Nickel bridging complexes, structure

Davies, S. C., Evans, D. J., Hughes, D. L., Longhurst, S. and Sanders, J. R. (1999) Synthesis and structure of a thiolate-bridged nickel-iron complex Towards a mimic of the active site of NiFe-hydrogenase. Chem. Commun., 1999, 1935-6. 

The series of nickel(II) complexes (Scheme 17) demonstrates the variety of structures found when the S denticity, the bridge length between the benzene-dithiolate units or the donor atoms D in DS ligands (D = additional donor atom) are varied. 

The known crystal structures of these complexes show no undue strain upon the geometry inflicted by the tether length. Apparently, an ethyl bridge between the indenyl (flu-orenyl) part of the ligand and the NHC unit is sufficiently long. In a comparative study between a tethered [135] and an untethered [194] nickel(II) complex (see Figure 4.64), no significant differences in the steric parameters were reported. However, a downfield shift of A8 = 4.1 ppm (from 166.8 to 170.9 ppm) in the C-NMR spectrum for the tethered complex was considered by the authors to be due to a chelate effect [135]. 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(II), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickel(II) complex was found to exhibit unusual spectrochemical behavior (365). 

We have reported the synthesis of functionalized calixarene 60. The crystal structure of the nickel azide complex of this dinuclear host indicates two nickel ions bound (one centered in each of the appended lower rim macrocycles), with three unique azide 1,1 end on bridging ligands cascaded in between (Fig. 6) (152). 

A trinuclear nickel(II) complex, [Ni3(dmpz)4(acac)2], 43, has been obtained by reacting Ni(acac)2 with an equimolar amount of Na(dmpz) (71). Its X-ray crystal structure emphasized that the geometry of each nickel atom is square planar and these planes provide the zigzag mode. The terminal and center nickel atoms are bridged by nitrogen atoms of two dmpz groups. Furthermore, the terminal metal centers are coordinated to two oxygen atoms of acac. 

In 1969 Trofimenko (183) prepared the triply bridged anionic complex [(CO)3Mn(pi-pz)3Mn(CO)3] , from BrMn(CO)s and Na(pz). The structure was assigned on the basis of IR and NMR data. The nickel nitrosyl complex [(NO)Ni( -dmpz)3Ni(NO)] had been reported 10 years later (184). The two Ni(NO) moieties are bridged by three dmpz ligands, with the nickel atom existing in a distorted tetrahedral coordination geometry. 

---