1. Introduction

One of
the major factors which implies molecular reactivity in chemistry is electron
affinity (EA). Explicitly, species with high EA typically act as strong
oxidizing agents in chemical processes and capture an excess electron to become
strongly bound negative ions. As it is well known, among the chemical elements
halogen atoms exhibit the largest EAs 
(fluorine 3.40 eV, chlorine 3.62 eV) 1. The unique
characteristic of halogen atoms has allowed for the creation of hypervalent
clusters consisting of an excess of these electronegative atoms, which, due to
collective effects, have a higher EA energies than that of the halogen atoms. These
hypervalent structures were given the name “superhalogens” by Gutsev and
Boldyrev who proposed their existence in the early 1980s  and have introduced a simple MXk+1
formula, where M is the main group or the transition metal atom, X is a halogen
atom, and  k is the maximal formal valence of the atom M.2 Up to now, a
great number of theoretical and experimental investigations have been carried
out to obtain novel superhalogens and find ways to enlarge their application
scope.3-18 The main purpose of investigating various novel superhalogen
species is to deliver reliable data and predictions considering the possible
use of such compounds as electron acceptors (oxidizing agents). Superhalogens due
to their strong oxidizing capability can be used to access the high oxidation
states otherwise inaccessible in conventional chemistry. In particular,
superhalogens were proved to be able to oxidize molecules with high ionization
potential, like nanoparticles 19 and noble gas atoms 20. Moreover,
the superhalogens have considerable application potential in the synthesis of
novel chemical compounds as well as in serving as promising building blocks of
new functional (nano)materials, such as nonlinear optical materials 10 and
magnetic materials.

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Polynuclear
superhalogens (MnXn·k+1), of which the number of central
atoms (n) is larger than one, possess the advantage of increasing the number of
ligands while maintaining their high stability and therefore exploring
polynuclear superhalogens has become a essential new research direction. Examples include magnesia halogens whose electron
affinities not only confirming their superhalogen nature but also reveals EA
values are strongly contingent on the number of central metal atoms. Namely,
the vertical extra electron detachment energies (VDEs) of the superhalogen MgF3?
(VDE=8.79 eV)7 and MgCl3? (VDE=6.68 eV)7 anions increase by 0.59, 1.02, and 1.41 eV (for respectively Mg2F5? (VDE=9.38 eV)9, Mg2Cl5? (VDE=7.70 eV)8, Mg3Cl7? (VDE=8.09 eV)8) when the number of central metal atoms (and halogen ligands, to match
the MgnF2n+1 formula) increased. Successful utilizing
magnesia halogens as superhalogens, capable of forming strongly bound anionic systems,
turn us to the extension of this concept for trinuclear Mg3F7
clusters. 

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