Recently, the sigma-aromaticity model proposed for cyclopropane by Dewar was employed to account for the stability of Group 8 trinuclear metal-carbonyl compds. [M3(CO)12] (M = Fe, Ru, Os). This paper further examines this hypothesis and provides the first quant. evidence for the sigma-arom./antiarom. nature of the [M3(CO)12]/[M4(CO)16] species based on structural and nucleus-independent chem.-shift anal. In addn., the extent of electron delocalization in tetrahedral [M4(CO)14] and butterfly [M4(CO)15] is analyzed and compared to prototype cycloalkanes. While remarkable analogies exist between metal-carbonyls and cycloalkanes, transition metals provide addnl. overlap possibilities that affect both the ring strain and the magnetic properties of metal-carbonyl rings and cages. [on SciFinder (R)]