The reaction of [(η 2-CCPh)Co 2(CO) 6](Me)Si(H)(Cl),III, with different substrates is described: III yield with 0.5 equivalents of Co 2(CO) 8, II, under oxidative addition, [η 2-CCPh)Co 2(CO) 6](Me) Si[Co(CO) 4](Cl), IV. IV may also be synthesized directly by reaction of (PhCC)(Me)Si(H)(Cl), I, with 1.5 equivalents of Co 2(CO) 8. Treatment of III or IV with an excess of ROH (R = H, Me, Et) affords, in high yields, complex [η 2-CCPh)Co 2(CO) 6](Me)Si(OR) 2, V, in which the H and Cl groups in III are substituted by OR. However, the hydrolysis of III with one equivalent of H 2O yields the disiloxane {[η 2-CCPh)Co 2(CO) 6](Me)(H)Si} 2O, VII. The formation of [(η 2-CCPh)Co 2(CO) 6](Me)Si(H)(OH), VI, as a reactive intermediate is discussed. Treatment of compound III withBrMgCCPh affords [(η 2-CCPh)Co 2(CO) 6](PhCC)Si(H)(Me), VIII. VIII reacts with further Co 2(CO) 8, II, to yield [(η 2-CCP h)Co 2(CO) 6] 2Si(H)(Me). IX, in which each of the two phenylethynyl units is η 2-side-on co-ordinated to “CO 2(CO) 6” to form dicobalta-tetrahedrane cluster units. [(η 2-CCPh)Co 2(CO) 6](PhCC)Si(H)(Cl), XI, shows a similar reaction behaviour as III: With H 2O selectively the silanol [(η 2-CCPh)Co 2(CO) 6](PhCC)Si(H)(OH), XII, is formed, which upon reaction with one equivalent of Co 2(CO) 8, II, yields [(η 2-CCPhCo 2(CO) 6] 2Si(H)(OH), XIV, XIV may also be synthesized by treatment of XI with II, yielding in the first step [(η 2-CCPh)Co 2(CO) 6] 2Si(H)(Cl), XIII. Hydrolysis of XIII in the second step then yields quantitatively XIV. All new compounds have been characterized by analytical and spectroscopic data (IR, 1H, 13C NMR, MS).