First principles density functional theory is utilized to describe electronic structure and magnetic interactions between Cr(III) ions in linear and bent geometry of [cis-[hydroxotetramminechromium-mu-hydroxopentamminechromium](4+). The effective exchange parameter J is extracted and compared with empirical values fitted to experimental data. The calculated value J = 48 K for bent geometry compares well with the 30 K experimental value : the magnetic interaction is an order of magnitude larger in the linear configuration. Low-lying optical transitions are analyzed, making use of the transition state scheme, and are in semiquantitative agreement with experimental absorption.