Ligand field theory planar fields

The first Ni Mossbauer spectrum of nickel in a bioinorganic compound with determinable EFG and isomer shift was reported for a nickel complex compound with planar [NiSJ core and considered as a model compound for hydrogenase. This Mossbauer spectrum from the formal Ni compound is presented in Fig. 7.16. The observed quadrupolar interaction can be understood in terms of ligand field theory. In this approach, the b g and levels (d y2 and d ) are not occupied which is expected to cause a large negative EFG contribution [32]. 

Any computational treatment of TM systems must account for the LFSE. QM methods achieve this implicitly but d-electron effects must be explicitly added to MM (4). Some effects can be modeled within conventional MM. For example, low-spin d8 complexes are planar by virtue of the LFSE (21,22), but a planar structure can also be enforced using a normal out-of-plane term (22). However, the simplest general model for describing d-orbital energies is ligand field theory (LFT) (23) which was itself derived from the earlier electrostatic crystal field theory (CFT) (24) approach. 

In the forthcoming discussion, it will be assumed that the reader is familiar with the basic concepts of ligand field theory in order to understand the preferences of a given central metal for certain coordination geometry. In general d5 and d6 complexes are octahedral and d8 are square-planar. 

Martin, D. S., Fenske, R. F., Ruedenberg, K. (1962) Ligand field theory of square planar platinum(II) complexes. Inorg. Chem. 1, 441. 

Square-planar complexes are extremely important in inorganie chemistry, and we will now discuss the bonding in these complexes from the perspective of ligand field theory. 

The trigonal planar complexes of the MX3 type, whether of the lanthanide or of the transition metal series, have a particular significance. In an amendment to the crystal field intensity theory, designed to accommodate the hypersensitive f—f transitions, the addition of the first-rank spherical harmonics, Yim to the ligand field was proposed in order to form, with the dipole operator, an effective quadrupole operator within the 4f shell i). Crystal field potentials of the Yim form are restricted, however, to metal complexes with a permanent dipole moment, that is, to the site symmetries Cpv and their subgroups ), and the proposal does not account for the relatively large oscillator strengths of the quadrupole-allowed d—d or f—f transitions of... 

The first two entries are examples of HTMCC with a planar metal framework. In the [AuFe4(CO)i6]" n = 1,2,3) system only the parent monoanion has so far been investigated by X-ray diffraction (Fig. la) whereas the corresponding di- and trianions have only been characterized spectroscopically. The [AuFe4(CO)i6] anion can be considered as a square-planar d Au- + complex stabilized by two bidentate [Fe2(CO)8] ligands, isolobal with diphosphine. Crystal-field theory... 

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