Chemical origin of blue and red shifts of C-H stretching vibrations in M+−C2H2 (M = V, Fe, Co, Ni) and M+−C6H6 (M = V, Si, Ni) complexes

M+−C2H2 (M = V, Fe, Co, Ni) and M+−C6H6 (M = V, Si, Ni) complexes were studied by using density functional theory at the B3LYP/6-311+G∗∗ and B3LYP/6-311++G∗∗ levels, respectively. In M+−C2H2 complexes, the C-H bond lengths display an increase with a concomitant red shift of C-H stretch frequencies, while in M+−C6H6 complexes the C-H bonds exhibit contraction accompanied by the blue shift of C-H stretch frequencies. These frequency shifts are well consistent with experimental results. To account for the interesting changes of C-H vibrations in M+−C2H2/C6H6, the natural bond orbital analysis was carried out. The NBO results suggest that both the red and blue shifts are attributed to changes of electron density in σ∗CH and s-character of carbon in C-H bond. Our conclusion is that electron density redistribution and rehybridization are the chemical origin of these two types of frequency shifts in M+−C2H2/C6H6 complexes.