Building upon our earlier results on the chemistry of diruthenium analogue of pentaborane (9) with heterocumulenes, we continued to investigate the reactivity of arachno-[(Cp∗Ru)2(B3H8)(CS2H)], 1, (Cp∗ = η5-C5Me5) towards group 7 and 8 transition metal carbonyl compounds under photolytic and thermolytic conditions. The metal carbonyl compounds show diverse reactivity pattern with arachno-1. For example, the photolysis of arachno-1 with [Re2(CO)10] yielded [(Cp∗Ru)2B3H5(CH2S2){Re(CO)4}2], 2, [(Cp∗RuCO)2(μ-H)2(CH2S2){Re(CO)4}{Re(CO)3}], 3 and [(Cp∗Ru)2(μ-CO)(μ-H)(CH2S2){Re(CO)3}], 4. The geometry of 2 with a nearly planar eight-membered ring containing heavier transition metals rhenium, ruthenium is unprecedented. Compounds 3 and 4 can be considered as M4-quadrilateral and M3-triangle with a methylenedithiolato ligand attached to the metal centres, respectively. [Mn2(CO)10], on the other hand, reacts with arachno-1 to yield heterometallic binuclear [(Cp∗RuCO){Mn(CO)4}(μ-H)(SCH3)], 5 and homocubane [(Cp∗Ru)2{Mn(CO)3}-(CS2H2)B3H4], 6. In an attempt to generate group 8 analogues of 2-5, we performed the reaction of arachno-1 with [Fe2(CO)9] and [Ru3(CO)12]. Although, the objective of isolating analogous compounds was not achieved, the reaction with [Fe2(CO)9] led to novel tetrahedral cluster [(Cp∗RuCO){(Fe(CO)3}2S(μ-H)], 7. [Ru3(CO)12], in contrast, yielded known compounds [{Cp∗Ru(CO)}2B2H6], 9 and [Cp∗Ru(CO)2]2, 10. All the cluster compounds have been characterized by mass spectrometry, IR, and 1H, 11B, and 13C NMR spectroscopy, and the geometric structures were unequivocally established by crystallographic analysis of 2-5 and 7. © The Royal Society of Chemistry.