RSeCCPh (1a, R=Et; 1b, R=n-Bu; 1c, R=Ph; 1d, R=2,4,6-Me 3 C 6 H 2 ) reacts with equimolar amounts of Fe 2 (CO) 9 (2) to give [(μ-SeR)(μ-σ,π-CCPh)]Fe 2 (CO) 6 (3a, R=Et; 3b, R=n-Bu; 3c, R=Ph; 3d, R=2,4,6-Me 3 C 6 H 2 ).Complexes 3a–3d exist as two isomers, depending on the axial or equatorial position of R at selenium.Addition of P(O i C 3 H 7 ) 3 (4) to 3d affords {(μ-Se-2,4,6-Me 3 C 6 H 2 )[μ-η 1 -CCPh(P(O i C 3 H 7 ) 3 )]}Fe 2 (CO) 6 (5) along with {(μ-Se-2,4,6-Me 3 C 6 H 2 )[μ-η 1 :η 1 -PhCC(P(O i C 3 H 7 ) 3 )]}Fe 2 (CO) 6 (6).The solid-state structures of 3d, 5 and 6 were determined by single X-ray structure analysis.In mononuclear 3d the Fe(CO) 3 fragments are bridged by a μ-Se-2,4,6-Me 3 C 6 H 2 and a μ-σ,π-CCPh unit, resulting in an over-all butterfly arrangement.Due to steric reasons, the mesityl group is pointing away from the PhCC entity and hence, is located in an equatorial position.Compounds 5 and 6, which co-crystallise in the ratio of 7:93, feature aμ-bridging 2,4,6-Me 3 C 6 H 2 Se unit and either a vinylidenic CCPh(P(O i C 3 H 7 ) 3 ) (complex 5) or a olefinic PhCC(P(O i C 3 H 7 ) 3 ) (complex 6) building block of which the latter entity is part of a diiron cyclobutene ring.