When oxygen is bound to $\mathrm{F}{\mathrm{e}}^{2+}$ in heme, all the d-electrons of the Fe atom are forced into the three lower energy ${\mathrm{t}}_{2\mathrm{g}}$ orbitals and it produces low spin oxyhaemoglobin.

In oxyhaemoglobin, $\mathrm{F}{\mathrm{e}}^{2+}=\left[\mathrm{A}\mathrm{r}\right]3{\mathrm{d}}^6$



The sharp high energy transition between ${\mathrm{t}}_{2\mathrm{g}}$ and ${\mathrm{e}}_{\mathrm{g}}$ levels of the d-electrons is responsible for the bright red colour of oxyhaemoglobin.