M-L-X bond angles

There are two types of torsional interaction that involve the metal. These are the M-L-X-X and the L-M-L-X interactions. The first type of torsional interaction is almost always included in the calculations. Like M-L-X bond angles, M-L-X-X torsion angle interactions specify the preferred orientation of the metal relative to the ligand donor group. They are commonly described by equation (5) which gives energy as a function of torsion angle, 0. Adjustable parameters are Vi, Vj, and V3. 

The ionic method has been used in MM calculations on complexes of Fe(III) , the alkali and alkaline earth cations, and the lanthanides. Unlike the VFF and POS methods, structures and energies calculated with the ionic method are extremely sensitive to the van der Waals parameters and the charge assigned to the metal and the ligand atoms. With the omission of interactions used in bonded methods, the ionic model has failed in some cases to accurately reproduce structural features such as L-M-L and M-L-X bond angles and M-L-X-X torsional interactions. ... 

The use of nonbonded 1,3-interactions in place of force constants also offers a number of practical advantages. First, it is not necessary to determine the values of L-M-L force constants. If the assumption that the M-L-X force constants are ind-pendent of the metal center is also made, then replacement of the L-M-L force constants with the 1,3-interactions results in a force field that is largely independent of the metal ion. Second, the coordination geometry is not predetermined by the choice of the undeformed bond angles. For instance, in the case of a five-coordinate com-... 

In nonbonded methods, the metal ion is not formally connected to the ligand donor atoms and the metal-dependent interactions present in bonded methods are not included. There are no explicit M-L bonds, M-L-X and L-M-L angles, or M-L-X-X and L-M-L-X torsional interactions. Instead the metal-ligand complex is modeled with a collection of pairwise electrostatic and van der Waals interactions between the metal and all ligand atoms, i.e., with M-L and M-X interactions. 

Jiang, X., C.-H. Yu, M. Cao, S. Q. Newton, E. F. Paulus, and L. Schafer. 1997. A / Torsional Dependence of Peptide and Protein Backbone Bond-Lengths and Bond-Angles Comparison of Crystallographic and Calculated Parameters, J. Mol. Struct. 

Compound Bond Bond length (A) L-M-Pb angle (deg) M-Pb-X angle (deg) Reference... 

Treatment of AgCl with 2,1 l-bis(diphenylphosphinemethyl)benzo[c] phenanthrene (P-P) in Me2CO yields [Ag(P-P)Cl)]. The X-ray structures of this and similar Cu and Au analogues show a remarkable change in P—M—P bond angle from 132° (Cu) to 141° (Ag) and 176° (Au). Conductivities of the complexes indicate that the degree of dissociation of Cl increases in the order Cumetal-chlorine bond lengths. 

---