Presented here is an investigation on the geometric (molecular) structures, spectroscopic properties, and electronic structures of copper(II)-nitrite complexes as a function of steric effects utilizing a set of closely related coligands. The prepared complexes with general formula [CuLR(2-ONO)2] where LR is N,N-dialkyl N'-benzyl-ethylenediamine and R is methyl, ethyl or isopropyl moieties were structurally characterized by physico-chemical and spectroscopic methods. The X-ray diffraction study on CuLMe(2-ONO)2] and CuLEt(2-ONO)2] reveals that the copper (II) center in both compounds located in a distorted octahedral environment through coordination of two amine nitrogen atoms and four oxygen atoms of the nitrite ligands. Depending on the steric crowding of the coligand, the coordination mode is either symmetric or asymmetric bidentate as evident from X-ray crystallography and IR spectroscopy. The relative stability of linkage isomers of these compounds along with a tert-butyl derivative complex were investigated using density functional theory (DFT) calculations. The calculated results demonstrated that in all cases the bidentate 2-ONO isomer is more stable than four-coordinated nitro isomer (1-NO2). However, the differences in the relative stability decrease as the steric hindrance in coligand increases. The vibrational spectra of the complexes were assigned using DFT calculations. In support with X-ray structure, the results reveal that as(N-O) splits in two bands that its separation increases from [CuLMe(2-ONO)2] through [CuLtert-buty(2-ONO)2]. That is at least partially dictated by steric factors within the molecule imposed by alkyl groups in coligand. The UV-Vis absorption spectra are presented and analyzed with the help of time-dependent density functional theory (TD-DFT) calculations.
