Interligand interaction

The availability of different metal ion binding sites in 9-substituted purine and pyrimidine nucleobases and their model compounds has been recently reviewed by Lippert [7]. The distribution of metal ions between various donor atoms depends on the basicity of the donor atom, steric factors, interligand interactions, and on the nature of the metal. Under appropriate reaction conditions most of the heteroatoms in purine and pyrimidine moieties are capable of binding Pt(II) or Pt(IV) [7]. In addition, platinum binding also to the carbon atoms (e.g. to C5 in 1,3-dimethyluracil) has been established [22]. However, the strong preference of platinum coordination to the N7 and N1 sites in purine bases and to the N3 site in pyrimidine bases cannot completely be explained by the negative molecular electrostatic potential associated with these sites [23], Other factors, such as kinetics of various binding modes and steric factors, appear to play an important role in the complexation reactions of platinum compounds. 

Rotamer distribution and interconversion rates give biochemically-relevant information on hydrogen-bonding and steric interligand interactions in [Pt(Me3dien)(guanine)]2+ (230). 

However, Tilley et al. reported an unusual d silylhydride ate-complex 25 that was suggested to have an interligand interaction Si-H. The 18e compound 25... 

Thus, the common assumption made in the identification and characterization of silane c-complexes that a larger H-Si coupling constant corresponds to a stronger interligand interaction (Section II.D) is not valid for the compounds with IHI. This unexpected result can be explained in the same way as it was in Section II.D for the compound [Mn(ri -HSiCl3)(CO)2Cp]. That is, although the increased chlorine substitution in 107-109 results in the decreased IHI, the relative contribution of silicon s character in the Si" H bond increases, which leads to the increase of Si-H coupling. 

The different conformations occurring in M84 ring systems have been explained by the varying degree of interligand interactions. In the... 

One of the unusual features of tris-dithiolenes is their preference for trigonal prismatic rather than octahedral coordination geometries.7 Examples of this structure are the dithiolenes (8) of Cr, Mo and W. Distortions towards octahedral structures are known to occur especially in anionic tris-dithiolenes, but the preference for trigonal prismatic arrangements is clearly established for the neutral species. The main reason is seen in favorable interligand interactions between the sulfur atoms.7 In Mo(edt)3 the intra- and inter-ligand S—S distances of 3.10 and 3.11 A are virtually identical.14 In the tris-dithiolene anions, the increased charge density on the S atoms leads to repulsive interactions for which the octahedral coordination is preferred. 

Relating to the work of Heber and Hoyer42 (see Section 16.5.2.4.1) on methyl ethers of the dithiolene ligand, Richter et al % determined the structure of tris(S-methylethylenedithiolato)Rh and found an octahedral structure, a cis arrangement of the methyl groups and no evidence for interligand interactions of the S atoms. 

In the previous section we developed the DMM methodology and derived formulae for the energy (force fields) for interactions of donor atoms with acceptors. A simplified representation of the acceptor with a single s -orbital was used there. Here we consider the metal-ligand interactions from a different point of view - that of the metal. The metal ion in a CC acquires some density not from one but from many lone pairs of the ligating donor atoms. Constructing a mechanistic or at least an economic QM description for such a case would possibly help to rationalize terms of interligand interaction force fields, which are sometimes included in the standard MM picture to assure proper description of the metal CC. 

Vogler A, Kunkely H. Excited state properties of organometallics based on interligand interactions between aromatic ligands. Coord Chem Rev 2005 249 1511-16. 

Topology of interligand interactions. Consider, for example, a tetrahedral molecule AL4, where A is the central atom and L are identical a-ligands. The topological matrix of such a molecule can be written in the form... 

There is evidence for interligand interactions, without inner-sphere metal-nucleic acid binding, in three different types of complexes. In the first, the potential metal-binding sites are blocked by a proton, for example, in saltlike stmc-tures of the complexes of protonated bases 9-ethylguantne,... 

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