Metal-nitrogen distances

Table 5. Metal-oxygen and metal-nitrogen distances. A, in ring complexes...

At the same time the bond angles (C—N—C) at the N7 position vary from 112 (-fp3 as expected for an amine ligand) in the manganese complex (where repulsion is least) up to a maximum value of 120 in the iron complex with maximum repulsions. The tertiary amine nitrogen atom (N7) corresponds to a three-ribbed umbrella that has been inverted by the wind (the handle is the lone pair directed at the metal). As the a, and If levels fill, the repulsions increase, the metal-nitrogen distance increases, and the umbrella begins to flatten ... 

Table 2 Observed Ranges of Metal-Nitrogen Distances (in pm) in Azole Coordination Compounds, Without Steric Effects...

The three structurally characterized examples of this type of bonding are shown in Table 16 and all would involve a twenty electron valence count for the metal if the hydrazido(2 —) ligand functioned as a four-electron donor. The structure of [Re( 7s-CsH5)(CO)2(NNMeC6H4OMe)]460 is typical and is shown in (34). The formal localization of a lone pair on Na causes a significant increase in the metal nitrogen distance consistent with decreased multiple bonding. 

The structure of iron(II) octaethylchlorin shows the iron and four nitrogen atoms to be rigorously planar, but the rest of the chlorin macrocycle to be significantly S4 ruffled.732 Hydroporphyrins intrinsically have larger cores than porphyrins, but the distortion from planarity leads to a reduction in core size and shorter metal-nitrogen distances. The enhanced ability of hydroporphyrins to undergo distortion so as to adjust their core size in response to the size of the metal may be responsible for the differences between iron porphyrins and hydroporphyrins.733... 

Table VI. Metal—Nitrogen Distances in Tetradentate Macrocyclic Complexes...

Metal-nitrogen distances, as observed in a very large number of crystal structure determinations, are almost the same as those observed for the azole ligands (see Table 2) within experimental error. Comparing pyridine with substituted ligands, it is observed that substituents at position 2 (and 6) next to the donor atom have a dramatic effect upon the stoichiometry and properties of the compounds formed. It appears that a stoichiometry M(Rpy)4(anion)2 with a trans orientation of the anions is not possible, because the methyl group effectively blocks the axial coordination sites. This results in square-planar low-spin compounds for Ni and tetrahedral geometry for some other metals such as Co " and... 

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