Adhikari et al.
responsible precursor in the process for desulfurization is a
bimetallic Ni(I) core bridged by two hydride ligands. Intuitively,
to understand the mechanism behind important transformations
such as C-C bond formation or desulfurization, it is necessary
to determine the electronic structure of the Ni(I) dimeric core,
especially since the Ni-Ni interaction appears weak enough
to promote redox events possibly via mononuclearity formation.
Bimetallic cores consisting of the most accessible oxidation state
of Ni, namely, Ni(II) centers, have been scrutinized in depth to
address the electronic interaction between the metal ions, more
specifically, the mechanism for the communication among the
two metal centers via superexchange through spin delocalization
or spin polarization.10 On the contrary, bimetallic centers
consisting of Ni(I) ions are a very scarce occurrence and hence
lack detailed scrutiny.3-8 As a result, investigating the electronic
and magnetic interactions between two Ni(I) centers should
provide clues to how these systems, if any, cooperatively
perform important chemical transformations relating to bond-
breaking and -forming reactions. In general, understanding the
interaction between two metal centers represents an important
paradigm in coordination chemistry since it can lead to
predictions on how dinuclear moieties operate, in concert and
along the way to performing unusual transformations.
In a bimetallic motif consisting of two d9 metal centers,
such as in a Ni(I)/Ni(I) dinuclear complex, one can categorize
three different types of electronic states that depend greatly
on the strength of metal-metal interactions.11 In one case,
the two metal centers in question can be virtually noninter-
active, essentially displaying properties amenable to an
isolated d9 unit and more consistent with a paramagnetic
monomer. In a second case scenario, whereby there is weak
interaction, the metal centers can be communicating very
weakly, thus leading to ferromagnetically and antiferromag-
netically coupled ground states, depending on whether the
total high-spin or low-spin state is attained. In such a case
of weak interaction, it is expected that the triplet excited state
should be close in energy to the singlet ground state and
that such a low-lying excited state should be populated
thermally. In the third case scenario, the two metal centers
could be interacting strongly, thereby forming a metal-metal
bond which eventually displays diamagnetic behavior. De-
ciphering these types of interactions can be challenging,
given the interplay between metal-metal bonds, ferro- and
antiferromagnetic coupling pathways through space or the
bridging ligands, and a possible caveat involving a monomer-
dimer equilibrium scenario in solution.
In this manuscript, we present a bimetallic system where
two Ni(I) centers are assembled in a diamond-like core form,
and where the metal ions in question are very weakly
interacting despite the connective amido bridges between
them. Intuitively, two 17-electron metal centers in proximity
and bridged by a strong field ligand such as amido are
inclined to interact strongly via the bridge or through space
along the way to metal-metal bond formation. Applying
X-ray crystallography and other spectroscopic techniques,
we elucidate our dinuclear Ni2(I,I) complex to be a rare and
special example of weakly coupled bimetallic framework.
Herein, we describe the synthesis of an electronically unusual
dinuclear Ni(I) complex, [Ni(µ2-PNP)]2, supported by the
pincer ligand PNP (PNP- ) N[2-P(CHMe2)2-4-methylphe-
nyl]2).12 Despite having a short Ni · · · Ni distance, this
complex exists at room temperature as a diradical having a
Ni2N2 diamond core resting state. Although the solution
magnetization data suggest this core to equilibrate to two
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10480 Inorganic Chemistry, Vol. 47, No. 22, 2008