In structural chemistry,it has been well accepted that,besides orbital interactions,geo-metrical factors e.g.size and shape of the bonded atoms and groups,may also play importantroles.However,a quantitative study of these factors has long been neglected.In the point that steric effects should become more remarkable in weak bond compounds,it is expected that coordination and organometallic chemistry of f-group elements,owing totheir low CFSE and high coordination numbers,and also to their poor directionality of ioniccharacter,is assumed to be an appropriate field for study on steric effects while a rapid de- In structural chemistry, it has been well accepted that, or orbital interactions, geo-metrical factors egsize and shape of the bonded atoms and groups, may also play importantroles. However, a quantitative study of these factors has long been neglected. The point that steric effects should become more remarkable in weak bond compounds, it is expected that coordination and organometallic chemistry of f-group elements, due totheir low CFSE and high coordination numbers, and also to their poor directionality of ioniccharacter, is assumed to be an appropriate field for study on steric effects while a rapid de-