Nickel vibrational coordinates

In order to illustrate the vibrational motions of a molecule belonging to a non-commutative symmetry point group, we return to the considerations of Section 2.3.2 and once more use as our example the square-planar complex, NiFj. A non-linear penta-atomic molecule has nine independent vibrational coordinates, distributed among the symmetry species of T>4h. These can be fully specified by standard methods [7], but the following simple qualitative considerations allow us to conclude that there are seven in-plane and two out-of-plane vibrations. Fig. 4.10 depicts several of the in-plane modes the motion of the nickel atom to conserve the center of mass is implied. 

Furthermore, the binding energy difference between the 4a and 5a carbon monoxide molecular orbitals, A(4o-5o), varies by only 0.3 eV ( 7 7, 82-88). The vibrational spectra show tremendous differences, however. Both nickel (89) and palladium (68) form multiply coordinated carbonyl species at low CO exposures and the atop species are only seen at high coverage. 

The nickel chloride complex(X) of the spiro-cyclic phosphazene(V) is diamagnetic suggesting a square planar coordination around nickel. The P=N stretching vibration for the complex appears as two split bands(1215 and 1183 cm-1) compared to a single band at 1200 cm-1 observed for the ligand. The phosphorus chemical shifts move upfield compared to those of the ligand the spiro phosphorus atom is the one most affected [complex SpR 21.9 Sp(spir0) 25.5 ... 

The results of these experiments form a picture of the dominant features of the methane-nickel surface interaction potential that control the mechanism of the dissociation of methane. We will find that there is indeed a barrier to the dissociative chemisorption of methane and that translational and vibrational energy of the incident methane molecule are effective in overcoming 1t. The identification of this barrier along the dissociative reaction coordinate allows the establishment of a link between low pressure, ultrahigh vacuum surface science and high pressure catalysis (ref. 3). 

It should be noted that the central atom of a highly symmetrical molecule (T, Oh, etc.) does not move during the totally symmetric vibration. Thus, no metal-isotope shifts are expected in these cases [90]. When the central atom is coordinated by several different donor atoms, multiple isotope labeling is necessary to distinguish different coordinate bond-stretching vibrations. For example, complete assignments of bis (glycino)nickel(II) require and/or isotope shift data as well as... 

Nickel is the only metal to react directly with carbon monoxide at room temperature at an appreciable rate, although iron does so on heating under pressure. Cobalt affords HCo(CO)4 with a mixture of hydrogen and carbon monoxide (p. 387). In general, therefore, direct reaction does not provide a route to metal carbonyls. The metal atom technique (p. 313) has been used to prepare carbonyls of other metals in the laboratory e.g. Cr(CO)g, but it offers no advantages over the reduction method discussed below. When metal vapours are cocondensed with carbon monoxide in frozen noble gas matrices at very low temperatures (4-20K) the formation of carbonyl complexes is observed. These include compounds of metals which do not form any stable isolable derivatives e.g. Ti(CO), Nb(CO) and Ta(CO)g as well as Pd(C0)4 and Pt(C0)4. Vibrational spectra of the matrix show that coordinatively unsaturated species such as Ni(CO) n = 1-3) or Cr(CO) (n = 3-5) are also formed under these conditions. 

There is now a growing literature of nickel organometallic complexes that contain carbon dioxide or related cumulene ligands that result from reactions with carbon monoxide. The first structurally characterized complex of carbon dioxide was the nickel complex Ni(G02)(PCy3)2 reported in 1975. A more recent study of this complex provides the complete assignments of the vibrational spectra and theoretical calculations of different isomers in support of a mechanism for CO2 fluxionality that involves end-on coordination. The tridentate pincer ligand 2,6-bis((diiso-propylphosphino)methyl)phenyl (PGP) has been used to form the square-planar Ni(ii) hydroxide complex Ni(OH)(PGP). The complex Ni(OH)(PCP) reacts with CO to give a binuclear /X-GO2 complex (Equation (2)). 

Figure 6.15. Transition state of CH4 dissociation on a nickel atom. Characteristic frequencies for the stretch vibrations of the bonds have been indicated the imaginary frequency represents the reaction coordinate (from Burghgraef et al.y 1993).

The IR spectrum of NiCO isolated in solid argon gave assignments to Vi, V3 and V5 modes, with isotopic shifts.The IR spectrum of CO adsorbed on Nin clusters shows the presence of 4 (vibrationally-coupled) CO molecules per cluster. FTIR spectra (vCO) were used to probe the effects of co-adsorption of on-top CO on bridge CO on a Ni(lll) surface.The FTIR spectrum of CO on an anodic nickel oxide surface had a band at 2112 cm assigned to CO adsorbed to Ni(II) or Ni(0) sites perturbed by oxidation of neighbouring nickel atoms. The geometry of CO or NO coordination on NiO(lll) thin films was deduced from the vCO and vNO values. 

The vibrational spectrum of single-crystal Ni (0EP) C104 contained the marker band expected for an OEP species. Resonance Raman spectra were used to probe axial coordination and conformational heterogeneity of Ni(TPP) in the presence of nitrogenous bases. Similar experiments gave information on the protein-induced changes in non-planarity of the prophyrin in nickel cytochrome... 

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