Theoretical Electronic Spectroscopy of Gas Phase Transition Metal Acetylide Cations (MCCH⁺, M = Sc···Zn)

C₂H radicals and C_nH polyynes are among the most abundant interstellar polyatomic molecules, predominantly observed in molecular clouds and planetary nebulae. While the C₂H^- anion is considered a plausible component in the interstellar medium (ISM), it has not yet been detected. Given the extensive study of interstellar polyynes, MCCH/MCCH⁺ species are also expected to exist in the ISM. Via fundamental metal-carbon bonding, MCCH/MCCH⁺ molecules exhibit strong polarity, enhancing the intensity of their rotational transitions and improving the chances of astronomical observation. Theoretical spectroscopy enables the generation of molecular reference data to aid in detections. In this study, highly accurate single reference coupled cluster theory [CCSD(T)] is compared to multireference configuration interaction theory (MRCISD+Q) to survey the electronic structure of 3d metal mono acetylide cations (MCCH⁺, where M = Sc to Zn) which could catalyze gas phase reactions in cold astrochemical environments. CCSD(T) and MRCISD+Q theories gave equivalent predictions of ground electronic states for all cations excluding CoCCH⁺. The MRCISD+Q ground state for CoCCH⁺ (⁴Φ) is multireference and multiconfigurational, and single reference coupled cluster energies could not be converged. The investigated electronic states of MCCH⁺ arise from interactions between the ¹Σ⁺ ground state of the C₂H^- anion and the metal dication (M²⁺). A thorough analysis of multireference character of electronic states is reported. The ³Π excited state of NiCCH⁺ was the variationally accessible electronic state with the strongest reported multireference character (T₁ diagnostic = 0.15, D₁ diagnostic = 0.55, C₀² = 0.26). Harmonic vibrational frequencies, infrared intensities, and dipole moments are computed for the identified ground states. Theoretical characterization of the ground and excited electronic states of these specific ions will assist in their astronomical detection, constrain laser spectroscopy experiments of the neutral species, and guide characterization in synthetic laboratories.