Bind the Ribbon:Button Tag Property to the ThemedDataGrid SelectedIndex Property

Good Afternoon,

I try to Bind the Tag Property of a ThemedDataGrid ContextMenu Button to the Grids SelectedIndex...

All I get is : Error: 4 : Cannot find source for binding with reference 'ElementName=processDataGrid'. BindingExpression:Path=SelectedIndex; DataItem=null; target element is 'Button' (Name=''); target property is 'Tag' (type 'Object')

The Goal: My Command should receive the SelectedIndex or better the SelectedItem as a Paramter.

Here is my approach

<datagrid:ThemedDataGrid x:Name="processDataGrid" AutoGenerateColumns="False" ItemsSource="{Binding ProcessList}" SelectionMode="Single">

	<datagrid:ThemedDataGrid.ContextMenu>
		<ribbon:ContextMenu>
			<StackPanel>
				<ribbon:Menu>
					<ribbon:Separator Label="Processes" />
					<ribbon:Button Label="Watch this" ImageSourceSmall="Ressources/Images/16/process-browse.png"
								   Command="{Binding AddWatchCommand}" 
                                                                   Tag="{Binding SelectedIndex, ElementName=processDataGrid}"/>
				</ribbon:Menu>
			</StackPanel>
		</ribbon:ContextMenu>
	</datagrid:ThemedDataGrid.ContextMenu>

	<datagrid:ThemedDataGrid.Columns>
		<DataGridTextColumn x:Name="ProcessName" Binding="{Binding ProcessName}" Header="Process Name" Width="*" />
		<DataGridTextColumn x:Name="Id" Binding="{Binding Id}" Header="Process Id" Width="75" />
		<DataGridTextColumn x:Name="Responding"
							Binding="{Binding Responding, Converter={StaticResource ProcessRespondingToTextConverter}}"
							Header="Responding" Width="100" />
		<DataGridTextColumn x:Name="TotalProcessorTime" Binding="{Binding TotalProcessorTime}"
							Header="Total Processor Time" Width="150" />
		<DataGridTextColumn x:Name="WorkingSet64"
							Binding="{Binding WorkingSet64, Converter={StaticResource FileSizeConverter}}"
							Header="Working-Set 64" Width="150" />
	</datagrid:ThemedDataGrid.Columns>

</datagrid:ThemedDataGrid>