Successive treatment of 9-(phenylethynyl)fluoren-9-ol (
1a), with HBr, butyllithium and chlorodiphenylphosphine furnishes 3,3-(biphenyl-2,2′-diyl)-1-diphenylphosphino-1-phenylallene (
5). Moreover, reaction of
1a directly with chlorodiphenylphosphine yields the corresponding allenylphosphine oxide (
6). The allenylphosphine (
5), and Fe
2(CO)
9 initially form the phosphine–Fe(CO)
4 complex,
11, which is very thermally sensitive and readily loses a carbonyl ligand. In the resulting phosphine–Fe(CO)
3 system,
12, the additional site at iron is coordinated by the allene double bond adjacent to phosphorus; the Fe(CO)
3 tripod in
12 exhibits restricted rotation on the NMR time-scale even at room temperature. The corresponding chromium complex, (
5)-Cr(CO)
5 (
9), has also been prepared. The gold complexes (
5)-AuCl (
13), and [(
5)-Au(THT)]
+ X
−, where (THT) is tetrahydrothiophene, and X = PF
6 (
14a), or ClO
4 (
14b), are analogous to the known triphenylphosphine–gold complexes. In contrast, in the (arene)(allenylphosphine)RuCl
2 system the allene double bond adjacent to phosphorus displaces a chloride, and the resulting cationic species undergoes nucleophilic attack by water yielding ultimately a five-membered Ru–P–C
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C–O ruthenacycle (
17). Thus, the allenylphosphine (
5), reacts initially as a conventional mono-phosphine but, when the metal centre has a readily displaceable ligand such as a carbonyl or halide, the allene double bond adjacent to the phosphorus can also function as a donor. X-ray crystal structures are reported for
5,
6,
11,
12,
13,
14a,
14b and
17.
