Thermal Transformation of cis - and trans -[{Mo(CO) 3 H} 2 {μ-η 5 :η 5 -C 5 H 3 (SiMe 2 ) 2 C 5 H 3 }] into [{Mo(CO) 3 } 2 {μ-η 5 :η 5 -C 5 H 4 (SiMe 2 SiMe 2 )- C 5 H 4 }]( Mo − Mo ) Promoted by Hydride Migration