therefore requires that these interactions be limited via the incorporation of a
suitable ‘‘surface’’ on a metal chalcogenide core. This surface is typically formed
by the use of heteroligands such as tertiary phosphines or amines or organochalcogenolate anions RE
À . These ligands kinetically stabilize the metal chalcogenide cluster core by bonding at the cluster surface, preventing further condensation to bulk materials.
Chalcogenide ligands typically adopt m 3 - and m 4 - and higher coordination interactions in high nuclearity clusters. The three- and four-coordinate bridging modes
are the most common for chalcogenide ligands. The ability to bridge metal centers
increases on going down the group from sulfur to selenium to tellurium, and this
tendency can be explained as a consequence of the larger ionic radii, and thus
larger polarizability of the heavier elements.
The organochalcogenolate ligands (RE
À ) constitute a very important ligand type
in nanometer-sized metal-chalcogen complexes when used in conjunction with E
2À
ligands. Like their chalcogenide counterparts, they demonstrate a strong tendency
to bridge metal centers. The potential to modify the coordinating ability of these
ligands by changing the organosubstituent (R) has resulted in these ligands being
coined ‘‘chalcogenide with a handle’’ [4]. The most common bonding mode of RE
À
is the doubly bridging (m 2 ) type, which is observed in clusters with thiolate, selenolate, and tellurolate ligands. Triply and higher bridging coordination modes are
more common for selenolate and tellurolate ligands than for thiolates, again
attributable to the larger size of the heavier chalcogens [2].
In the following sections, an overview of the recent developments in the
synthesis of structurally characterized late metal-chalcogen nanocluster semiconductor complexes is given. Relationships and contrasts of the structural and
photophysical properties of these molecules compared to solid materials are
also discussed. Late metal-chalcogen and -pnictogen clusters typically display condensed core structures, and can thus be contrasted with the more open frameworks observed with the earlier metals [10]. This point is highlighted with the
report of the nanometer-sized open iron–selenium framework of [Fe 20 Se 38 ]
18À (1,
Figure 13.1) [11].
13.2
Nickel Chalcogenides
The use of trialkylphosphine-selenide and -telluride ligands has been developed for
the generation of a series of high nuclearity complexes of nickel-chalcogenides,
building upon the successful isolation of lower-nuclearity metal-chalcogen complexes of Cr, Mn, Fe, Co and Pd [12–16]. The reagents R 3 PbE can be viewed as
soluble, organometallic sources of elemental chalcogen, where the selenide and
(especially) telluride derivatives have sufficiently poor EbP orbital overlap, such
that they can act as transfer agents of zerovalent chalcogen [17]. These reagents
have been used to transfer selenium or tellurium ligands to zerovalent metals with
sufficiently labile ligands (Eq. (1)) [18].
13.2 Nickel Chalcogenides 419
suitable ‘‘surface’’ on a metal chalcogenide core. This surface is typically formed
by the use of heteroligands such as tertiary phosphines or amines or organochalcogenolate anions RE
À . These ligands kinetically stabilize the metal chalcogenide cluster core by bonding at the cluster surface, preventing further condensation to bulk materials.
Chalcogenide ligands typically adopt m 3 - and m 4 - and higher coordination interactions in high nuclearity clusters. The three- and four-coordinate bridging modes
are the most common for chalcogenide ligands. The ability to bridge metal centers
increases on going down the group from sulfur to selenium to tellurium, and this
tendency can be explained as a consequence of the larger ionic radii, and thus
larger polarizability of the heavier elements.
The organochalcogenolate ligands (RE
À ) constitute a very important ligand type
in nanometer-sized metal-chalcogen complexes when used in conjunction with E
2À
ligands. Like their chalcogenide counterparts, they demonstrate a strong tendency
to bridge metal centers. The potential to modify the coordinating ability of these
ligands by changing the organosubstituent (R) has resulted in these ligands being
coined ‘‘chalcogenide with a handle’’ [4]. The most common bonding mode of RE
À
is the doubly bridging (m 2 ) type, which is observed in clusters with thiolate, selenolate, and tellurolate ligands. Triply and higher bridging coordination modes are
more common for selenolate and tellurolate ligands than for thiolates, again
attributable to the larger size of the heavier chalcogens [2].
In the following sections, an overview of the recent developments in the
synthesis of structurally characterized late metal-chalcogen nanocluster semiconductor complexes is given. Relationships and contrasts of the structural and
photophysical properties of these molecules compared to solid materials are
also discussed. Late metal-chalcogen and -pnictogen clusters typically display condensed core structures, and can thus be contrasted with the more open frameworks observed with the earlier metals [10]. This point is highlighted with the
report of the nanometer-sized open iron–selenium framework of [Fe 20 Se 38 ]
18À (1,
Figure 13.1) [11].
13.2
Nickel Chalcogenides
The use of trialkylphosphine-selenide and -telluride ligands has been developed for
the generation of a series of high nuclearity complexes of nickel-chalcogenides,
building upon the successful isolation of lower-nuclearity metal-chalcogen complexes of Cr, Mn, Fe, Co and Pd [12–16]. The reagents R 3 PbE can be viewed as
soluble, organometallic sources of elemental chalcogen, where the selenide and
(especially) telluride derivatives have sufficiently poor EbP orbital overlap, such
that they can act as transfer agents of zerovalent chalcogen [17]. These reagents
have been used to transfer selenium or tellurium ligands to zerovalent metals with
sufficiently labile ligands (Eq. (1)) [18].
13.2 Nickel Chalcogenides 419