diffraction studies where the various groups have been able to rationalise changes to
structure through following the molecular movement as a function of pressure. There
are many more studies using X-ray powder diffraction, Raman and IR that will
provide readers with a significant body of research to digest. That being said, there
are many more materials out there to be squeezed and probed with X-ray diffraction
methods, and we hope that we have provided, at least, a starting point from which to
begin your studies.
4 The Effect of Pressure on Metal-Containing Complexes
and Framework Materials
4.1 Introduction
The effect of pressure on metal complexes, frameworks, coordination polymers and
metal-organic framework (MOF) materials has been extensively reviewed, and we
would particularly like to point those of Moggach and Parsons [143], Tidey et al.
[144] and Gütlich et al. [145]. These reviews comprehensively cover the effect of
pressure on metal complexes and spin-crossover materials, whilst the review by
McKellar and Moggach [50] discusses the effect of pressure on MOFs. We will not
be discussing in detail effects such as negative linear compressibility (NLC behaviour), where an expansion along a particular crystallographic direction is observed
on increasing pressure [146], despite this effect being observed in a number of metalcontaining compounds recently. If the reader is interested in NLC behaviour, we
encourage them to read much more thorough and focussed reviews, such as the
excellent review by Cairns and Goodwin [147].
In this section, we will provide an overview of the science of metal-containing
compounds under high-pressure conditions conducted since the last review of the
area in 2015. As with the previous section on organic materials, metal-organic
materials are a broad and keenly studied area, and we have tried to highlight a few
topics that may be of interest to the reader.
One of the stark differences between the compressibility of organic compounds
and metal-containing materials in the solid state is the compressibility of the metalligand (M-L) bonds which are far more compressible than covalent bonds. For
example, Cu-ligand bond distances can vary from 1.9 Å to 3.1 Å and still be
considered a bonding interaction. As a consequence, far greater changes in intramolecular bonds can be observed. Surprisingly, not only do we see the occurrence of
bond formation in metal complexes but also the ability to break bonds on increasing
pressure. This ability to tune the formation and breakage of bonds is limited to
specific sets of organic materials that possess the correct functional groups to
facilitate the transition and hence is not so widely applicable (Sect. 3.2.2). Just as
in the organic solid state though, conformational changes occur, whilst changes in
intermolecular interactions in compounds that contain a metal often give rise to
178
S. A. Moggach and I. D. H. Oswald
structure through following the molecular movement as a function of pressure. There
are many more studies using X-ray powder diffraction, Raman and IR that will
provide readers with a significant body of research to digest. That being said, there
are many more materials out there to be squeezed and probed with X-ray diffraction
methods, and we hope that we have provided, at least, a starting point from which to
begin your studies.
4 The Effect of Pressure on Metal-Containing Complexes
and Framework Materials
4.1 Introduction
The effect of pressure on metal complexes, frameworks, coordination polymers and
metal-organic framework (MOF) materials has been extensively reviewed, and we
would particularly like to point those of Moggach and Parsons [143], Tidey et al.
[144] and Gütlich et al. [145]. These reviews comprehensively cover the effect of
pressure on metal complexes and spin-crossover materials, whilst the review by
McKellar and Moggach [50] discusses the effect of pressure on MOFs. We will not
be discussing in detail effects such as negative linear compressibility (NLC behaviour), where an expansion along a particular crystallographic direction is observed
on increasing pressure [146], despite this effect being observed in a number of metalcontaining compounds recently. If the reader is interested in NLC behaviour, we
encourage them to read much more thorough and focussed reviews, such as the
excellent review by Cairns and Goodwin [147].
In this section, we will provide an overview of the science of metal-containing
compounds under high-pressure conditions conducted since the last review of the
area in 2015. As with the previous section on organic materials, metal-organic
materials are a broad and keenly studied area, and we have tried to highlight a few
topics that may be of interest to the reader.
One of the stark differences between the compressibility of organic compounds
and metal-containing materials in the solid state is the compressibility of the metalligand (M-L) bonds which are far more compressible than covalent bonds. For
example, Cu-ligand bond distances can vary from 1.9 Å to 3.1 Å and still be
considered a bonding interaction. As a consequence, far greater changes in intramolecular bonds can be observed. Surprisingly, not only do we see the occurrence of
bond formation in metal complexes but also the ability to break bonds on increasing
pressure. This ability to tune the formation and breakage of bonds is limited to
specific sets of organic materials that possess the correct functional groups to
facilitate the transition and hence is not so widely applicable (Sect. 3.2.2). Just as
in the organic solid state though, conformational changes occur, whilst changes in
intermolecular interactions in compounds that contain a metal often give rise to
178
S. A. Moggach and I. D. H. Oswald