Sharing columns or rows in a grid - wpf

Question

I'm trying to create an asymmetrical layout using a Grid where i have 2 rows, 2 columns and an extra shared column as follows:
<Grid Background="Black">
<Grid.RowDefinitions>
<RowDefinition Height="200" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Rectangle
Grid.Row="0"
Grid.Column="0"
Width="200"
Height="200"
Fill="Red" />
<Rectangle
Grid.Row="0"
Grid.Column="1"
Grid.ColumnSpan="2"
Width="250"
Height="200"
Fill="Blue" />
<Rectangle
Grid.Row="1"
Grid.Column="0"
Grid.ColumnSpan="2"
Width="250"
Height="200"
Fill="Yellow" />
<Rectangle
Grid.Row="1"
Grid.Column="2"
Width="200"
Height="200"
Fill="Green" />
</Grid>
But however i try to set it up, the second column always collapses unless i explicitly set a fixed width (in this case 50px). Why is this happening?
Shouldn't the second column resize itself to the remainder of each rectangle?

Answer 1

Using a Converter i managed to manually calculate the shared column size by placing the content of the first column in a container and binding the width of the shared column to the ActualWidth of that container subtracted by the first column width.
e.g.
<Grid Background="Black">
<Grid.RowDefinitions>
<RowDefinition Height="200" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200" />
<ColumnDefinition Width="{Binding ElementName=Rect3, Path=ActualWidth, Converter={StaticResource SharedColumnConverter}, ConverterParameter=200}" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Rectangle
Grid.Row="0"
Grid.Column="0"
Width="200"
Height="200"
Fill="Red" />
<Rectangle
Grid.Row="0"
Grid.Column="1"
Grid.ColumnSpan="2"
Width="250"
Height="200"
Fill="Blue" />
<Rectangle
Name="Rect3"
Grid.Row="1"
Grid.Column="0"
Grid.ColumnSpan="2"
Width="250"
Height="200"
Fill="Yellow" />
<Rectangle
Grid.Row="1"
Grid.Column="2"
Width="200"
Height="200"
Fill="Green" />
</Grid>