Seven-coordinate species shape

Available structural details for these silicates, and for the wide range of other known species containing Mn" and silicate, are summarized in Table 23. Coordination numbers four (tetrahedral), five (both square pyramid and trigonal bipyramid and in-between structures), six (mainly octahedral and distorted, but including one trigonal prism), seven (of various shapes) and eight (dodecahe-dra and distorted rhombohedra) all are defined and there is evidence for three-coplanar in a zeolite the hydrated form is five-coordinate (3 x 2.28 A and 2 x 2.05 A) but, on dehydration of this Mn-exchanged zeolite, the solid contains planar three-coordinate Mn with Mn—O = 2.11 A, ... 

A nearly completed structure determination (4) of crystalline Ca[Fe(OHj)Z]j.9 H O reveals a sexadentate seven-coordinate [Fe(OHj)Z] which resembles that described (3) for [Fe(OH )Y] as closely as one could expect for analogous DCTA and EDTA chelates. Excepting the four outer carbon atoms in the cyclohexane ring skeleton of the DCTA complex, this latter is described quite adequately in respect to effective symmetry, shape, and bond parameters by the published data (3) for the EDTA complex to which the Reader is refened. The greater stability of the DCTA chelate is not reflected in shorter complexing bonds, but is attributable rather to a more favorable entropy of formation. [FefOH Z]", as with [Mn(OHj)Y] (2) and [Fe(OH )Y] (3), is expected to be the dominant species throughout an appropriate range of pH in aqueous solution. 

This finding is easily rationalized using Pearson s concept of hard and soft acids and bases (see Section 2.14). Whenever NCN is coordinated by chemically hard cations (for example, Ca +, with rj = 19.52 eV, see Table 2.3) there results a considerable amount of ionic bonding between the cation and a symmetrical N=C=N anion. Chemically softer cations such as Pb + (8.46 eV) or Ag+ (6.7 eV) will bond more covalently to one end of the NCN unit, inducing the less symmetrical (N-C=N ) cyanamide shape this species must be chemically softer. Figure 3.41(b) shows the PbNCN structure with a seven-... 

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