13 research outputs found
Perturbation of the Charge Density between Two Bridged Mo<sub>2</sub> Centers: The Remote Substituent Effects
A series
of terephthalate-bridged dimolybdenum dimers with various formamidinate
ancillary ligands, denoted as [Mo<sub>2</sub>(ArNCHNAr)<sub>3</sub>]<sub>2</sub>Â(μ-O<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>) (Ar = <i>p</i>-XC<sub>6</sub>H<sub>4</sub>, with X = OCH<sub>3</sub> (<b>1</b>), CH<sub>3</sub> (<b>2</b>), F (<b>3</b>), Cl (<b>4</b>), OCF<sub>3</sub> (<b>5</b>), and CF<sub>3</sub> (<b>6</b>)), has been
synthesized and studied in terms of substituent effects on electron
delocalization between the two dimetal sites. X-ray structural analyses
show that these complexes share the same molecular scaffold with the <i>para</i>-substituents (X) being about 8 Ã… away from the
Mo<sub>2</sub> center. It is found that the remote substituents have
the capability to tune the electronic properties of the complexes.
For the series <b>1</b> to <b>6</b>, the metal–metal
bond distances (<i>d</i><sub>Mo–Mo</sub>) decrease
slightly and continuously; the potential separations (Δ<i>E</i><sub>1/2</sub>) for the two successive one-electron oxidations
decrease constantly, and the metal to ligand transition energies (λ<sub>max</sub>) increase in order. More interestingly, the two types of
methine protons, H<sub>∥</sub> on the horizontal and H<sub>⊥</sub> on the vertical ligands with respect to the plane
defined by the Mo–Mo bond vectors and bridging ligand, display
separate resonant signals δ<sub>∥</sub> and δ<sub>⊥</sub> in the NMR spectra. The displacements of the chemical
shifts, Δδ<sub>∥–⊥</sub> = δ<sub>∥</sub> – δ<sub>⊥</sub>, are getting smaller
as the substituents vary from electron-donating to -withdrawing. These
results show that the peripheral groups on the [Mo<sub>2</sub>] units
function to fine-tune the metal–metal interactions crossing
the bridging ligand. The experimental parameters, Δ<i>E</i><sub>1/2</sub>, λ<sub>max</sub>, and Δδ<sub>∥–⊥</sub>, which are linearly related with the Hammett constants (σ<sub>X</sub>) of the X groups, can be used to probe the charge density
on the two [Mo<sub>2</sub>] units and the electronic delocalization
between them
Perturbation of the Charge Density between Two Bridged Mo<sub>2</sub> Centers: The Remote Substituent Effects
A series
of terephthalate-bridged dimolybdenum dimers with various formamidinate
ancillary ligands, denoted as [Mo<sub>2</sub>(ArNCHNAr)<sub>3</sub>]<sub>2</sub>Â(μ-O<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>) (Ar = <i>p</i>-XC<sub>6</sub>H<sub>4</sub>, with X = OCH<sub>3</sub> (<b>1</b>), CH<sub>3</sub> (<b>2</b>), F (<b>3</b>), Cl (<b>4</b>), OCF<sub>3</sub> (<b>5</b>), and CF<sub>3</sub> (<b>6</b>)), has been
synthesized and studied in terms of substituent effects on electron
delocalization between the two dimetal sites. X-ray structural analyses
show that these complexes share the same molecular scaffold with the <i>para</i>-substituents (X) being about 8 Ã… away from the
Mo<sub>2</sub> center. It is found that the remote substituents have
the capability to tune the electronic properties of the complexes.
For the series <b>1</b> to <b>6</b>, the metal–metal
bond distances (<i>d</i><sub>Mo–Mo</sub>) decrease
slightly and continuously; the potential separations (Δ<i>E</i><sub>1/2</sub>) for the two successive one-electron oxidations
decrease constantly, and the metal to ligand transition energies (λ<sub>max</sub>) increase in order. More interestingly, the two types of
methine protons, H<sub>∥</sub> on the horizontal and H<sub>⊥</sub> on the vertical ligands with respect to the plane
defined by the Mo–Mo bond vectors and bridging ligand, display
separate resonant signals δ<sub>∥</sub> and δ<sub>⊥</sub> in the NMR spectra. The displacements of the chemical
shifts, Δδ<sub>∥–⊥</sub> = δ<sub>∥</sub> – δ<sub>⊥</sub>, are getting smaller
as the substituents vary from electron-donating to -withdrawing. These
results show that the peripheral groups on the [Mo<sub>2</sub>] units
function to fine-tune the metal–metal interactions crossing
the bridging ligand. The experimental parameters, Δ<i>E</i><sub>1/2</sub>, λ<sub>max</sub>, and Δδ<sub>∥–⊥</sub>, which are linearly related with the Hammett constants (σ<sub>X</sub>) of the X groups, can be used to probe the charge density
on the two [Mo<sub>2</sub>] units and the electronic delocalization
between them
Electronic Coupling and Electron Transfer between Two Dimolybdenum Units Spaced by a Biphenylene Group
Three
symmetrical dimolybdenum dimers bridged by 4,4′-biphenyldicarboxylate
and the partially and fully thiolated derivatives have been synthesized
and studied with respect to electronic coupling and intramolecular
electron transfer. As generally denoted by [Mo<sub>2</sub>]–(ph)<sub>2</sub>–[Mo<sub>2</sub>], the complexes are differentiated
by the [Mo<sub>2</sub>] units but have a biphenylene spacer in common,
where [Mo<sub>2</sub>] = [Mo<sub>2</sub>(DAniF)<sub>3</sub>(EE′C)]
with auxiliary ligands DAniF (<i>N</i>,<i>N</i>′-diÂ(<i>p</i>-anisyl)Âformamidinate) and donor atoms
E and E′ (O or S). The radical cations {[Mo<sub>2</sub>]–(ph)<sub>2</sub>–[Mo<sub>2</sub>]}<sup>+</sup>, prepared by one-electron
oxidation of the corresponding neutral precursor, exhibit a characteristic
intervalence (IV) charge transfer absorbance in the near-IR spectra.
The electronic coupling matrix elements (<i>H</i><sub>ab</sub>) calculated from the Mulliken–Hush expression vary in the
range of 245–415 cm<sup>–1</sup> depending on the number
of sulfur atoms in the [Mo<sub>2</sub>] units. These parameters are
also calculated by CNS superexchange formalism, in which only the
electron-hopping pathway is taken into account because of the lack
of ligand to metal charge transfer absorptions in the spectra. The
results show remarkable alignment between the two different methods.
Thus, the mixed-valence complexes are assigned to weakly coupled Class
II in terms of Robin–Day’s classification. Under the
Marcus–Hush theoretical framework, the adiabatic electron transfer
rate constants (<i>k</i><sub>et</sub>) are optically determined
in the range of 10<sup>9</sup> – 10<sup>11</sup> s<sup>–1</sup>. The fastest electron transfer is observed in the fully thiolated
species. In comparison with the reported (ph)<sub>1</sub> series,
a relatively small attenuation factor, ca. βÂ(<i>H</i><sub>ab</sub>) 0.17, is also estimated for the tetrathiolated system.
Therefore, the introduction of sulfur atoms along the charge transfer
axis efficiently enhances the electronic coupling and facilitates
the electron transfer between the two dimetal centers
Spectroscopic Study of δ Electron Transfer between Two Covalently Bonded Dimolybdenum Units via a Conjugated Bridge: Adequate Complex Models to Test the Existing Theories for Electronic Coupling
Three symmetrical and one unsymmetrical
dimolybdenum dimers, namely,
[Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(E<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CE<sub>2</sub>) (DAniF = <i>N</i>,<i>N</i>′-diÂ(<i>p</i>-anisyl)Âformamidinate and
E = O or S), are structurally and electronically closely related.
The mixed-valence cation radicals display well-defined metal to ligand
(ML), ligand to metal (LM), and metal to metal (MM) charge transfer
absorption bands. Successive thiolations of the complexes result in
steady increases of the electronic coupling between the two [Mo<sub>2</sub>] units. The electronic coupling matrix elements (<i>H</i><sub>ab</sub>) calculated from the Hush model fall in the
range of 600–900 cm<sup>–1</sup>, which are remarkably
consistent with the results from the CNS superexchange formalism.
Spectroscopic analyses suggest that the intramolecular electron transfer
occurs by electron-hopping and hole-hopping in concert. The rate constants
(<i>k</i><sub>et</sub>) are estimated in the range of 10<sup>11</sup>–10<sup>12</sup> s<sup>–1</sup> for the symmetrical
analogues and 10<sup>7</sup> s<sup>–1</sup> for the unsymmetrical
species. The ultrafast electron transfer in such a weakly coupled
system (<i>H</i><sub>ab</sub> < 1000 cm<sup>–1</sup>) is attributed to the dÂ(δ)–pÂ(Ï€) conjugation between
the dimetal centers and the bridge
Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads
Three quadruply bonded
dimolybdenum complexes equipped with a triarylamine
pendant, [(DAniF)<sub>3</sub>Mo<sub>2</sub>(μ-O<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>NÂ(C<sub>6</sub>H<sub>4</sub>CH<sub>3</sub>)<sub>2</sub>] (DAniF = <i>N,N</i>′-diÂ(<i>p</i>-anisyl)Âformamidinate; [<b>OO–ph–N</b>]), [(DAniF)<sub>3</sub>Mo<sub>2</sub>(μ-OSCC<sub>6</sub>H<sub>4</sub>NÂ(C<sub>6</sub>H<sub>4</sub>CH<sub>3</sub>)<sub>2</sub>] ([<b>OS–ph–N</b>]), and [(DAniF)<sub>3</sub>Mo<sub>2</sub>(μ-S<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>NÂ(C<sub>6</sub>H<sub>4</sub>CH<sub>3</sub>)<sub>2</sub>] ([<b>SS–ph–N</b>]), have been synthesized
and characterized by single crystal X-ray diffraction. In electrochemical
measurements, the redox couple for the organic amine group becomes
irreversible, reflecting the substantially strong electronic interaction
between the dimetal center and organic redox site. The potential difference
for the two successive redox events, ca. Δ<i>E</i><sub>1/2</sub>(<i>E</i><sub>1/2</sub>(2)Â(N/N<sup><b>•</b>+</sup>) – <i>E</i><sub>1/2</sub>(1)Â(Mo<sub>2</sub><sup>IV/V</sup>)), falls in the range of 0.5–0.8
V as estimated from the differential pulse voltammograms. For the
monocation radicals [<b>OO–ph–N</b>]<sup>+</sup>, [<b>OS–ph–N</b>]<sup>+</sup>, and [<b>SS–ph–N</b>]<sup>+</sup>, obtained by chemical oxidation of the neutral precursor,
a broad ligand (amine) to metal (Mo<sub>2</sub>) charge transfer (LMCT)
absorption band is observed in the near-IR region. Interestingly,
analogous to the intervalence charge transfer (IVCT) bands for mixed-valence
complexes, the LMCT absorption bands, which are solvent dependent,
decrease in energy and bandwidth as the electronic coupling between
the two redox sites increases in an order of increasing S content
in the chelating group. The electronic coupling matrix elements (<i>H</i><sub>ab</sub>) are determined by optical analyses from
the generalized Mulliken–Hush (GMH) theory, falling in the
range of 400–800 cm<sup>–1</sup> in CH<sub>2</sub>Cl<sub>2</sub>. These results indicate that in these radical cations the
charge is localized. Time-dependent DFT calculations show that the
frontier molecular orbitals for these asymmetrical donor–acceptor
systems have unbalanced distribution of electron density, and the
LMCT bands arise from an electronic transition from the pendant ligand-based
to metal-based molecular orbitals, corresponding to donor (N)–acceptor
(Mo<sub>2</sub>) charge transfer
Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group
Using
2,6-naphthalenedicarboxylate and its thiolated derivatives
as bridging ligands, three Mo<sub>2</sub> dimers of the type [Mo<sub>2</sub>(DAniF)<sub>3</sub>]Â(E<sub>2</sub>CC<sub>10</sub>H<sub>6</sub>CE<sub>2</sub>)Â[Mo<sub>2</sub>(DAniF)<sub>3</sub>] (DAniF = <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate;
E = O, S) have been synthesized and characterized by X-ray diffraction.
These compounds can be generally formulated as [Mo<sub>2</sub>]–naph–[Mo<sub>2</sub>], where the complex unit [Mo<sub>2</sub>] ([Mo<sub>2</sub>(DAniF)<sub>3</sub>(μ-E<sub>2</sub>C)]) functions as an electron
donor (acceptor) and the naphthalene (naph) group is the bridge. The
mixed-valence (MV) complexes, generated by one-electron oxidation
of the neutral precursors, display weak, very broad intervalence charge-transfer
absorption bands in the near-to-mid-IR regions. The electronic coupling
matrix elements for the MV complexes, <i>H</i><sub>ab</sub> = 390–570 cm<sup>–1</sup>, are calculated from the
Mulliken–Hush equation, which fall between those for the phenyl
(ph) and biphenyl (biph) analogues reported previously. The three
series consisting of three complexes with the same [Mo<sub>2</sub>] units exhibit exponential decay of <i>H</i><sub>ab</sub> as the bridge changes from ph to biph via naph, with decay factors
of 0.21–0.17 Å<sup>–1</sup>. Therefore, it is evidenced
that while the extent of the bridge conjugacy varies, the electronic
coupling between the two [Mo<sub>2</sub>] units is dominated by the
Mo<sub>2</sub>···Mo<sub>2</sub> separation. The absorption
band energies for metal-to-ligand charge transfer are in the middle
of those for the ph and biph analogues, which is consistent with variation
of the HOMO–LUMO energy gaps for the complex series. These
results indicate that the interplay of the bridge length and conjugacy
is to affect the enegy for charge transfer crossing the intervening
moiety, in accordance with a superechange mechanism
Abnormally Long-Range Diamagnetic Anisotropy Induced by Cyclic d<sub>δ</sub>–p<sub>π</sub> π Conjugation within a Six-Membered Dimolybdenum/Chalcogen Ring
Incorporating two quadruply bonded dimolybdenum units
[Mo<sub>2</sub>(DAniF)<sub><sup>3</sup></sub>]<sup>+</sup> (ancillary
ligand DAniF
= <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate) with two hydroselenides (SeH<sup>–</sup>) gave rise to [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(μ-SeH)<sub>2</sub> (<b>1</b>). With the molecular scaffold remaining unchanged,
aerobic oxidation of <b>1</b>, followed by autodeprotonation,
generated [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>(μ-Se)<sub>2</sub> (<b>2</b>). The two complexes share a common cyclic
six-membered Mo<sub>2</sub>/Se core, but compound <b>2</b> is
distinct from <b>1</b> by having structural, electronic, and
magnetic properties that correspond with aromaticity. Importantly,
the aromatic behaviors for this non-carbon system are ascribable to
the bonding analogy between the δ component in a Mo–Mo
quadruple bond and the π component in a C–C double bond.
Cyclic π delocalization via d<sub>δ</sub>–p<sub>π</sub> conjugation within the central unit, which involves
six π electrons with one electron from each of the Mo<sub>2</sub> units and two electrons from each of the bridging atoms, has been
confirmed in a previous work on the oxygen- and sulfur-bridged analogues
(Fang, W.; et al. <i>Chem.î—¸Eur. J.</i> <b>2011</b>, <i>17</i>, 10288). Of the three members in this family,
compound <b>2</b> exhibits an enhanced aromaticity because of
the selenium bridges. The remote in-plane and out-of-plane <i>methine</i> (ArNC<i>H</i>NAr) protons resonate at
chemical shifts (δ) 9.42 and 7.84 ppm, respectively. This NMR
displacement, Δδ = 1.58 ppm, is larger than that for the
oxygen-bridged (1.30 ppm) and sulfur-bridged (1.49 ppm) derivatives.
The abnormally long-range shielding effects and the large diamagnetic
anisotropy for this complex system can be rationalized by the induced
ring currents circulating the Mo<sub>2</sub>/chalcogen core. By employment
of the McConnell equation {Δσ = ΔχÂ[(l –
3 cos 2θ)/3<i>R</i><sup>3</sup><i>N</i>]},
the magnetic anisotropy (Δχ = χ<sub>⊥</sub> – χ<sub>||</sub>) is estimated to be −414 ppm
cgs, which is dramatically larger than −62.9 ppm cgs for benzene,
the paradigm of aromaticity. In addition, it is found that the magnitude
of Δχ is linearly related to the radius of the bridging
atoms, with the selenium analogue having the largest value. This aromaticity
sequence is in agreement with that for the chalcogen-containing aromatic
family, e.g., furan < thiophene < selenophene
Optical Determination of Electron Transfer Dynamics and Kinetics for Asymmetrical [Mo<sub>2</sub>]–ph–[Mo<sub>2</sub>] Systems
We
report the study, in terms of electronic coupling (EC) and electron
transfer (ET), on three asymmetrical Mo<sub>2</sub> dimers [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-(NH)ÂOCC<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>] ([<b>NO–ph–OO</b>]) (DAniF = <i>N</i>,<i>N</i>′-diÂ(<i>p</i>-anisyl)Âformamidinate), [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-(NH)<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>] ([<b>NN–ph–OO</b>]), and [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-(NH)<sub>2</sub>CC<sub>6</sub>H<sub>4</sub>CÂ(NH)ÂO] ([<b>NN–ph–NO</b>]), which are closely related to the three symmetrical analogues
[<b>OO–ph–OO</b>], [<b>NO–ph–NO</b>], and [<b>NN–ph–NN</b>] reported earlier. The
mixed-valence (MV) complexes [<b>NO–ph–OO</b>]<sup>+</sup>, [<b>NN–ph–OO</b>]<sup>+</sup>, and [<b>NN–ph–NO</b>]<sup>+</sup> exhibit metal to ligand
and ligand to metal charge transfer bands, along with an intervalence
(IV) transition absorption in the Near-IR region. The free energy
change (Δ<i>G</i>°) for ET is determined by comparing
the redox potential splitting (Δ<i>E</i><sub>1/2</sub>) and IV transition energy (<i>E</i><sub>IT</sub>) with
the data for the symmetrical species. The reorganization energy (λ)
is estimated from the Hush model (Δν<sub>1/2</sub> = [16ln(2)<i>λRT</i>]<sup>1/2</sup>). Significantly, electrochemical
and optical analyses verify <i>E</i><sub>IT</sub> = Δ<i>G</i>° + λ, the core energetic relationship underlying
the semiclassical theories. With the electronic coupling parameters
calculated from the method suggested by Creutz, Newton and Sutin (<i>H</i><sub>MM′</sub> = ∼ 500 cm<sup>–1</sup>), the adiabatic ET rate constants <i>k</i><sub>et</sub> (<i>f</i>) are determined to be ∼10<sup>10</sup> s<sup>–1</sup> for [<b>NO–ph–OO</b>]<sup>+</sup> and [<b>NN–ph–NO</b>]<sup>+</sup>, smaller
than <i>k</i><sub>et</sub> (<i>r</i>) for the
backward reaction and <i>k</i><sub>et</sub> for the symmetrical
analogues by 1 order of magnitude, and ∼10<sup>9</sup> s<sup>–1</sup> for [<b>NN–ph–OO</b>]<sup>+</sup>. This work illustrates that the redox asymmetry in D–B–A
systems controls the ET rate and direction
Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group
Using
2,6-naphthalenedicarboxylate and its thiolated derivatives
as bridging ligands, three Mo<sub>2</sub> dimers of the type [Mo<sub>2</sub>(DAniF)<sub>3</sub>]Â(E<sub>2</sub>CC<sub>10</sub>H<sub>6</sub>CE<sub>2</sub>)Â[Mo<sub>2</sub>(DAniF)<sub>3</sub>] (DAniF = <i>N</i>,<i>N</i>′-di-<i>p</i>-anisylformamidinate;
E = O, S) have been synthesized and characterized by X-ray diffraction.
These compounds can be generally formulated as [Mo<sub>2</sub>]–naph–[Mo<sub>2</sub>], where the complex unit [Mo<sub>2</sub>] ([Mo<sub>2</sub>(DAniF)<sub>3</sub>(μ-E<sub>2</sub>C)]) functions as an electron
donor (acceptor) and the naphthalene (naph) group is the bridge. The
mixed-valence (MV) complexes, generated by one-electron oxidation
of the neutral precursors, display weak, very broad intervalence charge-transfer
absorption bands in the near-to-mid-IR regions. The electronic coupling
matrix elements for the MV complexes, <i>H</i><sub>ab</sub> = 390–570 cm<sup>–1</sup>, are calculated from the
Mulliken–Hush equation, which fall between those for the phenyl
(ph) and biphenyl (biph) analogues reported previously. The three
series consisting of three complexes with the same [Mo<sub>2</sub>] units exhibit exponential decay of <i>H</i><sub>ab</sub> as the bridge changes from ph to biph via naph, with decay factors
of 0.21–0.17 Å<sup>–1</sup>. Therefore, it is evidenced
that while the extent of the bridge conjugacy varies, the electronic
coupling between the two [Mo<sub>2</sub>] units is dominated by the
Mo<sub>2</sub>···Mo<sub>2</sub> separation. The absorption
band energies for metal-to-ligand charge transfer are in the middle
of those for the ph and biph analogues, which is consistent with variation
of the HOMO–LUMO energy gaps for the complex series. These
results indicate that the interplay of the bridge length and conjugacy
is to affect the enegy for charge transfer crossing the intervening
moiety, in accordance with a superechange mechanism
Electronic Coupling in [Mo<sub>2</sub>]–Bridge–[Mo<sub>2</sub>] Systems with Twisted Bridges
In
order to evaluate the impact of bridge conformation on electronic
coupling in donor–bridge–acceptor triad systems, two
Mo<sub>2</sub> dimers, [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-1,4-{CÂ(O)ÂNH}<sub>2</sub>-Naph] (<b>1</b>, DAniF = <i>N</i>,<i>N</i>′-diÂ(<i>p</i>-anisyl)Âformamidinate
and Naph = naphthalenyl) and [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-1,4-(CS<sub>2</sub>)<sub>2</sub>-2,5-Me<sub>2</sub>C<sub>6</sub>H<sub>2</sub>] (<b>2</b>), have been synthesized
and structurally characterized. These two compounds feature a large
dihedral angle (>60°) between the central aromatic ring and
the plane defined by the Mo–Mo bond vectors, which is distinct
from the previously reported phenylene bridged analogues [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-1,4-{CÂ(O)ÂNH}<sub>2</sub>-ph] (<b>I</b>) and [Mo<sub>2</sub>(DAniF)<sub>3</sub>]<sub>2</sub>[μ-1,4-(CS<sub>2</sub>)<sub>2</sub>-C<sub>6</sub>H<sub>4</sub>] (<b>II</b>), respectively. Unusual optical behaviors
are observed for the mixed-valence (MV) species (<b>1</b><sup>+</sup> and <b>2</b><sup>+</sup>), generated by single-electron
oxidation. While <b>2</b><sup>+</sup> exhibits a weak intervalence
charge transfer (IVCT) absorption band in the near-IR region, the
IVCT band is absent in the spectrum of <b>1</b><sup>+</sup>,
which is quite different from what observed for <b>I</b><sup>+</sup> and <b>II</b><sup>+</sup>. Optical analyses, based
on superexchange formalism and Hush model, indicate that, in terms
of Robin–Day classification, mixed-valence species <b>1</b><sup>+</sup> belongs to the electronically uncoupled Class I and
complex <b>2</b><sup>+</sup>, with <i>H</i><sub>ab</sub> = 220 cm<sup>–1</sup>, is assigned to the weakly coupled
Class II. Together with <b>I</b><sup>+</sup> and <b>II</b><sup>+</sup>, the four MV complexes complete a transition from Class
I to Class II–III borderline as a result of manipulating the
geometric topology of the bridge. Given the structural and electronic
features for the molecular systems, the impacts of electrostatic interaction
(through-space) and electron resonance (through-bond) on electronic
coupling are discussed