Molecular mechanics calculations on imine and mixed-ligand systems of Co-III, Ni-II and Cu-II
UNSPECIFIED. (1997) Molecular mechanics calculations on imine and mixed-ligand systems of Co-III, Ni-II and Cu-II. JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS (4). pp. 537-546. ISSN 0300-9246Full text not available from this repository.
The force field for the cellular ligand field stabilisation energy/molecular mechanics (CLFSE/MM) method has been applied to 28 transition-metal complexes. Computed and experimental structures are Compared for 12 ML(x)Cl(6-x) species (M = Co-III or Ni-II;. Z = amine donor; x = 6 or 4), 12 MA(x)B(6-x) compounds (M = Ni-II or Cu-II, A = imine, B = amine; x = 6, 4 or 3), one five-co-ordinate copper(II) imine-amine complex and three four-coordinate copper(II) imine and imine-amine molecules. For pi-bonding-ligands a stronger donor interaction is associated with a larger positive value of the CLF e(x) parameter but, due to the use bf a crystal field type barycentre, the CLFSE actually goes up. The CLFSE thus has the wrong form for treating the pi contributions to bond stretching and distance-dependent e(x) parameters are-inappropriate. However, the desired bond lengths can be obtained by modifying the Morse function and e(sigma) terms. The pi contribution to the L-M-L angle; bending operates in the correct sense but is small and can also be accommodated by altering the magnitude of e(sigma). For asymmetric pi interactions (e(nx) not equal e(xy)) there is no effect on the M-L torsional potential for low-spin d(6), high-spin d(8) and d(9) configurations where the pi-symmetry d orbitals are completely filled. Hence, only the sigma-bonding contributions to the CLFSE are retained. This approach still gives good agreement with experimental structures; even for formally pi-bonding ligands, with average root-mean-square errors in M-L lengths and L-M-L angles of about 0.02 Angstrom and 3 degrees for Co-III, Ni-II and four co-ordinate Cu-II, excluding [Cu(bipy)(2)](2+) (bipy = 2,2':bipyridyl), and about 0.05 Angstrom and 4 degrees respectively for six-co-ordinate Cu-II, excluding [Cu(terpy)(2)](2+) (terpy = 2,2': 6',2''-terpyridyl). The subtle interplay between the axial Ni-Cl and equatorial Ni-N distances in trans-[NiN4Cl2] macrocyclic species is reproduced for the first time by an MM-based approach. However, the model appears to give relatively poor agreement for [Cu(bipy)(2)(NH3)](2+), [Cu(terpy(2))(2+) and [Cu(bipy)(2)](2+). For the five-co-ordinate complex this is due to the intrinsic plasticity of five-co-ordinate copper(II) species. The energy difference between the limiting trigonal-bipyramidal and square-pyramidal geometries is only a few kcal mol(-1). For [Cu(terpy)(2)](2+) the limiting geometries of tetragonally elongated-and compressed octahedra are also within a few kcal mol(-1) although the present set of parameters overestimates the ligand contribution and predicts a compressed geometry. The; calculated structure of [Cu(bipy)(2)](2+) is too flat but for four-co-ordinate species it; is shown, using [CuCl4](2-) as an example, that there are several ways to induce a tetrahedral distortion. The most satisfactory method is to include charges on Cu and the ligand donors whereupon the geometries of [CuCl4](2-) and [Cu(bipy)(2)](2+) distort to the required flattened tetrahedral structures.
|Item Type:||Journal Article|
|Subjects:||Q Science > QD Chemistry|
|Journal or Publication Title:||JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS|
|Publisher:||ROYAL SOC CHEMISTRY|
|Date:||21 February 1997|
|Number of Pages:||10|
|Page Range:||pp. 537-546|
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