielectric strength for solvating small fragments
such as CN• and CN-. In MeCN formation of [MnII(CN)4]2-
occurs after a several minute induction period, consistent with
formation of an intermediate. One intermediate is ascribed to
peak VI, Figure 2, since this species goes through a maximum
concentration partway through the photolysis before being
consumed. There is evidence to suggest that this is [MnIII(CN)6]3-,
Figure 1. (a) IR spectra of a 10.0 mM CH2Cl2 solution of [PPN]2-
[MnIV(CN)6] exposed to ambient light for 810 min. Peak I, [MnIV-
(CN)6]2-, loses intensity while peaks II, [MnII(CN)4]2-, III and IV,
[C12N12]2-, grow in. Peaks marked with an asterisk (*) are due to the
aromatic rings of the [PPN]+. (b) Integrated areas for peaks I-III vs
t. Curve fits (solid lines) for peak I (exponential) and II (linear) are
extrapolated beyond the fitted range.
νCN ) 2100 cm-1 12b
because it can be crystallized from these
,
solutions in significant amounts by addition of a counter solvent.
Peak VII appears to be a side product that is likely dimeric/
oligomeric with Mn-CN-M linkages that gives rise to the 2141
cm-1 absorption. This absorption is within the region known
for several cyano-bridging M[MnIII(CN)6] compounds,3a,14 but
is too high in energy for isolated [Mn(CN)6]n- (n ) 4,12a 3,12b
25). The formation of peak IX later in the reaction correlates
with the consumption of peak VI and is consistent with forming
NCCN, but a [MnII(CN)6]4- species2 cannot be discounted.
The photolysis in CH2Cl2 proceeds with formation of only
the two final products, [MnII(CN)4]2-, II, and [C12N12]2-, III
and IV, Figure 1. The production of [MnII(CN)4]2- also occurs
after an induction period of several minutes suggesting that it
is formed from an intermediate as well. An important compari-
son to make between the photolysis in CH2Cl2 and MeCN is
that the disappearance of [MnIV(CN)6]2- is first order in both
cases even though the rates differ by an order of magnitude
and the detectable intermediates and products are not all the
same. This is evidence that loss of cyanide from the photoexcited
state species, {[MnIV(CN)6]2-}*, is the rate-limiting step and
for the disappearance of V fits best to an exponential first-order
rate constant during the first 48 min [ln(A0/A) vs t has a linear
slope of 0.115], and the production of [MnII(CN)4]2-, VIII, fits
best to a zero-order rate constant with a linear slope of 0.003 74
over the first 60 min after an induction period of ∼6 min. In
this case, the concentration of the light-absorbing [MnII(CN)4]2-
does not increase appreciably before [MnIV(CN)6]2- is consumed
by photolysis. There are, however, two species, peaks VI and
VII, that are formed as soon as photolysis has begun. Peak VII
grows to a constant intensity in ∼75 min, but peak VI first
increases, plateaus, and then decreases continuously. Peak IX
is observed after [MnIV(CN)6]2-, V, is nearly undetected and
peak VI reaches its maximum intensity.
The observed rate of photoinduced decomposition for [MnIV-
(CN)6]2- is first order in both CH2Cl2 and MeCN with nearly
an order of magnitude greater rate in MeCN. This difference in
rate is likely due to MeCN being a coordinating solvent that
can stabilize the intermediate manganocyanide species and also
(14) Ently, W. R.; Girolami, G. S. Inorg. Chem. 1994, 33, 5165; 1995, 34,
2262.

1930 Inorganic Chemistry, Vol. 40, No. 8, 2001
Manson et al.
Figure 4. Electronic spectra of [MnIV(CN)6]2- in MeCN (b ) 5 cm)
with (s) [NEt4]+ and (+) [PPN]+ cations.
CN bond lengths (Å) are as follows: C(22)-N(22) 1.48(3),
C(22)-C(23) 1.60(2), C(24)-N(22) 1.263(14), and C(24)-N(3)
1.032(6) Å. The large standard deviations are a result of the
positional disorder, however, the observed bond distances are
consistent with the [(CtN)2-CdN-(CtN)]- formulation.
Distortion of sp2-hybridized carbons and nitrogens from the ideal
angle of 120° is readily apparent from selected bond angles such
as N(3)-C(24)-N(21), N(2)-C(23)-N(22), and C(24)#2-
C(22)-C(23) which are 161.7(7)°, 145.9(13)°, and 134.6(11)°,
respectively. Attempts to obtain additional material for deter-
mination of the UV-vis spectra, as well as magnetic studies,
were unsuccessful.
Figure 3. ORTEP and atom-labeling diagram of [C4N4]-, 3.
that the first-formed intermediate is not detected in either
solvent. It is proposed that initial loss of a CN• leads to a very
reactive [MnIII(CN)5(solvent)]2- intermediate that quickly un-
dergoes reductive elimination of a second CN•, dimerization,
or recombination with CN• or CN-. Coordinating solvents such
as MeCN better stabilize such an intermediate and should
increase the rate of [MnIV(CN)6]2- photolysis. The rate of
[MnII(CN)4]2- production differs only by a factor of 3.5 between
solvents, further supporting the rate-limiting production of an
undetected intermediate species. This photolysis mechanism
appears to proceed through a stepwise elimination of CN• and
requires more detailed studies to elucidate it fully.
Photochemical decomposition of [MnIV(CN)6]2- in MeCN
or CH2Cl2 is induced by an absorption into a LMCT band in
the visible part of the spectrum (λmax ) 25 700 cm-1, ꢀ ) 3700
cm-1 M-1).5 The same qualitative behavior is observed when
either the [PPN]+ or tetraethylammonium, [NEt4]+, cations are
used, showing that the cation is not directly involved in the
decomposition process. The electronic spectrum of [NEt4]2-
[MnIV(CN)6] is reported here in Figure 4 to show the higher
energy CT bands that are obscured by [PPN]+. Above the bands
at 31 850 cm-1 (ꢀ ) 5000 cm-1 M-1) and 34 300 cm-1 (ꢀ )
8300 cm-1 M-1) are three more overlapping bands positioned
near 36 000 cm-1 (ꢀ ∼ 8000 cm-1 M-1), 38 900 cm-1 (ꢀ ∼
14 700 cm-1 M-1), and 40 500 cm-1 (ꢀ ) 16 200 cm-1 M-1).
Only in CH2Cl2 is the production of [C12N12]2- observed in
the time of photolysis, and it is presumably formed by
recombination and oligomerization of CN• and CN-.15 In
addition to formation and isolation of [C12N12]2- (2), a small
amount of a second polycyanide oligomer, [C4N4]- (3), has been
identified, and its infrared spectra and structure have been
determined.
The overall spectrum looks like that of [MnIII(CN)6]3- 12b,c
but
,
shifted to lower energy. The three lowest energy CT transitions
(25 700, 31 850, 34 300 cm-1) are shifted by 5300, 5250, and
6700 cm-1, respectively, to lower energy going from MnIII to
MnIV, and the higher energy transitions are resolved. Full
analysis of this data is outside the scope of this work. The
important connection to point out here is that the combination
of a LMCT transition being shifted into the visible and the
reduction potential of the complex being favorable (MnIII/IV E1/2
) 0.14 V in MeCN)5 allows for the photoinduced reductive
The structure of [PPN][C4N4] was determined from a single-
crystal X-ray diffraction study. The [PPN]+ cation shows bond
lengths (P-N ) 1.583 Å) and angles (P-N-P ) 136.2°) typical
of this cation.5,6,7c,12b [C4N4]-, 3, is planar within experimental
error and is positionally disordered over four equivalent sites,
Figure 3. At first glance, the molecule appears to be isomorphous
to TCNE (tetracyanoethylene) or [TCNE]•-, but has different
elimination of cyanide from [MnIV(CN)6]2-
.
Thermal decomposition of [MnIV(CN)6]2- in the solid state
shows a simple transformation, with the loss of cyanogen, to
produce [MnII(CN)4]2-. Monitoring the solid state IR spectrum
of [PPN]2[MnIV(CN)6] in Nujol at 140 °C shows the disappear-
ance of the νCN absorption at 2132 cm-1 and the growing in of
a broad absorption around 2185 cm-1. After a bulk sample is
heated to 140 °C in vacuo, the IR spectrum of the crude product
shows a strong, split absorption at 2202 and 2192 cm-1 (Nujol).
The IR spectrum of the crude product dissolved in MeCN
exhibits the νCN absorption at 2202 cm-1 and after recrystalli-
zation at 2205 cm-1 (Nujol), consistent with the photochemical
ν
CN absorptions as well as a deep red color. We also considered
that the proposed [C4N4]- molecule could simply be two
orientationally disordered [C(CN)3]- anions; however, formation
of a “naked” carbon atom is unlikely to occur under these
circumstances. Hence, reaction of CN• radicals in solution could
readily lead to [C4N4]-, in addition to [C12N12]2-. The CC and
(15) Ferris, J. P.; Donner, D. B.; Lotz, W. J. Am. Chem. Soc. 1972, 94,
6968.

Tetracyanomanganate(II) and Its Salts
Inorganic Chemistry, Vol. 40, No. 8, 2001 1931
Figure 5. TGA (upper) and DSC (lower) curves for [PPN]2[MnIV-
(CN)6] with heating rates of 5 and 2 °C/min, respectively.
Figure 6. Electronic spectra of [MnII(CN)4]2- in MeCN (b ) 5 cm)
with (O) [PPN]+, 1.00 mM; (0) [PPN]+, 0.097 mM; and (4) [NEt4]+,
1.00 mM.
preparation of [PPN]2[MnII(CN)4]. Monitoring this process by
TGA consistently shows a 3.4% weight loss at Tonset ) 136 °C,
about 85% of the expected 4.04% weight loss for two CN’s
per Mn center, Figure 5. The DSC response shows two
irreversible exotherms at Tonset ) 122 °C (104.2 kJ/mol) and
Tonset ) 156 °C (8.2 kJ/mol). Above 250 °C there is significant
decomposition and weight loss. The IR spectrum of the red
thermolysis product produced in the TGA, by heating to 225
°C, exhibits single absorption bands of comparable intensity at
2198 and 1730 cm-1 (Nujol). The 1730 cm-1 absorption appears
to be due to a thermolysis byproduct that can be eliminated by
prolonged heating or recrystallization and accounts for the 15%
excess residual mass in the TGA experiment. A curious feature
of the single-crystal X-ray structure of the [MnIV(CN)6]2- is
that one of the three crystallographically unique cyanide ligands,
C(3)N(3), has thermal parameters that are significantly greater
than those of the other two cyanides.5 It may be reasonable to
expect that this C(3)N(3) is involved with the thermal Mn-
CN bond homolysis.
[PPN]2[MnII(CN)4] is most easily prepared from the reductive
decomposition of [PPN]2[MnIV(CN)6] by thermolysis in vacuo
and subsequent recrystallization. It is fairly oxygen stable in
the solid state and in dry aprotic solvents, but readily hydrolyzes
in protic media as evidenced by a change in color from deep
red to pale green. The anion also demonstrates redox stability
over a wide range of potentials (ca. (2 V) in MeCN. Details
of the tetrahedral [PPN]2[MnII(CN)4] structure, high-spin state,
and room temperature solution EPR have been published
previously.6 The issue of cyanide ligand lability will be
discussed here due to its significance during the formation of
lattice structures with divalent first-row transition metals.
however, unlike [MnII(CN)4]2- the solution species of Co are
detectable by IR and are thought to be monomeric.17
The solution EPR spectrum of [PPN]2[MnII(CN)4] in
CH2Cl2 changes with concentration at room temperature as
previously described.6 Below 1 mM concentration the EPR
spectrum exhibits the expected isotropic six-line hyperfine
interaction for 55Mn (I ) 5/2) centered at g ) 2.003 ( |A| ) 71
G). Above 1 mM concentration the Mn hyperfine signal
broadens significantly and a very narrow 11-line resonance (|A|
) 1.6 G) of much weaker intensity grows in and is centered at
the same g value. When a solution of [A]2[MnII(CN)4] (A )
PPN, NEt4N) (e1 mM) is quenched to 78 K, the spectrum
changes shape and the g value increases to 2.012 ( |A| ) 71
G), Figure 7a. The concentration and temperature-dependent
responses of the electronic and EPR spectra are reversible.
The strongest evidence for aggregation is in the EPR spectra.
The frozen solution EPR spectrum for [PPN]2[MnII(CN)4],
Figure 7a, is nearly the same as for [PPN]2[MnIV(CN)6] (g )
1.996, |A| ) 68 G), Figure 7b, except the g value is greater.
[PPN]2[MnIV(CN)6] has no dramatic temperature dependence
on its EPR line shape.5 The similarity in line shapes and dif-
ference in g values between the [MnII(CN)6]2- and [MnIV(CN)4]2-
frozen EPR spectra strongly suggest that [MnII(CN)4]2- can
become octahedral by some mechanism of aggregation. This
could be either axial nitrile coordination of two [MnII(CN)4]2-
ions, i.e., 4, or forming a [MnII(CN)6]4- (low-spin, S )1/2)
species by scavenging CN- via linkage isomerization3a,18 of
bridging cyanides in 4 or free CN- in solution. For a low-spin
octahedral or square planar d5 configuration contributions of
spin-orbit coupling are greater than for an octahedral d3
configuration and will result in an increase in g-value.19a,b
Isolated square planar [MnII(CN)4]2- is ruled out because it
should exhibit an anisotropic spectrum with axial and equatorial
g tensors. Nonetheless, the reaction of [PPN]2[MnII(CN)4] with
pyridine or [PPN]CN in CH2Cl2 yields only the tetrahedral MnII
upon recrystallization. This observation supports the notion that
a hexacoordinate species is relatively short-lived in solution.
In contrast to [MnIV(CN)6]2-, Figure 4, and [MnIII(CN)6]3- 12b
,
the electronic spectrum of [MnII(CN)4]2- changes with cation
substitution and concentration, Figure 6. Significant differences
appear at energies lower than 33 000 cm-1 and prevent
assignments of d-d ligand-field transitions. The concentration-
dependent absorptions are intense relative to the normal ligand-
field transition intensities expected (ꢀ < 10 cm-1 M-1),
suggesting the presence of aggregation or a change in geometry
in solution. The difference in electronic spectra between cations
may be due to differences in the cations’ ability to solubilize
different types/sizes of aggregates. In any case the electronic
spectrum observed for [MnII(CN)4]2- is atypical for isolated
tetrahedral or octahedral MnII ions.16 Solvent and cation effects
have been reported for CoII cyanide solutions in MeCN;
(17) Carter, S. J.; Foxman, B. M.; Stuhl, L. S. Inorg. Chem. 1986, 25,
2888.
(18) Reguera, E.; Bertra´n, J. F.; Nun˜ez, L. Polyhedron 1994, 10, 1619.
Brown, D. B.; Shriver, D. F. Inorg. Chem. 1969, 8, 37. Shriver, D.
F.; Shriver, S. A.; Anderson, S. E. Inorg. Chem. 1965, 4, 725. (b)
Brown, D. B.; Shriver, D. F.; Schwartz, L. H. Inorg. Chem. 1968, 7,
77. Buschmann, W. E.; Ensling, J.; Gu¨tlich, P.; Miller, J. S. Chem.s
Eur. J. 1999, 5, 3019.
(19) (a) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders
College Pub.: Ft. Worth, TX, 1992; Chapter 10. (b) Figgis, B. N. In
ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.; Perga-
mon: New York, 1987; Vol. 1, Chapter 6.
(16) Figgis, B. N. Introduction to Ligand Fields; John Wiley-Interscience:
New York, 1966; Chapter 9.

1932 Inorganic Chemistry, Vol. 40, No. 8, 2001
Manson et al.
Mn. The homometallic R-MnII[MnII(CN)4] is the first ex-
ample of a paramagnetic sphalerite lattice. Details of the
structure and magnetic behavior have previously been pub-
lished.21 The reaction of [PPN]2[MnII(CN)4] and [MnII(NCMe)6]-
[TFPB]2 in MeCN forms R-MnII[MnII(CN)4] as a red-brown
precipitate. The IR spectrum of the initial sample suggests a
pure structurally ordered material of high symmetry, i.e., a
single, sharp νCN band is observed at 2170 cm-1. This is reduced
by 32 cm-1 relative to [MnII(CN)4]2-, suggesting weakening
of the CtN bond upon coordination of its nitrogen atom to
high-spin MnII. This is the opposite effect observed for ZnII-
(CN)2 (2217 cm-1), which significantly increases in frequency
relative to that found for K2[ZnII(CN)4] (2149 cm-1).22 Ther-
mogravimetric analysis of initially formed R-MnII[MnII(CN)4]
between 25 and 500 °C shows a continuous weight loss of about
27% between ∼40 and 500 °C. Mass spectrometry has
confirmed the loss of CN-, HCN, and (CN)2 with no evidence
for the presence or loss of solvents.
The unit cell parameter of a ) 6.123 Å has been determined
for R-MnII[MnII(CN)4] using the P4h3m space group of ZnII-
(CN)2 (a ) 5.901 Å) as a structural model.21,23a This assumes
an ordered arrangement of cyano ligands, i.e., MnC4 and MnN4
coordination spheres. Alternatively, a random orientation of
cyano ligands, i.e., MnCnN4-n (n ) 1, 2, 3, 4) coordination
sphere, may occur resulting in the lower-symmetry space group
Pn3hm, which has been proposed for ZnCnN4-n23a and observed
for the CdCnN4-n by 113Cd NMR. The unit cell, primitive
23b
space group, and experimentally determined density21 are
consistent with an interpenetrating 3-D sphalerite-type network
structure for R-MnII[MnII(CN)4]. The lack of solvent molecules
in the lattice after preparation is consistent with this lattice type
having no large cavities. By comparison, a noninterpenetrating
lattice yields a face-centered cubic structure (space group F4h3m)
and a larger unit cell, such as 11.609 Å, reported for [NMe4]-
[CuIZnII(CN)4] in which the [NMe4]+ cation occupies the open
lattice cavities.20
R-MnII[MnII(CN)4] exhibits strong antiferromagnetic coupling
(θ ) -240 K) and order as an antiferromagnet below its Ne´el
temperature, TN ≈ 65 K.21 A complication in evaluating this
material is the presence of what is estimated to be about 9.5%
paramagnetic, or weakly coupled, high-spin MnII impurity. As
discussed above, there is significant oligomerization of
[MnII(CN)4]2- in solution and such multinuclear species are
subject to being coprecipitated with the bulk of the lattice
compound. Dimers or trimers of Mn centers would correspond
to 4.7% and 3.2%, respectively, of chemical impurity in the
material. The estimated level of MnII impurity seems to be
reasonable in light of the significant changes in electronic and
EPR responses for [MnII(CN)4]2- with concentration, vide infra.
A second, â-Mn[Mn(CN)4] phase was isolated from CH2Cl2,
albeit on one occasion, and has a νCN absorption at 2159
cm-1 along with a trace amount of coordinated MeCN showing
weak bands at 2308 and 2279 cm-1. The magnetic susceptibility
of the â-phase, Figure 8, is similar to the R-phase, showing
that there is not a significant change in the spin states of, or
Figure 7. Frozen solution (1.00 mM, CH2Cl2) EPR spectra of [PPN]2-
[MnII(CN)4] (a), g ) 2.012, |A| ) 71 G, and [PPN]2[MnIV(CN)6]
(b), g ) 1.996, |A| ) 68 G. Spectra (offset) were measured under the
same conditions at 78 K and 9.646 GHz with the relative intensities
preserved.
As noted above, a unique EPR active species becomes apparent
above 1 mM concentration at room temperature while the
isotropic MnII hyperfine signal broadens and becomes unobserv-
able above ∼1.5 mM.6 These observations are consistent with
spin relaxation on Mn becoming faster than the time scale of
the experiment due to either intermolecular spin-spin or dipolar
relaxation processes. The dramatic dependence of the EPR
response on both temperature and concentration is consistent
with a hexacoordinate MnII equilibrium species due to inter-
molecular significant interactions.
3-D MII[MnII(CN)4] Lattice Compounds. The inherent
lability of the [MnII(CN)4]2- ion in solution is a challenging
issue in the preparation of single-component M[Mn(CN)4] (M
) Cr, Mn, Fe, Co, Ni) sphalerite (diamond-like) lattices, akin
to ZnII(CN)2,20 as potential magnetic materials. In numerous
cases more than one cyano-containing product is formed when
equimolar amounts of [PPN]2[MnII(CN)4] and [MII(NCMe)6]2+
salts are combined as indicated by IR spectroscopy and sig-
nificant variations in magnetic behavior from sample to sample.
Many attempts were made by varying the solvent conditions,
concentrations, and reaction temperatures to isolate pure, single-
phase materials. The three best-characterized systems (M ) Mn,
Co, Ni) are presented.
(20) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546.
(21) Manson, J. L.; Buschmann, W. E.; Miller, J. S. Angew. Chem., Int.
Ed. 1998, 37, 783; Angew. Chem.1998, 110, 815.
(22) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Co-
ordination Complexes, 4th ed.; Wiley: New York, 1986; pp 274-
279.
(23) (a) Kitazawa, T.; Nishikiori, R.; Kuroda, R.; Iwamoto, T. J. Chem.
Soc., Dalton Trans. 1994, 1029. (b) Nishikiori, S. I.; Ratcliffe, C. I.;
Ripmeester, J. A. Can J. Chem. 1990, 68, 2270. Nishikiori, S. I.;
Ratcliffe, C. I.; Ripmeester, J. A. J. Chem. Soc., Chem. Commun. 1991,
735.

Tetracyanomanganate(II) and Its Salts
Inorganic Chemistry, Vol. 40, No. 8, 2001 1933
Co complexes due to low-lying excited d orbitals that result in
higher effective g values.19b A θ value of -137 K, extrapolated
from 1/ø(T), is indicative of strong antiferromagnetic coupling
between nearest neighboring MII centers through the bridging
cyanides. The X-ray powder diffraction data of this material
exhibits very broad features, indicative of significant structural
disorder and small particle size, with a low angle peak maximum
around 21.6° 2θ. This corresponds to an average M‚‚‚M
separation of 4.11 Å that is 1.19 and 0.96 Å shorter than the
Mn‚‚‚Mn separation in MnII[MnII(CN)4]21 and K2MnII[MnII-
(CN)6],14 respectively. It is even shorter than the range found
for other disordered, nonaqueous M[Mn(CN)6] examples.3a,25
The first-formed material has a color characteristic of
tetrahedral CoII complexes,19b,25a or square planar [CoII(CN)4]2-,7c
and the IR spectra suggest a mixture of CN bridging motifs.
The 2114 cm-1
νCN absorption is consistent with either partial
Figure 8. Temperature-dependent magnetic susceptibility of R-Mn-
[Mn(CN)4], (O) øT, (4) 1/ø, and â-Mn[Mn(CN)4], (b) øT, (2) 1/ø.
or full linkage isomerization (or cyanide abstraction by Co) to
give the CoII-CN-MnII bridging motif while the weak 2210
cm-1 absorption is consistent with the tetrahedral MnII-CN-
CoII isomer. The shoulder at 2158 cm-1 is also consistent with
the CoII-CN-MnII linkage isomer, but likely due to a different
geometry. The magnetic susceptibility data supports the for-
mulation of MnII[CoII(CN)4] (square planar CoII) as the primary
structural component. The presence of tetrahedral CoII and the
low-lying excited states of square planar CoII are likely to be
contributions to the observed room temperature øT value being
13.5% greater than the calculated. The short M‚‚‚M separation
given by X-ray diffraction would require unreasonably short
M-C or M-N bond distances for bridging cyanide, but may
reflect an interlayer separation of quasi-2-D domains structurally
induced by the presence of square planar [CoII(CN)4]2-. An
example of this type of layered structure has been reported for
Fe[Ni(CN)4], which has an Fe-NC-Ni distance of 5.20 Å.26
The above evidence leads to the assignment of the first-formed
material as R-MnII[CoII(CN)4] with a noninterpenetrating dis-
ordered lattice network.
Figure 9. Temperature-dependent magnetic susceptibility of R-MnII-
[CoII(CN)4], (O) 1/ø, (4) øT, and â-MnII[CoII(CN)4], (b) 1/ø, (2) øT.
short-range coupling between, Mn centers. There is no evidence,
however, of long-range order above 2 K. The IR data suggests
that there is open space or coordination sites in the material
(by the presence of solvent) that are not available in the
interpenetrating sphalerite-type structure of R-MnII[MnII(CN)4].
These observations and the results for â-MnII[CoII(CN)4] (νCN
) 2158 cm-1) described below are good indications that â-MnII-
[MnII(CN)4] is a noninterpenetrating network solid with tetra-
hedral [MnII(CN)4]2- ions. Moreover the [Mn(CN)4]2- anion
is surprisingly robust in that good Lewis bases such as MeCN
or pyridine do not coordinate.
Upon heating, R-MnII[CoII(CN)4] completely transforms,
irreversibly, to a second phase assigned as â-MnII[CoII(CN)4].
This transformation is monitored by heating an IR sample to
135 °C causing the νCN absorptions at 2210 and 2114 cm-1 to
disappear while the only remaining absorption at 2158 cm-1
becomes more intense. This â-phase is considerably darker in
color, and its observed room temperature øT value is 6.40 emu
K/mol, Figure 9, nearly matching the above calculated value
of 6.25 emu K/mol for both high-spin MII tetrahedral centers.
The øT(T) for â-MnII[CoII(CN)4] is significantly different than
for the R-phase in that it continuously decreases down to 2 K,
Figure 9.
Co. Linkage isomerization of cyano ligands is not expected
to be an issue for spin states or geometries in R-MnII[MnII(CN)4]
since both MnII sites are tetrahedral and high-spin, although the
local coordination sphere may contain MnN4, MnN3C, MnN2C2,
and MnNC3. The situation is different when exploring hetero-
metallic materials. The reaction of [PPN]2[MnII(CN)4] and [CoII-
(NCMe)6][SbF6]2 in MeCN immediately forms a blue-green
precipitate. At room temperature, this material displays its
primary νCN absorption band at 2114 cm-1 with a shoulder
around 2158 cm-1 and an additional weak absorption at 2210
cm-1. Temperature-dependent magnetic susceptibility data,
Figure 9, is evaluated for a 1:1:4 ratio of Co/Mn/CN. The
observed room temperature øT value of 4.94 emu K/mol, much
less than the calculated value (g ) 2.00) of 6.25 emu K/mol
for uncoupled high-spin MnII (S ) 5/2) and CoII (S ) 3/2)
tetrahedral centers, is slightly higher than the calculated 4.35
Significant differences between R- and â-MnII[CoII(CN)4]
exist in the IR and susceptibility data and are thought to arise
from the two geometric isomers possible for [CoII(CN)4]2-, i.e.,
square planar CoII for the R-phase and tetrahedral for the
â-phase. Formation of the R-phase occurs first by either a
linkage isomerization or CN- scavenging mechanism in the
presence of the labile [MnII(CN)4]2- ion. Monomeric square
planar [CoII(CN)4]2- has been previously characterized, and its
[PPN]+ salt exhibits two νCN absorptions in the solid state at
2102 and 2095 cm-1 7c whereas a 1:4, CoII/CN- mixture in
,
MeCN is reported to be in a dynamic equilibrium with several
(24) Buschmann, W. E.; Miller, J. S. Inorg. Chem. 2000, 39, 2411.
(25) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry; Wiley:
New York, 1988; pp 724-732. (b) Cotton, F. A.; Wilkinson, G.
AdVanced Inorganic Chemistry; Wiley: New York, 1988; p 743.
(26) Kitazawa, T.; Funkunaga, M.; Takahashi, M.; Takeda, M. Mol. Cryst.
Liq. Cryst. 1994, 244, 331.
5
1
emu K/mol for MnII (S ) /2) and square planar CoII (S ) /2)
ions. Observed øT(T) values are often increased for square planar

1934 Inorganic Chemistry, Vol. 40, No. 8, 2001
Manson et al.
temperature øT value are consistent with the formulation of MnII-
[NiII(CN)4]. Production of this material is likely driven by the
formation of very thermodynamically stable, square planar
[NiII(CN)4]2- 2a,25b
.
Reactions of [PPN]2[Mn(CN)4] and VII, FeII, CrII, and CuI
ions under nonaqueous conditions lead to materials of unre-
solved composition and structure. IR spectra show multiple νCN
bands ranging from ∼2050 to 2220 cm-1 indicating multiple
phases, i.e., materials that possess a distribution of square planar
and tetrahedral metal coordination geometries as well as CN
linkage isomers.
Conclusion
Photochemical and thermal decomposition of [PPN]2[MnIV-
(CN)6] leads to several characterized products, namely, [PPN]2-
[MnII(CN)4] (1), [PPN]2[C12N12] (2), and [PPN][C4N4] (3), as
well as several uncharacterized products. This is the only route
found thus far to synthesize [MnII(CN)4]2- as direct combination
of MnII salts and CN- in nonaqueous media do not form this
complex. Photochemical decomposition of [MnIV(CN)6]2- in
MeCN or CH2Cl2 is induced by a LMCT absorption whereas
thermolysis of [MnIV(CN)6]2- is a clean, quantitative route to
[MnII(CN)4]2-. By either method, the end result is a two-electron
reduction of MnIV with homolytic cleavage of two cyanide
radicals. Only in CH2Cl2 are the conditions suitable for the
formation of [C12N12]2- (2) in the time period of photolysis,
and it is presumably formed by recombination and oligomer-
ization of CN• and CN-. Longer periods of time and more dilute
Figure 10. Temperature-dependent magnetic susceptibility of MnII-
[NiII(CN)4], (O) 1/ø, (2) øT.
“CoII(CN)nn-2” (n ) 2-5) species.17 A mixture of MnII with
excess CN- in MeCN solution does not lead to the direct
formation of [MnII(CN)4]2- and shows that the formation of
[CoII(CN)4]2- is more favorable. Therefore the chemical dynam-
ics in solution should lead to the formation of square planar
[CoII(CN)4]2- first. Upon heating, however, the observed
changes show that the geometric isomerization from square
planar to tetrahedral CoII, in this case, is energetically accessible
and irreversible in the solid state.
solutions lead to the isolation of [C4N4]- (3), and other [CN]x
z-
Ni. Another example that demonstrates the lability of cyanide
in [MnII(CN)4]2- is its reaction with NiII salts. Reaction of
[PPN]2[MnII(CN)4] with either [NiII(NCMe)6][TFPB]2 or
[NiII(NCMe)4][CF3SO3]2 in THF forms a pale yellow precipitate,
upon addition of CH2Cl2, that exhibits a single, strong νCN band
at 2159 cm-1. The color of this material does not reflect the
presence of tetrahedral [MnII(CN)4]2-, as in R-MnII[MnII(CN)4],
species are also expected to be present. These results show that
many unusual products can be formed as a consequence of C-C
and C-N bond formation via the reaction of cyanogen and CN-
mediated by the presence of a cyanide-labile transition metal
ion.
[MnII(CN)4]2- is a building block for the preparation of four-
coordinate extended network structures. Several new materials
have been synthesized by nonaqueous routes and show that a
wide variety of products are possible due to the lability of the
[MnII(CN)4]2- ion in solution. R-MnII[MnII(CN)4] is an inter-
penetrating sphalerite-type network structure with tetrahedral
Mn centers and orders as an antiferromagnet below 65 K.
â-MnII[MnII(CN)4] appears to be a disordered noninterpenetrat-
ing network structure with tetrahedral [MnII(CN)4]2- centers and
no long-range magnetic ordering above 2 K. When MII cations
besides Mn are reacted with [MnII(CN)4]2-, there is either
cyanide abstraction or linkage isomerization due to the lability
of tetracyanomanganate(II). R-MnII[CoII(CN)4] is a disordered
network structure with square planar [CoII(CN)4]2- that is
thermally isomerized to a â-phase in the solid state, at elevated
temperatures. The â-MnII[CoII(CN)4] has tetrahedral CoII and
the same IR νCN absorption band as â-MnII[MnII(CN)4]. MnII-
[NiII(CN)4] is stable as a disordered network solid with square
planar Ni centers. The lack of other products attests to the
stability of [NiII(CN)4]2-. In each of these materials there is
antiferromagnetic coupling between nearest neighbor ions except
for MnII[NiII(CN)4] that requires coupling of Mn ions through
the diamagnetic [NiII(CN)4]2- bridge.
but is consistent with yellow, square planar [NiII(CN)4]2-
.
Temperature-dependent magnetic susceptibility data for this
material, Figure 10, is evaluated for a 1:1:4 ratio of Ni/Mn/
CN. The observed room temperature øT value of 4.40 emu
K/mol is in good agreement with a calculated value of 4.35
emu K/mol for uncoupled MnII (S ) 5/2) ions in MnII[NiII(CN)4]
(square planar NiII, S ) 0). A much greater øT value of 5.38
emu K/mol is expected for the presence of high-spin, tetrahedral
NiII (S ) 1) expected for NiII[MnII(CN)4]. 1/ø, fit to the Curie-
Weiss expression, gives a θ value of -5 K with g ) 2.02,
indicative of weak antiferromagnetic coupling between MnII
centers through the bridging [NiII(CN)4]2- ion. This is consistent
with a significant decrease in øT(T) below 90 K as antiferro-
magnetic coupling interactions increase. The X-ray powder
diffraction data of this material exhibits very broad features,
indicative of significant structural disorder and small particle
size, with a low angle peak maximum around 18.3° 2θ. This
corresponds to an average M‚‚‚M separation of 4.85 Å that is
0.45 and 0.22 Å shorter than the Mn‚‚‚Mn separation in MnII-
[MnII(CN)4]21 and K2MnII[MnII(CN)6],14 respectively. It is,
however in the range of the average M‚‚‚M separation found
in other disordered, nonaqueous M[Mn(CN)6] examples.3a,24
This short distance could reflect either a distorted Ni-CN-
Mn linkage or an interlayer separation of quasi-2-D domains
analogous to the reported Fe[Ni(CN)4] layered structure.26
The above observations lead to the conclusion that square
planar [NiII(CN)4]2- forms upon reaction with [MnII(CN)4]2-
by either abstraction or linkage isomerization3a,18 of cyanide.
The resulting material’s color, νCN absorption, and room
Acknowledgment. The authors appreciate the determination
of the structure of [PPN][C4N4] by M. J. Scott and S. J. Lippard
(MIT) and the constructive comments and insight provided by
Prof. E. Coronado (Universidad de Valencia), Prof. A. J. Epstein,
Dr. C. M. Wynn, and Mr. M. Girtu (The Ohio State University)
and gratefully acknowledge the support from the DOE DMS

Tetracyanomanganate(II) and Its Salts
Inorganic Chemistry, Vol. 40, No. 8, 2001 1935
anisotropic thermal parameters, bond distances and angles, and hydrogen
atom positions nonbonding distances for [PPN]3 in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
(Grant No. DE FG 03-93ER45504) and the American Chemical
Society PRF (Grant No. 30722-AC5).
Supporting Information Available: An ORTEP atom-labeling
diagram for [PPN][C4N4], [PPN]3. A summary of the crystallographic
data, tables of fractional coordinates and isotropic thermal parameters,
IC0012726