  We explore the nature of the metal-insulator transition in
$\beta^{\prime\prime}$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Cl by x-ray, electrical
transport, ESR, Raman and infrared investigations. Magnetic and vibrational
spectroscopy concurrently reveal a gradual dimerization along the stacking
direction $a$, setting in already at the crossover temperature of 150K from the
metallic to the insulating state. A spin gap $\Delta_\sigma = 47$meV is
extracted. From the activated resistivity behavior below 55K a charge gap
$\Delta_\rho=60$meV is derived. At $T_{\rm CO} = 72$K the C=C vibrational modes
reveal the development of a charge-ordered state with a charge
disproportionation of $2\delta_{\rho}=0.34e$
