N-donor two-center

Among coordination compounds of d transition metals with n-donor two-center ligands, peroxo complexes have been studied in the greatest detail the search for methods of molecular nitrogen fixation has stimulated studies on the structures of complexes with hydrazine derivatives. As a rule, only X-ray diffraction data are available for these complexes [10],... 

DYNAMIC BEHAVIOR OF d TRANSITION METAL COMPLEXES WITH n-DONOR TWO-CENTER LIGANDS... 

The observed analogy in the nature of bonding of the coordinated ligands to the central atom in the complexes with olefins as well as in those with n-donor two-center ligands provides a better understanding of the factors responsible for the similarities and differences in the structure, properties, reactivity and dynamic behavior of these complexes. The use of the same models... 

The orientation of the olefin in a -ir complex (Zeise s salt) is known [86] to be mainly defined by steric factors. In the study of tungsten complexes with n-donor two-center ligands, ketimines, and amines, we concluded that the electronic rather than steric factor exerts the crucial effect on the ligand orientation in the complex. 

We considered the structure and dynamic behavior of Group 5 and 6 d transition metal fluoro complexes with n-donor 0,N- and N,N-two-center ligands in nonaqueous solutions, which allowed us to suggest a model of bonding of n-donor two-center ligands in d transition metal complexes. The survey of available data allowed us to draw an analogy between the structure and dynamic behavior of these complexes and those of olefin rr complexes. 

The formation conditions and interconversions of the stereoisomeric complexes with coordinated n-donor two-center ligands were considered. The mechanism of these transformations was elucidated, stating that a complex with open structure, i.e., a complex with ligand coordination through only one n-donor atom, is formed in the transition state. 

Figure 3.91 A schematic depiction ofNBO donor-acceptor relationships connecting one-center (n), two-center (c), and three-center (tu and t) orbital types. ...

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

Two general differences are apparent. TheFock-matrix element general reduction of a-orbital amplitude at each center (by the normalization factor 2-1/2) as well as the more directed p-rich character of n compared with a. For the present comparison this results in a difference... 

In n symmetry, there is a stabilizing donor-acceptor interaction involving four electrons between the doubly degenerate nHOMO and itLUMo causing two partial n bonds (8). They are opposed by the Pauli repulsive iHomo - homo two-center four-electron (2c-4e) interaction. Note the difference in nature between c and the n bonds the former is an electron pair bond between singly occupied orbitals, whereas the latter evolves from a donor-acceptor or charge transfer interaction between occupied and unoccupied orbitals. 

The peculiarities of classical localized coordination bonds (two-electron and two-center [1,4,5]) are displayed most clearly in MCC. Mostly, the elements of the first period of the Periodic Table (C, N, O) participate as electron donors in the formation of such bonds. In complexes of this type, the role of Ji-dative interactions is significant. These interactions are revealed in coordination compounds of ligands containing the elements of the next periods as donor centers (P, As, Sb S, Se, Te Cl, Br, I). We note that the examined complexes are the most successful objects to study the influence of ligand and metal nature on the character of the coordination bond, since, in this case, the factors which could distort this influence (chelate, macrocyclic, and other effects [117,135]) are absent. 

Among radical cations of n-donors we mention briefly those of 1,4-diazabicy-clo[2.2.2]-octane (99) and of the tricyclic tetraaza compound (100). For the bicyclic system a perfect correspondence has been reported between the AEs of the radical ion and the AIs of its precursor [276], The radical cation of the tetracyclic system, on the other hand, is significantly distorted. While the parent system has D2d symmetry and a b2 HOMO, the radical cation is distorted towards two equivalent structures of C2y symmetry (2E), with a two-center three-electron N — N bond [281, 282]. The dioxetane radical cations (101), invoked as intermediates in oxygenations via oxygen capture (Scheme 6), and characterized by ESR spectroscopy [8] contain analogous three-electron O—O bonds. 

An important geometric parameter for four-coordinate systems is the tetrahedral twist angle 9 (Fig. 17.7.3). We can use this parameter to inspect the geometry around one of the coordinated amine donors. Define a new reference plane with one of the N donors and two of the atoms bound to it (e. g., the Co center and one of the FI s). Then, click on the Angle between two planes button and follow the instruction to define the other plane (the same N donor again and the other two substituents, i. e., the second H and the C atom bound to the N). The value of the tetrahedral distortion (0 = 88.5°) will then be displayed (the value of 9 obviously depends on the choice of planes). 

An important feature of five-membered chelate rings is the amount of puckering. You can check this by computing the angle between the plane defined by the Co center and two N donors of one of the chelate rings, and the line defined by the two C atoms of this ring. For /e/3-[Co(cn)3]3+ this angle is 27.3°. 

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