AbstractPentacarbonyl(pentamethylenevinylidene)chromium, [(CO)5CrCC(CH2)5] (1), reacts with 1‐methylthio‐1‐propyne (2a) and N‐(1‐propynyl)phenothiazine (2b) by regio‐specific addition of the CC bond of the alkyne to the CC bond of the vinylidene ligand of 1. Initially, complexes with a spirocyclic cyclobutenylidene ligand are formed, [(CO)5Cr(CH2)5] (3) and [(CO)5‐Cr(CH2)5] (10). Subsequent thermally initiated ring expansion affords the 2‐methyl‐4,5,6,7‐tetrahydroindenyl(methyl)thioether complex [(CO)5‐Cr–S(Me)(Me‐C9H10)] (4) and the N‐(2‐methyl‐4,5,6,7‐tetrahydroindenyl)phenothiazine complex [(CO)5Cr–S(C6H4)2‐N–(Me‐C9H10)] (11), respectively. In both complexes, 4 and 11 the ligand is coordinated to the metal via the sulfur atom. The constitutions were established by (H,H)‐, (H,C)‐ and NOE‐correlated NMR spectroscopy. In the reaction of 1 with 2a, in addition to 3 and 4, the methyl(propynyl)thioether complex [(CO)5Cr–S(Me)CCMe] (5) was formed. An isolable 3‐methylthio‐substituted cyclobutenylidene complex, [(CO)5CrPh2] (9) was obtained from [(CO)5CrCCPh2] (8) and 2a. The 4,5,6,7‐tetrahydroindenylthioether ligand of 11 was cleaved from the metal by exposure of 11 adsorbed on silica gel to air or by treatment of 11 with [NEt4]Br in dichloromethane.