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Spin states (d electrons)
Spin states (d electrons)
Spin states when describing transition metal coordination complexes refers to the potential spin configurations of
the metal centers d electrons. In many molecules these spin states vary between high-spin and low-spin
configurations. These configurations can be understood through the two major models used to describe coordination
complexes; ligand field theory, which is an application of molecular orbital theory to transition metals, and crystal
field theory, which has roots in VSEPR theory.[1]
High-spin vs Low-spin
Octahedral complexes
The Δ splitting of the d-orbitals plays an important role
in the electron spin state of a coordination complex.
There are three factors that affect the Δ: the period of
the metal center, the charge of the metal center, and the
field strength of the complex's ligands as described by
the spectrochemical series.
In order for low spin splitting to occur, the energy cost
Low-spin [Fe(NO2)6]3− crystal field diagram
of placing an electron into an already singly occupied
orbital must be less than the cost of placing the
additional electron into an eg orbital at an energy cost of Δ. If the energy required to pair two electrons is greater than
the energy cost of placing an electron in an eg, Δ, high spin splitting occurs.
If the separation between the orbitals is large, then the lower energy orbitals are completely filled before population
of the higher orbitals according to the Aufbau principle. Complexes such as this are called "low-spin" since filling an
orbital matches electrons and reduces the total electron spin. If the separation between the orbitals is small enough
then it is easier to put electrons into the higher energy orbitals than it is to put two into the same low-energy orbital,
because of the repulsion resulting from matching two electrons in the same orbital. So, one electron is put into each
of the five d-orbitals before any pairing occurs in accord with Hund's rule resulting in what is known as a "high-spin"
complex. Complexes such as this are called "high-spin" since populating the upper orbital avoids matches between
electrons with opposite spin.
Within a transition metal group moving down the series
corresponds with an increase in Δ. The observed result
is larger Δ splitting for complexes in octahedral
geometries based around transition metal centers of the
second or third row, periods 5 and 6 respectively. This
Δ splitting is generally large enough that these
complexes do not exist as high-spin state. This is true
High-spin [FeBr6]3− crystal field diagram
even when the metal center is coordinated to weak field
ligands. It is only octahedral coordination complexes
which are centered around first row transition metals that fluctuate between high and low-spin states.
The charge of the metal center plays a role in the ligand field and the Δ splitting. For example, Fe2+ and Co3+ are
both d6; however, the higher charge of Co3+ creates a stronger ligand field than Fe2+. All other things being equal,
Fe2+ is more likely to be high spin than Co3+.
Ligands also affect the magnitude of Δ splitting of the d-orbitals according to their field strength as described by the
spectrochemical series. Strong-field ligands, such as CN− and CO, increase the Δ splitting and are more likely to be
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Spin states (d electrons)
low-spin. Weak-field ligands, such as I− and Br− cause a smaller Δ splitting and are more likely to be high-spin.
Tetrahedral complexes
The Δ splitting energy for tetrahedral metal complexes (four ligands), Δtet is smaller than that for an octahedral
complex. Therefore, it is rare to have a Δtet large enough to cause electrons to pair before filling high orbitals. Thus,
tetrahedral complexes are usually high-spin. "There are no known ligands powerful enough to produce the
strong-field case in a tetrahedral complex" (Transition metals and Coordination Chemistry:The Crystal field Model
by Steven S. Zumdahl. Chemical Principles)
Square planar complexes
Most spin state transitions are between the same geometry, namely octahedral. However, in the case of d8 complexes
is a shift in geometry between spin states. There is no possible difference between the high and low-spin states in the
d8 octahedral complexes, however d8 complexes are able to shift from paramagnetic tetrahedral geometry to a
diamagnetic low-spin square planar geometry.
Ligand field theory vs Crystal field theory
The rationale for why the spin states exist according to ligand field theory is essentially the same as the crystal field
theory explanation. However the explanation of why the orbitals split is different accordingly with each model and
requires translation.
High-spin and low-spin systems
The first d electron count (special version of electron configuration) with the possibility of holding a high spin or low
spin state is octahedral d4 since it has more than the 3 electrons to fill the non bonding d orbitals according to ligand
field theory or the stabilized d orbitals according to crystal field splitting. The spin state of the complex also affects
an atom's ionic radius.[2]
d4
Octahedral high-spin: 4 unpaired electrons, paramagnetic, substitutionally labile. Includes Cr2+ ionic radius 80
pm, Mn3+ ionic radius 64.5 pm.
Octahedral low-spin: 2 unpaired electrons, paramagnetic, substitutionally inert. Includes Cr2+ ionic radius 73
pm, Mn3+ ionic radius 58 pm.
d5
Octahedral high-spin: 5 unpaired electrons, paramagnetic, substitutionally labile. Includes Fe3+ ionic radius
64.5 pm.
Octahedral low-spin: 1 unpaired electron, paramagnetic, substitutionally inert. Includes Fe3+ ionic radius 55
pm.
d6
Octahedral high-spin: 4 unpaired electrons, paramagnetic, substitutionally labile. Includes Fe2+ ionic radius 78
pm, Co3+ ionic radius 61 pm.
Octahedral low-spin: no unpaired electrons, diamagnetic, substitutionally inert. Includes Fe2+ ionic radius 62
pm, Co3+ ionic radius 54.5 pm, Ni4+ ionic radius 48 pm.
d7
Octahedral high-spin: 3 unpaired electrons, paramagnetic, substitutionally labile. Includes Co2+ ionic radius
74.5 pm, Ni3+ ionic radius 60 pm.
2
Spin states (d electrons)
Octahedral low-spin:1 unpaired electron, paramagnetic, substitutionally labile. Includes Co2+ ionic radius 65
pm, Ni3+ ionic radius 56 pm.
d8
Octahedral high-spin: 2 unpaired electrons, paramagnetic, substitutionally labile. Includes Ni2+ ionic radius 69
pm.
Square planar low-spin: no unpaired electrons, diamagnetic, substitutionally inert. Includes Ni2+ ionic radius
49 pm.
References
[1] Miessler, Gary L.; Donald A. Tarr (1998). Inorganic Chemistry (2nd edition). Upper Saddle River, New Jersey: Pearson Education, Inc.
Pearson Prentice Hall. ISBN 0-13-841891-8.
[2] Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides Shannon R.D. Acta Cryst. A32
751-767 (1976) doi:10.1107/S0567739476001551
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Article Sources and Contributors
Article Sources and Contributors
Spin states (d electrons) Source: http://en.wikipedia.org/w/index.php?oldid=491609113 Contributors: John of Reading, Kmarinas86, NameIsRon, OMCV, Petergans, Prari, Quadell, V8rik, 15
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Contributors: YanA
File:CFT - High Spin Splitting Diagram 2.png Source: http://en.wikipedia.org/w/index.php?title=File:CFT_-_High_Spin_Splitting_Diagram_2.png License: GNU Free Documentation
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