Carbonyl stretching frequencies ν(CO)
C–O stretching frequencies of free CO, the isoelectronic hexacarbonyls and [Ni(CO)₃L]. The more electron-rich the metal, the stronger the π back-donation and the lower ν(CO).
| [Ti(CO)₆]²⁻ | isoelectronic hexacarbonyl | 1748.0 | −395.0 | 0.665 |
|---|---|---|---|---|
| [V(CO)₆]⁻ | isoelectronic hexacarbonyl | 1859.0 | −284.0 | 0.753 |
| Cr(CO)₆ | isoelectronic hexacarbonyl | 2000.0 | −143.0 | 0.871 |
| [Mn(CO)₆]⁺ | isoelectronic hexacarbonyl | 2100.0 | −43.0 | 0.960 |
| [Fe(CO)₆]²⁺ | isoelectronic hexacarbonyl | 2204.0 | 61.0 | 1.058 |
| CO | free molecule (gas) | 2143.0 | 0.0 | 1.000 |
| [Ni(CO)₃L], L = P(t-Bu)₃ | Tolman electronic parameter (A₁) | 2056.1 | −86.9 | 0.921 |
| [Ni(CO)₃L], L = PMe₃ | Tolman electronic parameter (A₁) | 2064.1 | −78.9 | 0.928 |
| [Ni(CO)₃L], L = PPh₃ | Tolman electronic parameter (A₁) | 2068.9 | −74.1 | 0.932 |
| [Ni(CO)₃L], L = P(OEt)₃ | Tolman electronic parameter (A₁) | 2076.3 | −66.7 | 0.939 |
| [Ni(CO)₃L], L = PCl₃ | Tolman electronic parameter (A₁) | 2097.0 | −46.0 | 0.958 |
| [Ni(CO)₃L], L = PF₃ | Tolman electronic parameter (A₁) | 2110.8 | −32.2 | 0.970 |
Force-constant ratios assume one C–O oscillator, k ∝ ν̃², ignoring coupling to other vibrations. Hexacarbonyl values are the IR-active T₁u band; they vary by a few cm⁻¹ between media.