has been studied in several Au and Ag complexes. In the tetranuclear Ag
I -Cu
I
complex Ag 2 Cu 2 L 4 (L ¼ 2-diphenylphosphino-3-methylindole) [151], shortening
of the Ag-Ag bond was observed, decreasing from 3.042(2) Å under ambient
conditions to 2.773(3) Å at 3.47(4) GPa. This is accompanied by a blue-shift to
red-shift switch in the absorption and emission spectrum on increasing pressure
(between 2 and 3 GPa). The shortening of the Ag-Ag bond was not solely responsible for this, with defects and non-hydrostatic conditions thought to play a role. In
the tris(μ 2 -3,5-diisopropyl-1,2,4- triazolato-κ
2 N
1 :N
2 )trigold(I) complex, a number of
Au. . .Au intermolecular contacts are also made, with the molecules forming dimers
in the solid state [152]. In this study, the compound exhibits four successive phase
transitions under hydrostatic pressure, driven by these aurophilic interactions. In this
example, the closest Au. . .Au contacts are between trimeric units where they
decrease from 3.4070(5) Å to 3.0273(15) Å, between ambient pressure and
3.31 GPa. A rare observation in this system is the re-emergence of a lower-pressure
phase (Form II, 1.69 GPa) at higher pressure (3.3 GPa) after it had undergone
transitions to two further phases at 2.18 and 2.70 GPa; this behaviour is referred to
as a re-entrant phase transition. Conformational changes in the molecule during the
transitions result not only in these shorter Au. . .Au contacts, but they also correlate
with shifts of the luminescence maxima, from a band maximum at 14040 cm
À1 at
2.40 GPa, decreasing to 13,550 cm
À1 at 3 GPa.
Spectroscopic changes are not just limited to Au and Ag complexes. A reduction
in length of Ni-Ni and Ni-L distances in the complexes bis(3-fluorosalicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) has
resulted in a blue shift of the UV-Vis leading to an example of piezochromism,
with the crystals changing colour from green to red at 5 GPa as the octahedral
crystal-field splitting is decreased. In the fluoro system, this equated to a decrease in
the Ni. . .Ni distance from 3.19 to 2.82 Å at 5.4 GPa [153]. The bis
(dimethylglyoximato)nickel(II) complex [154] possesses a structure similar to the
salicylaldoximato complexes; hence, the changes in the structure (in an equivalent
pressure range, to 5.1 GPa) are similar. The glyoximato complex also changes colour
but at a lower pressure of 2 GPa. However, the latter complex becomes conducting at
high pressure [155] in contrast to the salicyloximato complex that remain
non-conducting. Piezochromism, caused by compression of molecular compounds,
has also been observed in the photomagnetic compound [Y(DMF) 4 (H 2 O) 3 (μ-CN)Fe
(CN) 5 ]H 2 O (DMF ¼ dimethylformamide), on increasing pressure to 7.60 GPa,
where a reversible yellow-to-red transition is induced via a charge transfer mechanism through the cyanide ligand, though structural evidence for this was lacking as
the models could not be refined against the data above 0.7 GPa [156]. A reduction in
length of M-M and M-L bonds might seem unsurprising; however, on compression
of the compound Co 2 (CO) 6 (PPh 3 ) 2 , the length of the Co-Co bond increases from
2.67(1) to 2.72(1) Å due to a change in the geometry of the CO and PPh 3 ligands
where they move from a staggered to eclipsed conformation [157].
Conformational flexibility in organic and metal-organic compounds is commonplace within the pressure regime (0.001–10 GPa). In the Pd oxathioether macrocyclic complex PdCl 2 ([9]aneS 2 O), three reported phases of the mononuclear Pd
II
Crystallography Under High Pressures
181
I -Cu
I
complex Ag 2 Cu 2 L 4 (L ¼ 2-diphenylphosphino-3-methylindole) [151], shortening
of the Ag-Ag bond was observed, decreasing from 3.042(2) Å under ambient
conditions to 2.773(3) Å at 3.47(4) GPa. This is accompanied by a blue-shift to
red-shift switch in the absorption and emission spectrum on increasing pressure
(between 2 and 3 GPa). The shortening of the Ag-Ag bond was not solely responsible for this, with defects and non-hydrostatic conditions thought to play a role. In
the tris(μ 2 -3,5-diisopropyl-1,2,4- triazolato-κ
2 N
1 :N
2 )trigold(I) complex, a number of
Au. . .Au intermolecular contacts are also made, with the molecules forming dimers
in the solid state [152]. In this study, the compound exhibits four successive phase
transitions under hydrostatic pressure, driven by these aurophilic interactions. In this
example, the closest Au. . .Au contacts are between trimeric units where they
decrease from 3.4070(5) Å to 3.0273(15) Å, between ambient pressure and
3.31 GPa. A rare observation in this system is the re-emergence of a lower-pressure
phase (Form II, 1.69 GPa) at higher pressure (3.3 GPa) after it had undergone
transitions to two further phases at 2.18 and 2.70 GPa; this behaviour is referred to
as a re-entrant phase transition. Conformational changes in the molecule during the
transitions result not only in these shorter Au. . .Au contacts, but they also correlate
with shifts of the luminescence maxima, from a band maximum at 14040 cm
À1 at
2.40 GPa, decreasing to 13,550 cm
À1 at 3 GPa.
Spectroscopic changes are not just limited to Au and Ag complexes. A reduction
in length of Ni-Ni and Ni-L distances in the complexes bis(3-fluorosalicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) has
resulted in a blue shift of the UV-Vis leading to an example of piezochromism,
with the crystals changing colour from green to red at 5 GPa as the octahedral
crystal-field splitting is decreased. In the fluoro system, this equated to a decrease in
the Ni. . .Ni distance from 3.19 to 2.82 Å at 5.4 GPa [153]. The bis
(dimethylglyoximato)nickel(II) complex [154] possesses a structure similar to the
salicylaldoximato complexes; hence, the changes in the structure (in an equivalent
pressure range, to 5.1 GPa) are similar. The glyoximato complex also changes colour
but at a lower pressure of 2 GPa. However, the latter complex becomes conducting at
high pressure [155] in contrast to the salicyloximato complex that remain
non-conducting. Piezochromism, caused by compression of molecular compounds,
has also been observed in the photomagnetic compound [Y(DMF) 4 (H 2 O) 3 (μ-CN)Fe
(CN) 5 ]H 2 O (DMF ¼ dimethylformamide), on increasing pressure to 7.60 GPa,
where a reversible yellow-to-red transition is induced via a charge transfer mechanism through the cyanide ligand, though structural evidence for this was lacking as
the models could not be refined against the data above 0.7 GPa [156]. A reduction in
length of M-M and M-L bonds might seem unsurprising; however, on compression
of the compound Co 2 (CO) 6 (PPh 3 ) 2 , the length of the Co-Co bond increases from
2.67(1) to 2.72(1) Å due to a change in the geometry of the CO and PPh 3 ligands
where they move from a staggered to eclipsed conformation [157].
Conformational flexibility in organic and metal-organic compounds is commonplace within the pressure regime (0.001–10 GPa). In the Pd oxathioether macrocyclic complex PdCl 2 ([9]aneS 2 O), three reported phases of the mononuclear Pd
II
Crystallography Under High Pressures
181