Dinickel II Compounds

The -1-2 oxidation state of nickel (and also for Pd and Pt) gives rise to a stable closed-shell d electronic configuration, thus it is the most prevalent for nickel in general inorganic chemistry. However, where metal-metal bonds are concerned, and from a MO perspective, the dinickel(ll) Ni moiety has an electronic configuration of which leads to a net bond order of 0 which will clearly be 

In the last decade, a few of dinickel(ll) compounds with a Ni(ll)-Ni(ll) bond were reported, although they are not that prevalent compared to their dinickel(l) analogs. In the following section, a summary of these advances will be presented. 

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When complex lb, containing a ( j,-aquo)bis( j,-carboxylato) core, was reacted with 1 equiv of trimethylsilyl triflate, followed by the addition of urea, complex 50 could be isolated in 55% yield (Scheme 10) (110). A mononuclear species [Ni(OAc)(urea)2(tmeda)](OTf) was produced as a minor side product in this reaction. Complex 50 represented the first example of a synthetic compound with urea coordinated to a dinickel(II) site. The urea molecule binds in its most common mode, namely, through its carbonyl O, to one of the Ni(II) ions (Fig. 8). 

Dithiolate compounds are similar to the carboxylates, with sulfur atoms taking the place of the oxygen atoms in the polar head. Each of the tetrakis(alkyldithiolato)dinickel(II) complexes ((101) R = C H2 +i, n = 4- ) shows a monotropic lamellar mesophase between 80and 90°C, which has been described as discotic lamellar (Dl) phase (see Figure 22). In the case where R is a branched chain, an enantiotropic Colh and a monotropic Dl mesophases have been found for tetrakis( 1 -ethylpentyldithiolato)dinickel(II) complex. ... 

Reactivity studies of the dinickel(I) compound 13 were carried out (Scheme 10.6) [10]. The reaction between 13 and CO gas produces the dinickel(I) carbonyl compound 15, with the Ni(I)-Ni(I) bond length elongated from 2.2908(11) in 13 to 2.437(1) A in 15. When 13 was treated with trimethylsilyl azide (MejSiNj), a dinickel(II) azide 16 was obtained. The two Ni(II) centers... 

Another example of this type of neutrtil NHC-supported dinickel(I) compound is dinickel(I) p-iodide p-nitrosyl compound 57, in which the two Ni(I) centers were bridged by an iodide and a nitrosyl, and capped by the NHCs. Here, the Ni(I)-Ni(I) bond length is 2.314(1)A (Entry 25, Table 10.2). Complex 57 was made from the reduction of the Ni(II) nitrosyl iodide monomer [Ni(II)( PPr)(NO)(l)] by sodium amalgam (Scheme 10.24) [24]. 

Related dinickel(II) p-N bridging compounds with NHC ligands were reported by Hillhouse et al. in 2009 (Scheme 10.31) [40]. [Ni(I)(p-Cl)(l Pr)]2 underwent one-electron oxidation by MesNj to afford a dinickel(II) bridging imide [ Ni(ll)(l Pr)(Cl) 2(p-NMes)] 66, in which one of the chlorides can be abstracted by NaBAr, to produce the dinickel(ll) ion pair compound 67. The Ni(ll)-Ni(ll) bond lengths in 66 and 67 are 2.5767(15) and 2.2911(8) A, respectively (Entries 5 and 6, Table 10.3). Complex 66 can also undergo one-electron reduction of the [(Ni(ll)Cl 2(p-NMes)] core, to afford a mixed-valent Ni(I)-Ni(II) compound 68, which upon treating with a one-electron oxidant gives 67 as the product. It is noteworthy that 67 and 68 bear similar core structures, but the Ni-Ni bond... 

A dinickel(ll) p-methylene compound 71 supported by a bulky Cp derivative was reported in 2011 (Scheme 10.33) [42]. The formation of 71 waspostulatedtobeasaresult ofa-H abstraction of a Ni(II) methyl compound and following intermolecular proton abstraction. Complex 71 is a diamagnetic compound. The valence-electron counting requires a Ni(ll)=Ni(ll) double bond, and the bond length is 2.3158(10) A (Entry 9, Table 10.3). 

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