By J.C.A. Boeyens, S.N. Bose, J.C. Hawes, D.M.P. Mingos, K. Nag, S.-C. Tam, R.J.P. Williams
Content material: Complementary round electron density version / D.M.P. Mingos, J.C. Hawes -- Molecular mechanics and the constitution speculation / J.C.A. Boeyens -- Electrostatics and organic platforms / S.-C. Tam, R.J.P. Williams -- Chemistry of tetra- and pentavalent chromium / ok. Nag, S.N. Bose
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Extra resources for Bond and Structure Models (Structure and Bonding)
This localisation process is useful for a perturbation analysis of the bonding in for example octahedral MLsX complexes. In the limit of X being a very weakly held ligand the situation clearly approximates to a square-pyramidal ML5 complex59). Ligand lone pair orbitals represent the most obvious of the alternative modes of generating localised regions of electron density on the surface of the ligand sphere. A linear combination of atomic orbitals on the central atom of the type as + bpz + cdo has a localised region of electron density which when projected onto to the ligand sphere is concentrated around the polar region (see Fig.
Although, the s and p orbitals of the central atom in such compounds represent a spherically symmetric situation filling of the ligand D ~ shell can lead to an asymmetric charge distribution. For example, in a trigonal bipyramid the ligand D~ function is localised more extensively on the axial than the equatorial ligands, consistent with the higher negative charge associated with the axial ligand compared with the equatorial ligands. This conclusion has been confirmed by molecular orbital calculations at the ab initio level and has important ramifications for the site preferences in substituted trigonal bipyramidal compounds 55), In octahedral compounds the ligand D~ and DR orbitals are occupied and the resultant electron distribution has cubic symmetry.
The situation is the converse of that described in the context of n-acceptor ligands, where filled orbitals on the central atom are stabilised by empty orbitals on the ligands. Consequently, in n-donor complexes the driving force is the acievement of the inert gas configuration for the peripheral atomssT'ss~ and a spherical electron distribution for the central metal atom. This leads to the following simple rules for main group and transition metal n-donor complexes, MLN. (a) Main group compounds have the following characteristic geometries as a function of electron count: 8N: closo-, 8N + 2: nido-, 8N + 4: arachno-, 8N + 6: hypho-.
Bond and Structure Models (Structure and Bonding) by J.C.A. Boeyens, S.N. Bose, J.C. Hawes, D.M.P. Mingos, K. Nag, S.-C. Tam, R.J.P. Williams