Substituent effects at nitrogen/phosphorus atoms of dialkaline earth metal complexes: Excess electron and large second-hyperpolarizability

A number of imido-, amido-, and phosphido-bridged dialkaline earth metal (M = Be, Mg, and Ca) complexes and their alkali metal (Li and Na) derivatives have been considered to study the ground state structure and the second-hyperpolarizability. The calculated ground state geometries contain four-membered M N(P) M N(P) ring having either planar or butterfly-like bent structure. The second-hyperpolarizability has been calculated at the HF and CCSD(T) levels using Sadlej's pol and aug-pc-2 basis sets, respectively. The addition of second hydrogen/alkali metal atom on nitrogen/phosphorus (N/P) atom substantially reduces the charge transfer from the alkaline earth metal atoms as the high negative charge on N/P exerts stronger push effect on the outermost electron pair in the “ns” sub-shell of M. The excess electron density on the alkaline earth metal atoms plays a crucial role in the enhancement of second-hyperpolarizability. The sum-over-state method calculated two-photon contribution of second-hyperpolarizability has been found to be significant. The variation of second-hyperpolarizability has been explained satisfactorily in terms of the TD-CAMB3LYP calculated spectroscopic properties in the light of two-state model. The calculated mean second-hyperpolarizabilities of alkali substituted amido- and phosphido-bridged complexes are in the order of 10⁷ au.