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We report ab initio and experimental studies of HeClF and $(ClF)2$. Microwave electric resonance optothermal spectroscopy guided by ab initio potential energy surface and the resultant calculated vibrational and rotational states were used to observe rotational transitions of the two complexes. The HeClF potential energy surface was calculated at the MP4 level using a large basis set containing bond functions. The surface is characterized by three distinct minima with the global minimum at the linear He-Cl-F and two others at a nearly T-shaped and at the anti-linear He-F-Cl configurations. The bound state calculations were performed using the collocation method with a large basis set. Predicted bound states fall into three catagories with the He atom localized either in the linear He-Cl-F, a mixture of the T-shaped and anti-linear He-F-Cl, or the anti-linear He-F-Cl configurations. These calculations were then used to guide the experimental search for transitions. The $J=1\leftarrow 0andJ=2\leftarrow 1$ transitions for overall rotation of the linear structure were observed within 2% of theoretical values. The resolved hyperfine structure allowed determination of the quadrupole coupling constants for the two excited rotational states. For $He35ClF$ the spectroscopic constants are $B=5586.417(20)MHz,D=1.370(5)MHz,eqQJ=2=-133.70(6)MHz,andeqQJ=1=-133.79(6)MHz$. Rotational transitions of the three most abundant $(ClF)2$ isotopomers were also observed experimentally. These are attributed to $(ClF)2$ on the basis of MP2 level geometry optimizations. The calculated equilibrium structure is L shaped with nearly linear $F-Cl\cdots F,aCl\cdots F$ van der Waals bond distance $2.8\backslash AA$ and the $Cl\cdots F-Cl$ angle $112\circ$. For $(35ClF)2J=6\leftarrow 5$ and $J=7\leftarrow 6$ transitions were observed at 13.0 GHz and 15.1 GHz respectively.