The reactions of [Fe 3 (CO) 12 ] or [Ru 3 (CO) 12 ] with RNC (R ¼ Ph, C 6 H 4 OMe-p or CH 2 SO 2 C 6 H 4 Me-p) have been investigated using electrospray mass spectrometry. Species arising from substitution of up to six ligands were detected for [Fe 3 (CO) 12 ], but the higher-substituted compounds were too unstable to be isolated. The crystal structure of [Fe 3 (CO) 10 (CNPh) 2 ] was determined at 150 and 298 K to show that both isonitrile ligands were trans to each other on the same Fe atom. For [Ru 3 (CO) 12 ] substitution of up to three COs was found, together with the formation of higher-nuclearity clusters. [Ru 4 (CO) 11 (CNPh) 3 ] was structurally characterised and has a spiked-triangular Ru 4 core with two of the CNPh ligands coordinated in an unusual l 3 -g 2 mode.