Crystal field splitting energy is the energy difference between the d-orbitals of a transition metal ion in a complex due to the presence of ligands. In an octahedral complex, the d-orbitals split into two sets: the lower energy t2g set dxy, dyz, and dxz and the higher energy eg set dx^2-y^2 and dz^2 . The magnitude of the splitting energy depends on the nature of the metal ion and the ligands.For Co II and Ni II complexes with strong field ligands, the crystal field splitting energy is relatively large. Co II has a 3d^7 electronic configuration, while Ni II has a 3d^8 configuration. When strong field ligands are present, the electrons in the d-orbitals will pair up in the lower energy t2g orbitals before occupying the higher energy eg orbitals, leading to a low-spin configuration.The color of these complexes is influenced by the crystal field splitting energy. When light is absorbed by the complex, electrons can be excited from the t2g orbitals to the eg orbitals. The energy difference between these orbitals corresponds to the energy of the absorbed light, which in turn determines the color of the complex. A larger splitting energy corresponds to the absorption of higher energy shorter wavelength light, while a smaller splitting energy corresponds to the absorption of lower energy longer wavelength light.Experimental evidence for the crystal field splitting energy and the color of Co II and Ni II complexes can be obtained through UV-Vis spectroscopy. For example, the absorption spectrum of [Co NH3 6]^2+, a Co II complex with strong field ligands NH3 , shows a maximum absorption at around 490 nm, corresponding to a blue-green color. Similarly, the absorption spectrum of [Ni CN 4]^2-, a Ni II complex with strong field ligands CN- , shows a maximum absorption at around 420 nm, corresponding to a violet color. These absorption maxima can be used to calculate the crystal field splitting energy using the formula: = h * c / where h is Planck's constant 6.626 x 10^-34 Js , c is the speed of light 3.00 x 10^8 m/s , and is the wavelength of maximum absorption.For [Co NH3 6]^2+: = 6.626 x 10^-34 Js * 3.00 x 10^8 m/s / 490 x 10^-9 m 4.05 x 10^-19 JFor [Ni CN 4]^2-: = 6.626 x 10^-34 Js * 3.00 x 10^8 m/s / 420 x 10^-9 m 4.74 x 10^-19 JThese values of crystal field splitting energy support the observation that Co II and Ni II complexes with strong field ligands exhibit low-spin configurations and distinct colors due to the absorption of light corresponding to the energy difference between the t2g and eg orbitals.