Nickel carbonyl cluster complexes

Metal carbonyl clusters of groups 8-10: synthesis and catalysis

In this review article, we discuss advances in the chemistry of metal carbonyl clusters (MCCs) spanning the last three decades, with an emphasis on the more recent reports and those involving groups 8-10 elements. Synthetic methods have advanced and been refined, leading to higher-nuclearity clusters and a wider array of structures and nuclearities. Our understanding of the electronic structure in MCCs has advanced to a point where molecular chemistry tools and other advanced tools can probe their properties at a level of detail that surpasses that possible with other nanomaterials and solid-state materials. MCCs therefore advance our understanding of structure-property-reactivity correlations in other higher-nuclearity materials. With respect to catalysis, this article focuses only on homogeneous applications, but it includes both thermally and electrochemically driven catalysis. Applications in thermally driven catalysis have found success where the reaction conditions stabilise the compounds toward loss of CO. In more recent years, MCCs, which exhibit delocalised bonding and possess many electron-withdrawing CO ligands, have emerged as very stable and effective for reductive electrocatalysis reactions since reduction often strengthens M-C(O) bonds and since room-temperature reaction conditions are sufficient for driving the electrocatalysis.

Heterometallic Ni-Pt Chini-Type Carbonyl Clusters: An Example of Molecular Random Alloy Clusters

The direct reactions of homometallic [Ni₆(CO)₁₂]^2– and [Pt₆(CO)₁₂]^2– Chini carbonyl clusters result in heterometallic Ni–Pt Chini-type clusters of the general formula [Pt_6–xNi_x(CO)₁₂]^2– (x = 0–6). Their molecular structures have been determined by single-crystal X-ray diffraction (SC-XRD), showing a common octahedral (staggered, D_3d) structure analogous to that of [Ni₆(CO)₁₂]^2–, whereas [Pt₆(CO)₁₂]^2– displays a trigonal-prismatic (eclipsed, D_3h) structure. This structural change after replacing one single Pt with Ni may be classified as an alloying effect, and it has been theoretically investigated by DFT methods. Spectroscopic (IR and ¹⁹⁵Pt and ¹³C NMR) and ESI-MS studies indicate that mixtures of [Pt_6–xNi_x(CO)₁₂]^2– (x = 0–6) clusters are actually present in solution, whose compositions may be varied in an almost continuous way. Thus, they may be viewed as random alloy clusters whose overall compositions depend on the stoichiometry of the reagents.

Molecular Pt−Ni random alloy clusters adopting a Chini-type structure have been obtained upon mixing related homometallic clusters and investigated by structural, spectroscopic, and theoretical methods.

Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni11P(CO)18]3- and [H6-nNi31P4(CO)39]n- (n = 4 and 5)

The reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.5 equiv of PCl₃ affords the monophosphide [Ni₁₁P(CO)₁₈]^3- that in turn further reacts with PCl₃ resulting in the tetra-phosphide carbonyl cluster [HNi₃₁P₄(CO)₃₉]^5-. Alternatively, the latter can be obtained from the reaction of [NEt₄]₂[Ni₆(CO)₁₂] in thf with 0.8-0.9 equiv of PCl₃. The [HNi₃₁P₄(CO)₃₉]^5- penta-anion is reversibly protonated by strong acids leading to the [H₂Ni₃₁P₄(CO)₃₉]^4- tetra-anion, whereas deprotonation affords the [Ni₃₁P₄(CO)₃₉]^6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni₃₁P₄(CO)₃₉]^7- hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [Ni₁₁P(CO)₁₈]^3-, [HNi₃₁P₄(CO)₃₉]^5-, and [H₂Ni₃₁P₄(CO)₃₉]^4-. The reversible formation of the stable [Ni₁₁P(CO)₁₈]^4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni₁₁P(CO)₁₈]^3-. The redox changes of [HNi₃₁P₄(CO)₃₉]^5- show features of chemical reversibility and the vibrational spectra in the ν_CO region of the nine redox states of the cluster [HNi₃₁P₄(CO)₃₉]^n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H₂Ni₃₁P₄(CO)₃₉]^4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H₂Ni₃₁P₄(CO)₃₉]^n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi₃₁P₄(CO)₃₉]^5- and [H₂Ni₃₁P₄(CO)₃₉]^4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by ¹H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.

Selective synthesis of the [Ni36Co8C8(CO)48]6- octa-carbide carbonyl cluster by thermal decomposition of the [H2Ni22Co6C6(CO)36]4- hexa-carbide

The thermal decomposition in thf solution of [H2Ni22Co6C6(CO)36](4-) results in the new [HNi36Co8C8(CO)48](5-) bimetallic Ni-Co octa-carbide, which can be converted into the closely related [H6-nNi36Co8C8(CO)48](n-) (n = 3-6) polyhydrides by means of acid-base reactions. The structure of the [Ni36Co8C8(CO)48](6-) hexa-anion has been established via X-ray crystallography, showing that the eight interstitial carbide atoms are lodged within different metal cages. Thus, two C-atoms are enclosed within regular square anti-prismatic Ni8C cages, four within irregular Ni8C square anti-prismatic cages, and the last two within mono-capped trigonal prismatic Ni5Co2C cages. The structure of [Ni36Co8C8(CO)48](6-) is non-compact and closely related to [Ni32C6(CO)38](6-) and [HNi38C6(CO)44](5-). [Ni36Co8C8(CO)48](6-) approaches the nanosize regime and the whole molecular ion has a diameter (measured from the outer oxygen atoms) of ca. 1.61 nm.

Nickel poly-acetylide carbonyl clusters: structural features, bonding and electrochemical behaviour

The reactions of [NEt(4)](2)[Ni(6)(CO)(12)] with miscellaneous carbon halides (e.g. CCl(4), C(4)Cl(6)) in CH(2)Cl(2) have been extensively investigated particularly focusing on the stoichiometric ratio of the reagents and reaction temperature. This allowed the preparation of the previously known acetylide clusters [Ni(16)(C(2))(2)(CO)(23)](4-), [HNi(25)(C(2))(4)(CO)(32)](3-) and [Ni(22)(C(2))(4)(CO)(28)Cl](3-), as well as isolation and full characterisation of the closely related [Ni(17)(C(2))(2)(CO)(24)](4-) and [Ni(25)(C(2))(4)(CO)(32)](4-) tetraanions. From a structural point of view, all these clusters are based on a Ni(16) square orthobicupola which contain interstitial C(2), Ni(η(2)-C(2))(4) or Ni(2)(μ-η(2)-C(2))(4) moieties, displaying rather short C-C bonds. Electrochemical studies reveal that all these species undergo different redox processes, even if their stability is rather limited. This is corroborated by an extensive analysis of the interaction between interstitial C(2) acetylide units and the metal cluster cage by Extended Huckel Molecular Orbital (EHMO) calculations, which indicates that tightly bonded C-C units are less effective than isolated C-atoms in stabilising the cluster cage.