Dinickel bridging ligands

Structural characterization of the lantern-type dipalladium(II) (9) and dinickel(II) (10) complexes appeared shortly before the report of K2[Pt2(S04)4(H20)2]. Their chemistry did not develop as much as that of the platinum complexes, however. This is probably because of the limitation of the bridging ligands to give the lantern-type Pd and Ni complexes and also of a lesser trend of these metal ions to give stable complexes with higher oxidation states. 

A dinickel(I) compound 17 was made from the reaction between metallacyclic Ni(n) carboxylate ( nickelalactone ) and bis(diphenylphosphino)methane (dppm) (Scheme 10.7) [11]. The Ni(I)-Ni(I) bond length in 17 is 2.563(1) A (Entry 4, Table 10.2), and features three different bridging ligands (dppm, carboxylate, diphenylphosphido). The formation of 17 was proposed to proceed via the mechanism depicted in Scheme 10.8, and is remarkable because it acts as a model for the key step in the formation of acrylic acid from COj and ethylene. 

Pyrazolato-bridged bis(tacn) ligands and their dinickel complexes are mentioned in Sections 6.3.4.12.6 and 6.3.4.12.7. 

The flexible ligand (731) forms stable dinickel(II) complexes both in solution and in the solid state. The crystal structure of [Ni2(731)Cl3] reveals the bridging phenolato-O, a secondary Cl bridge, and two terminal Cl ligands, i.e., six-coordinate metal ions.1885... 

An even shorter Ni Ni distance of 3.17 A was observed in a related dinickel complex (5) that has longer chelate arms of the ligand scaffold and a ()0,-phenolato)-( a,-hydroxo) core (56). Addition of acid was suggested to protonate the bridging hydroxide with subsequent coordination of water molecules and six-coordinate metal ions, with pAia 8.5 for the Ni-bound water derived from a pH-metric titration. Unfortunately, the protonated product was not further characterized. 

In methanol solution, the terminal water ligands in the phtalazine-based dinickel complex 32 (see Section 11.B.2) can be replaced by O-bound urea to give 75 (Fig. 15). When acetonitrile is used as the solvent instead of methanol, one of the substrate molecules shifts to an unusual single atom O-bridging position in 76 (the only other known example in dinickel chemistry being 56). Solid-state (IR) spectra reveal a shift of the C=0 stretching frequency of urea from 1690 to 1663 cm in 75 or 1661 cm in 76upon coordination to the dinickel sites. Due to... 

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