66
F. Sagan and M. P. Mitoraj
1 Introduction
Chemical bonding in transition metal complexes is usually described in terms of
Dewar–Chatt–Duncanson model which accounts for donation (ligand → metal) and
back-donation (metal → ligand) charge transfer processes [1, 2]. They are crucial for
the molecular stability as well as for the determination of various catalytic and spectroscopic properties [1–5]. Very recently, however, more and more attention is paid
to London dispersion forces and different types of non-covalent interactions which
can, additionally to typical donor/acceptor phenomena, influence the chemistry of
transition metal complexes [6, 7]. One shall emphasize that the DFT method together
with the recent breakthrough developments of semi-empirical dispersion corrections
by Grimme [8, 9] allows for identification and better understanding a number of very
important physical phenomena in real materials [6, 7].
Transition metal complexes often contain sterically demanding ligands which traditionally are associated with the source of repulsion—however, the recent topical
review by Schreiner and Wagner has suggested the necessity for “…reconsidering
steric effects” because in many cases sterically demanding hydrophobic groups, often
leading to formation of untypical homopolar C–H•••H–C non-covalent interactions,
are truly London dispersion donors which can easily overcompensate Pauli repulsion [6]. Furthermore, Liptrot and Power nicely reviewed the importance of London
dispersion forces in organometallic and inorganic complexes [7]. One could also
reference that bulky ligands might control directly catalytic activity [10].
Despite significant progress in the identification of untypical homopolar
X–H•••H–X non-covalent interactions in various systems including transition metal
complexes, one must admit that this subject is still the matter of some discussion in
the literature particularly as far as intramolecular X–H•••H–X contacts are taken into
account. Here, one can cite the following debates on the stability of biphenyl (planar versus bent), [11–16] 2-butene isomers [17–19] or the nature of inter-molecular
homopolar B–H•••H–B (hydride-hydride) and other similar contacts in hydrogen
storage systems [20–27]. Apart from the above non-covalent interactions and well
established now polar dihydrogen X–H
δ+ •••
–δ H–Y (X Y) bonds (named also as
proton–hydride contacts) [28–32], there are other non-conventional and quite unintuitive weak interactions, which have been discovered in recent time, such as anion•••π
interactions [33–35] or various types of σ/π-hole bonds [36–40]. All these types of
weak interactions as well as typical hydrogen bonds, π•••π stacking and others
are now crucial forces for various branches of chemistry including transition metal
complexes [41–45].
F. Sagan and M. P. Mitoraj
1 Introduction
Chemical bonding in transition metal complexes is usually described in terms of
Dewar–Chatt–Duncanson model which accounts for donation (ligand → metal) and
back-donation (metal → ligand) charge transfer processes [1, 2]. They are crucial for
the molecular stability as well as for the determination of various catalytic and spectroscopic properties [1–5]. Very recently, however, more and more attention is paid
to London dispersion forces and different types of non-covalent interactions which
can, additionally to typical donor/acceptor phenomena, influence the chemistry of
transition metal complexes [6, 7]. One shall emphasize that the DFT method together
with the recent breakthrough developments of semi-empirical dispersion corrections
by Grimme [8, 9] allows for identification and better understanding a number of very
important physical phenomena in real materials [6, 7].
Transition metal complexes often contain sterically demanding ligands which traditionally are associated with the source of repulsion—however, the recent topical
review by Schreiner and Wagner has suggested the necessity for “…reconsidering
steric effects” because in many cases sterically demanding hydrophobic groups, often
leading to formation of untypical homopolar C–H•••H–C non-covalent interactions,
are truly London dispersion donors which can easily overcompensate Pauli repulsion [6]. Furthermore, Liptrot and Power nicely reviewed the importance of London
dispersion forces in organometallic and inorganic complexes [7]. One could also
reference that bulky ligands might control directly catalytic activity [10].
Despite significant progress in the identification of untypical homopolar
X–H•••H–X non-covalent interactions in various systems including transition metal
complexes, one must admit that this subject is still the matter of some discussion in
the literature particularly as far as intramolecular X–H•••H–X contacts are taken into
account. Here, one can cite the following debates on the stability of biphenyl (planar versus bent), [11–16] 2-butene isomers [17–19] or the nature of inter-molecular
homopolar B–H•••H–B (hydride-hydride) and other similar contacts in hydrogen
storage systems [20–27]. Apart from the above non-covalent interactions and well
established now polar dihydrogen X–H
δ+ •••
–δ H–Y (X Y) bonds (named also as
proton–hydride contacts) [28–32], there are other non-conventional and quite unintuitive weak interactions, which have been discovered in recent time, such as anion•••π
interactions [33–35] or various types of σ/π-hole bonds [36–40]. All these types of
weak interactions as well as typical hydrogen bonds, π•••π stacking and others
are now crucial forces for various branches of chemistry including transition metal
complexes [41–45].