Nature of the Metal-Ligand Bond in Some Complex Cyanides from a Study of Raman Intensities

A study of the Raman spectra of solutions of Zn (CN)₄⁼, Cd (CN)₄⁼, Hg (CN)₄⁼, Co (CN)₆⁻³, Cu (CN)₄⁻³, Ag (CN)₄⁻³, and Fe (CN)₆⁻⁴ shows that these complex ions fall into two distinct sets. For the first set, consisting of the first four species, the A₁CN frequencies are 2143, 2145, 2148, and 2152 cm⁻¹ and the ${ᾱ}_{{\mathrm{CN}}^{\prime }}$ lie intermediate between those determined for free CN⁻ and CH₃CN. For the latter three species, the corresponding frequencies are 2094, 2097, and 2094 cm⁻¹. Furthermore, members of the latter set have abnormally large Raman intensities, i.e., large ${ᾱ}_{{\mathrm{CN}}^{\prime }}$ , and an additional ultraviolet absorption band at 2600 A. For the first group, the assumption of partial sigma bonding of metal to C, consistent with the principle of essential electroneutrality, explains both the observed force constants and the values of ${ᾱ}_{{\mathrm{CN}}^{\prime }}$ . The low intensity found for the ν₂ mode is explained in terms of the effect on this quantity of the large amount of s character in the carbon sigma orbital. The second group cannot be simply treated and appears to present an example of the breakdown of bond localized polarizabilities.