**4. Conclusion**

232 Current Trends in X-Ray Crystallography

considerably decreases when an azo group (which has greater π–acidity) coordinates to the ruthenium atom, because the decrease of the *k* value caused by ligand-based oneelectron reduction corresponds to 0.56 N cm–1 in [Ru(bpy)2(CO)(quinoline)]2+ (Wada et al., 2004). Thus, it can be concluded that complex formation with azopyridyl ligands induces relatively high *k* values. This observation can be ascribed to the decrease of the π backdonation from the ruthenium center to the CO group. As a result, this suggests that the azopyridyl ligands serve as prominent electron reservoirs compared with other

polypyridines\* *d*(Ru–C) *d*(C–O) νCO *k*

4,4'-Me2bpy/5,6'-Me2phen 1.867(5) 1.112(7) 1978 15.81 dpk/4,7'-Me2phen 1.820(11) 1.146(13) 1976 15.80 10/bpy 1.867(5) 1.145(6) 1975 15.76 dpa/5,5'-Me2bpy 1.917(4) 1.026(6) 1972 15.72 bpy/dpk 1.880(5) 1.119(7) 1970 15.68

4,4'-Me2bpy/4,7'-Me2phen 1.95(2) 1.05(2) 1967 15.64

\* 4,4'-Me2bpy = 4,4'-dimethyl-2,2'-bipyridine; 5,5'-Me2bpy = 5,5'- dimethyl-2,2'-bipyridine; 5,6'-Me2phen = 5,6'-dimethyl-1,10-phenanthroline; 4,7'-Me2phen = 4,7'- dimethyl-1,10-phenanthroline; dpk = di(2-

Table 1. Bond distances (*d*: Å), stretching frequencies (ν: cm–1) and force constant (*k*: N cm–1)

The N–N bond distance in an azo moiety is an excellent indicator of the charge on an azo group (Kaim, 2001). The values for unreduced azo N–N bonds are 1.22–1.31 Å in metal complexes (1.23–1.26 Å for metal-free ligands). The one-electron reduced (anion radical) ligands have a bond distances of 1.31–1.41 Å, whereas the two-electron reduced (hydrazido) forms have single bonds with 1.41–1.50 Å distances (Fig. 26) (Sarkar et al., 2008). As shown in Table 2, the azo N–N bond distance of [Ru(bpy)2(CO)(3–*N*)]2+ is dramatically shorter than those of other complexes: the distance (1.188(4) Å) is shorter than that of a typical N=N

pyridyl)ketone; dpa = di(2-pyridyl)amine; phen = 1,10-phenanthroline; biq = 2,2'-biquinoline.

of the CO groups in *cis*-[Ru(polypyridine)2(CO)Cl]+.

double bond (1.23 Å) (Oyama, Fujita & Yui, 2008).

Fig. 26. Three oxidation states of an azo moiety.

bpy/dpa 1.883(16) 1.08(2) 1967 15.64 bpy/phen 1.857(4) 1.126(6) 1966 15.62 bpy/bpy 1.861(29) 1.122(32) 1966 15.62 biq/biq 1.91(2) 1.01(2) 1964 15.59

3/3 1.897(5) 1.136(6) 2031 16.67 4/bpy 1.889(3) 1.116(4) 2002 16.20 3/bpy 1.891(2) 1.122(3) 2001 16.18

polypyridyl ligands such as bpy.

Single crystal X-ray diffraction studies provide a valuable probe to visualize molecules. Although some problems such as disorder and twining exist in measurements and analyses, it still represents the most important analytical method for coordination chemists. The author believes that advances in this technology will lead to an increase in the use of single crystal X-ray diffraction, including the X-ray snapshot technique which enables the capture of frame-by-frame movies of chemical reactions as they proceed *in situ*, now ubiquitous in NMR (Inokuma, Kawano & Fujita, 2011).
