Partial quadrupole splitting

Partial quadrupole splittings in inorganic chemistry. G. M. Bancroft. Coord. Chem. Rev., 1973,11, 247-262 (53). 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

The QS in the Fe Mdssbauer spectra is determined by the geometry of the iron(II) coordination polyhedron and has been utilized to gain information on the structure of the complexes from the spectral parameter vs structure correlation. Conversely, the data available on the structure of clathrochelates employed for working out a modern version of the partial quadrupole splitting (PQS) concept permits one to obtain absolute PQS values and to analyse the results for macrobicyclic complexes [264, 265]. 

A very simple concept of partial quadrupole splitting (PQS) was developed in order to explain the peculiarities of QS in various compounds [275-277]. For example, the substitution of one or two ligands B in an octahedral low-spin iron complex FeBe (QS=0) results in the following QS values ... 

The observed lack of a quadrupole splitting for [FeCI(H)(depe)2] suggests that (might be an additive function of the ligands. It should therefore be possible to assign scales of partial quadrupole splitting parameters to the ligands, and this is illustrated below. Such scales allow... 

The simplest treatment of this situation is analogous to the crystal field approach for transition metal complexes. The ligands are considered to act as point charges, and each is assigned a partial quadrupole splitting parameter p.q.s.(L), which is assumed to be independent of the other ligands present, and to have a constant value p.q.s.(L) thus corresponds to an effective value of and different values may apply for different... 

It is usually found that the two methods lead to analogous expressions. It is therefore convenient to use the point charge treatment, which appears also to be able to take account of distortions from regular geometry (Parish Johnson, 1971 Sham Bancroft, 1975). The hybridisation method has the virtue of emphasising that different scales of partial quadrupole splitting values are needed for different coordination geometries. 

Of the main group elements, extensive partial quadrupole splitting scales have been evaluated for tin(IV) (Table 2.27). As indicated above, different... 

Table 2.25. Partial quadrupole splitting values for ligands coordinated to...

Table 2.26. Derivation of the partial quadrupole splitting scale for ruthenium(ll)...

Table 2.27. Partial quadrupole splitting scales for ligands coordinated to tin(IV) in four-, five-, and six-coordination...

Both parameters are sensitive to the donor-acceptor characteristics of a ligand, but they respond differently. The partial isomer shift represents the contribution to s electron density at the nucleus, which is increased both by o-donation, and by Ti-withdrawal of d electron density. The partial quadrupole splitting represents charge placed in the d orbitals, and becomes more negative as a-donation increases, but more positive with increase in metal to ligand rr-bonding. This means that... 

Fig. 2.15. Correlation diagrams (partial quadrupole splitting against partial isomer shift) for (a) iridium(III), and (b) ruthenium(II).

Nuclear quadrupole coupling constants and asymmetry parameters [in brackets] are from single crystal studies. NQCC( Mn) values have also been predicted from partial quadrupole splitting parameters for [Mn(CO)5L] (L = CN, CNR, PPhj, NCS, N3, SbPhj, and and calculated from Wy2 of the NMR resonance for [MnH(CO)5 ... 

Calculated from partial quadrupole splitting parameters. 

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