Section 9.6.4. Discussion

Table 9.6.3.3 has been derived from the CSD, and, as a result, does not contain every precisely determined metal–ligand interatomic distance. For example, there are many ammine (M—NH₃), carbonyl (M—CO), halide (M—Cl etc.), and aqua (M—OH₂) complexes that do not fall within the scope of the CSD. For such bond types, and other metal–non-metal bond-length information, the interested reader is referred to the Inorganic Crystal Structure Database (Bergerhoff, Hundt, Sievers & Brown, 1983).

The tabulation given here is a first attempt to obtain average dimensions for (d- and f-block) metal–ligand and intraligand bonds. Inspection of Table 9.6.3.3 shows that, in general, the sample standard deviations of metal–contact-atom interatomic distances are typically larger than those of the intraligand distances [e.g. for Fe—PPh₃ complexes (section 8.5.3), Fe—P mean 2.237, σ 0.038 Å, cf. P—C mean 1.834, σ 0.011 Å]. There are several factors that cause this phenomenon. Firstly, in many (but not all) cases no account has been taken of substituent effects at the metal, such as the trans influence of other ligands. In contrast, the substituent pattern at the ligand is usually well defined; therefore, the chemical causes for variation in the metal–ligand and intraligand distances are different. Secondly, it is likely that metal–ligand bonds are softer, i.e. have lower force constants, than the intraligand bonds, leading to a broader distribution of distances whatever the cause of variation. Finally, it should be noted that a substantial contribution to the standard deviation of both metal–ligand and intraligand distances comes from random errors arising most importantly from the rather poor location of light (B, C, N, O, F) atoms in the presence of 5d, 4f, and 5f metals (La–U). For example, C—O bond lengths in carbonyl complexes (section 3.7.1) of the individual 3d metals show sample standard deviations (σ) in the range 0.011–0.024 Å, while the 5d metals have σ in the range 0.023–0.035 Å. This last effect is somewhat reduced by the screening on the AS flag, as described above. While other contributions to the variance in interatomic distances undoubtedly play a part, readers should be aware of these various factors when making use of the averages and other statistics of Table 9.6.3.3. In the longer term, as more structures are determined, it will become possible to derive more precise averages by further subdivision of the distributions represented in Table 9.6.3.3.