chemical shifts, the frequencies of the asymmetric CO stretches Gco(flu) are given in Table 1, because Raman spectra have so far not been reported for the two carbonylate anions. For the remaining four molecular species from [W(CO),] to [1r(cO),l3 + complete vibrational data are available. [". 20-231 The trend shown by the.fl, vibrations, that the CO bond strength increases with increasing formal charge of the metal, is quantified by the increasing stretching force constants ,/lo (Table 1). The value of the force constant for [Ir(C0),l3' (20.8 x 10' Nm-') is only slightly below that for [Hg(CO),]2+ ( 2 1 . 0 ~ lo2 Nm-')[l21 and that for HCO'
( 2 1 . 3 ~ 10' Nm-').[51 The I3C chemical shift of 6 = 121 is the lowest that has been measured for carbonyl complexes. The rather smooth trend in both po(fi,) and I3C chemical shifts illustrates that for the six isostructural and isoelectronic (d6)[M(CO),l species x back donation gradually decreases with a concomitant increase in o donation as the formal charge on the metal gradually increases from -2 to + 3. For [Ir(C0),l3', x back bonding makes only a small, insignificant contribution to the Ir-CO bonding. A similar correlation can be made for carbonyl derivatives of iridium where Gco for terminal CO groups increases from 1642 cm-' for [Ir(C0)3]3-[41 to 2295 cm-' for the a l g mode of [Ir(CO)J3 +. Both trends clearly demonstrate the great versatility of the CO ligand, which coordinates to metallic centers of formal charges from -3 to + 3 form stable metal carbonyl species.
As more cationic carbonyl complexes like [M(C0),I2' (M = Ru, OS)['~], [Ir(CO),CI]* +, and [1r(CO),l3+ become known, together with accurate structural and vibrational data, and as smooth correlations between the formal charge on the metal and the bonding characteristics in the CO ligands emerge, it becomes increasingly obvious that any arbitrary subdivision using vague terms like "classical" and "nonclassical (o-only)" carbonyl complexes as proposed sometime ago for the thermally unstable [Ag(CO),]' (n = 1, 2) spe~ies''~' is incorrect, inappropriate, and unnecessary. The first metal carbonyl derivative [Pt(CO),CI,] reported in 1868 ["] with an average Gco value of 2175 cm-' [well above that (2143 cm-') for free CO[261] would be assigned "nonclassical", which is in our view absurd.