Collinear complexes

Coordination number 2 collinear. Collmear complexes are common in the case of heavy metal cations of electron configuration. Examples of collinear complexes are [Au(CN)2], formed during the extraction of gold from its ore, and [Ag(NHj)2] formed when AgCl dissolves in ammonia solution. 

For example, the rate constant of the collinear reaction H -f- H2 has been calculated in the temperature interval 200-1000 K. The quantum correction factor, i.e., the ratio of the actual rate constant to that given by CLTST, has been found to reach 50 at T = 200 K. However, in the reactions that we regard as low-temperature ones, this factor may be as large as ten orders of magnitude (see introduction). That is why the present state of affairs in QTST, which is well suited for flnding quantum contributions to gas-phase rate constants, does not presently allow one to use it as a numerical tool to study complex low-temperature conversions, at least without further approximations such as the WKB one. ... 

Figure 11. The minimum energy path of the OH + CH3F reaction, not including zero-point energy. The four labeled structures are (A), the central barrier TS (B), the nearly collinear backside well complex [HOCH3 F] (C) the transition of the F atom toward the OH moiety (D) the hydrogen-bonded [CH3OH F ] structure. Reprinted from [63] with permission from the American Association for the Advancement of Science.

The main difference between hydrogen bond and the halogen bond lies in the propensity of the hydrogen bond to be non-linear [28,29], when symmetry of the complex is appropriate (molecular point group Cs or Ci). In so far as complexes B- ClF are concerned, the nuclei Z Cl - F, where Z is the acceptor atom/centre in B, appear to be nearly collinear in all cases, while the nuclei Z- H - Cl in complexes B- HC1 of appropriate symmetry often show significant deviations from collinearity. This propensity for the hydrogen-bonded species B- HC1 to exhibit non-linear hydrogen bonds can be understood as follows. 

First, the Dy3 triangle reported by Powell et al., showing SMM behaviour of thermally excited spin states, can be regarded as the archetype of the non-collinear Ising antiferromagnetic arrangement in triangular lattices [28], as seen in several coupled Dy3 (Dy6) complexes [13, 29, 30]. Second, the excellent... 

The 1 1 reaction of 76 or 77 (R = Me, R = H) with bromine resulted in oxidation and the formation of a carbene complex of SBr2 (79) or SeBr2 (80), respectively (55, 58-62). The X-ray crystal structure of 79 has been reported and showed that this sulfurane possesses an almost collinear axial three-center geometry with elongated S-Br... 

Note The Ne-He An11 bond is almost linear (theH-bond angle ZN6H6Aun= 175.2 ). The very large polarity of this complex (dtot = 8.28 D) results from the fact that the Aui2-Auio-Ni bond dipole is almost collinear to the dipole moment of adenine ... 

When again a second order model was used for the data it appeared that there still was some lack of fit. Therefore models were constructed which included third order model terms. Further examination of these more complicated models however revealed that there was a much too high collinearity in these models. This means that there are systematical effects in the data which cannot be explained in a completely satisfactory manner. It was decided to use the second order model and to neglect further model complexities. The model validation results of the second order model can be found in Table 6.5. 

Note that both the collinearity problem and the requirement of having more samples than sensors can be solved by using regression techniques which can handle collinear data, such as factor-based methods such as PCR and PLS. These use linear combinations of all the variables and reduce the number of regressor variables. PCR or PLS are usually preferred instead of ILS, although they are mathematically more complex. 

---