Why does [math][Fe(CN)_{6}]^{-3}[/math] undergoes [math]d^{2}sp^{3}[/math] hybridisation, while [math][Fe(H_{2}O)_{6}]^{3}[/math] undergoes [math]sp^{3}d^{2}[/math], though both are paramagnetic and have octahedral geometry? What math comes after geometry?